Fig. 30.—Diagram of absorber table. 1 and 2 contain sulphuric acid; K contains potash-lime; G, sodium bicarbonate can; F, rotary blower for maintaining air-current; H, valves for closing either side; and D, mercury manometer and valve for diverting air to U-tubes on table. Air leaves A, passes through the meter, and then through drying tower B and through C to ingoing air-pipe. At the left is the regulating rheostat and motor and snap-switch. General direction of ventilation is indicated by arrows.Fig. 30.—Diagram of absorber table. 1 and 2 contain sulphuric acid; K contains potash-lime; G, sodium bicarbonate can; F, rotary blower for maintaining air-current; H, valves for closing either side; and D, mercury manometer and valve for diverting air to U-tubes on table. Air leaves A, passes through the meter, and then through drying tower B and through C to ingoing air-pipe. At the left is the regulating rheostat and motor and snap-switch. General direction of ventilation is indicated by arrows.
The absorber table, with theU-tubes and meter for residual analyses, is shown in the foreground in fig. 2. The two white porcelain vessels with a silver-plated can between them are on the middle shelf. The sodium bicarbonate can, for removing traces of acid fumes, is connected in an uprightposition, while the motor, the controlling rheostat, and the blower are supported by the legs near the floor. The two rubber pipes leading from the table can be used to connect the apparatus either with the bed or chair calorimeter. In fig. 4 the apparatus is shown connected with the bed calorimeter, but just above the lowest point of the rubber tubing can be seen in the rear the coupling for one of the pipes leading from the chair calorimeter. The other is immediately below and to the left of it.
The residual air inside of the chamber amounts to some 1,300 liters and contains about 250 liters of oxygen. Consequently it can be seen that in an 8-hour experiment the subject could easily live during the entire time upon the amount of oxygen already present in the residual air. It has been repeatedly shown that until the per cent of oxygen falls to about 11, or about one-half normal, there is no disturbance in the respiratory exchange and therefore about 125 liters of oxygen would be available for respiration even if no oxygen were admitted. Inasmuch as the subject when at rest uses not far from 14 to 15 liters per hour, the amount originally present in the chamber would easily suffice for an 8-hour experiment. Moreover, the difficulties attending an accurate gas analysis and particularly the calculation of the total amount of oxygen are such that satisfactory determinations of oxygen consumption by this method would be impossible. Furthermore, from our previous experience with long-continued experiments of from 10 days to 2 weeks, it has been found that oxygen can be supplied to the system readily and the amount thus supplied determined accurately. Consequently, even in these short experiments, we adhere to the original practice of supplying oxygen to the air and noting the amount thus added.
The oxygen supply was formerly obtained from small steel cylinders of the highly compressed gas. This gas was made by the calcium-manganate method and represented a high degree of purity for commercial oxygen. More recently we have been using oxygen of great purity made from liquid air. Inasmuch as this oxygen is very pure and much less expensive than the chemically-prepared oxygen, extensive provisions have been made for its continued use. Instead of using small cylinders containing 10 cubic feet and attaching thereto purifying devices in the shape of soda-limeU-tubes and a sulphuric-acid drying-tube, we now use large cylinders and we have found that the oxygen from liquid air is practically free from carbon dioxide and water-vapor, the quantities present being wholly negligible in experiments such as these. Consequently, no purifying attachments are considered necessary and the oxygen is delivered directly from the cylinder. The cylinders, containing 100 cubic feet (2,830 liters), under a pressure of 120 atmospheres, are provided with well-closing valves and weigh when fully charged 57 kilograms.
Fig. 31.—Diagram of oxygen balance and cylinder. At the top is the balance arrangement, and at the center its support. At the left is the oxygen cylinder, with reducing valve A, rubber tube D leading from it, F the electro-magnet which opens and closes D, K the hanger of the cylinder and support for the magnet, R the lever which operates the supports for the cylinder and its counterpoise S, T' a box which is raised and lowered by R, and T its surrounding box.Fig. 31.—Diagram of oxygen balance and cylinder. At the top is the balance arrangement, and at the center its support. At the left is the oxygen cylinder, with reducing valve A, rubber tube D leading from it, F the electro-magnet which opens and closes D, K the hanger of the cylinder and support for the magnet, R the lever which operates the supports for the cylinder and its counterpoise S, T' a box which is raised and lowered by R, and T its surrounding box.
It is highly desirable to determine the oxygen to within 0.1 gram, and we are fortunate in having a balance of the type used frequently in this laboratory which will enable us to weigh this cylinder accurately with a sensitiveness of less than 0.1 gram. Since 1 liter of oxygen weighs 1.43 grams, it can be seen that the amount of oxygen introduced into the chambercan be measured by this method within 70 cubic centimeters. Even in experiments of but an hour's duration, where the amount of oxygen admitted from the cylinder is but 25 to 30 grams, it can be seen that the error in the weighing of the oxygen is much less than 1 per cent.
The earlier forms of cylinders used were provided with valves which required some special control and a rubber bag was attached to provide for any sudden rush of gas. The construction of the valve and valve-stem was unfortunately such that the well-known reduction valves could not be attached without leakage under the high pressure of 120 atmospheres. With the type of cylinder at present in use, such leakage does not occur and therefore we simply attach to the oxygen cylinder a reduction-valve which reduces the pressure from 120 atmospheres to about 2 or 3 pounds to the square inch. The cylinder, together with the reduction valve, is suspended on one arm of the balance. The equipment of the arrangement is shown in fig. 31. (See also fig. 5, page 4.) The cylinder is supported by a clamp K hung from the balance arm, and the reduction-valve A is shown at the top. The counterpoise S consists of a piece of 7-inch pipe, with caps at each end. At a convenient height a wooden shelf with slightly raised rim is attached.
In spite of the rigid construction of this balance, it would be detrimental to allow this enormous weight to remain on the knife-edges permanently, so provision is made for raising the cylinders on a small elevator arrangement which consists of small boxes of wood, T, into which telescope other boxes, T'. A lever handle, R, when pressed forward, raises T' by means of a roller bearing U, and when the handle is raised the total weight of the cylinders is supported on the platforms.
The balance is attached to an upright I-beam which is anchored to the floor and ceiling of the calorimeter laboratory. Two large turnbuckle eye-bolts give still greater rigidity at the bottom. The whole apparatus is inclosed in a glass case, shown in fig. 5.
The use of the reduction-valve has made the automatic control of the oxygen supply much simpler than in the apparatus formerly used. The details of the connections somewhat schematically outlined are given in fig. 32, in which D is the oxygen cylinder, K the supporting band, A the reduction-valve, and J the tension-equalizer attached to one of the calorimeters. Having reduced the pressure to about 2 pounds by means of the reduction-valve, the supply of oxygen can be shut off by putting a pinch-cock on a rubber pipe leading from the reduction-valve to the calorimeters. Instead of using the ordinary screw pinch-cock, this connection is closed by a spring clamp. The spring E draws on the rod which is connected at L and pinches the rubber tube tightly. The tension at E can bereleased by an electro-magnet F, which when magnetized exercises a pull on the iron rod, extends the spring E, and simultaneously releases the pressure on the rubber tube at L. To make the control perfectly automatic, the apparatus shown on the top of the tension-equalizer J is employed. A wire ring, with a wire support, is caused to pass up through a bearing fastened to the clamp above J. As the air inside of the whole system becomes diminished in volume and the rubber cap J sinks, there is a point at which a metal loop dips into two mercury cups C and C', thus closing the circuit, which causes a current of electricity to pass through F. This releases the pressure at L, oxygen rushes in, and the rubber bag J becomes distended. As it is distended, it lifts the metal loop out of the cups, C and C', and the circuit is broken. There is, therefore, an alternate opening and closing of this circuit with a corresponding admission of oxygen. The exact position of the rubber diaphragm can be read when desired from a pointer on a graduated scale attached to a support holding the terminals of the electric wires. More frequently, however, when the volume is required, instead offilling the bag to a definite point, as shown by the pointer, a delicate manometer is attached to the can by means of a pet-cock and the oxygen is admitted by operating the switch B until the desired tension is reached.
Fig. 32.—Part of the oxygen cylinder and connections to tension-equalizer. At the left is shown the upper half of the oxygen cylinder with a detail of the electro-magnet and reducing-valve. D is the cylinder; K, the band supporting the oxygen cylinder and electro-magnet arrangement; F, the electro-magnet; E, the tension spring; and L, the rubber tubing at a point where it is closed by the clamp. The tension-equalizer and the method of closing the circuit operating it are shown at the right. C and C' are two mercury cups into which the wire loop dips, thus closing the circuit. B is a lever used for short-circuiting for filling the diaphragm J. G is a sulphuric-acid container; H, the quick-throw valve for shutting off the tension equalizer J; M, part of the ingoing air-pipe; N, a plug connecting the electric circuit with the electro-magnet; and O, a storage battery.Fig. 32.—Part of the oxygen cylinder and connections to tension-equalizer. At the left is shown the upper half of the oxygen cylinder with a detail of the electro-magnet and reducing-valve. D is the cylinder; K, the band supporting the oxygen cylinder and electro-magnet arrangement; F, the electro-magnet; E, the tension spring; and L, the rubber tubing at a point where it is closed by the clamp. The tension-equalizer and the method of closing the circuit operating it are shown at the right. C and C' are two mercury cups into which the wire loop dips, thus closing the circuit. B is a lever used for short-circuiting for filling the diaphragm J. G is a sulphuric-acid container; H, the quick-throw valve for shutting off the tension equalizer J; M, part of the ingoing air-pipe; N, a plug connecting the electric circuit with the electro-magnet; and O, a storage battery.
In order to provide for the maximum sensitiveness for weighing D and its appurtenances, the electric connection is broken at the cylinder by means of the plug N and the rubber tube is connected by a glass connector which can be disconnected during the process of weighing. Obviously, provision is also made that there be no leakage of air out of the system during the weighing. The current at F is obtained by means of a storage battery O. The apparatus has been in use for some time in the laboratory and has proved successful in the highest degree.
The rigid walls of the calorimeter and piping necessitate some provision for minor fluctuations in the absolute volume of air in the confined system. The apparatus was not constructed to withstand great fluctuations in pressure, and thin walls were used, but it is deemed inadvisable to submit it even to minor pressures, as thus there would be danger of leakage of air through any possible small opening. Furthermore, as the carbon dioxide and water-vapor are absorbed out of the air-current, there is a constant decrease in volume, which is ordinarily compensated by the admission of oxygen. It would be very difficult to adjust the admission of oxygen so as to exactly compensate for the contraction in volume caused by the absorption of water-vapor and carbon dioxide. Consequently it is necessary to adjust some portion of the circulating air-current so that there may be a contraction and expansion in the volume without producing a pressure on the system. This was done in a manner similar to that described in the earlier apparatus, but on a much simpler plan.
To the air-pipe just before it entered the calorimeter was attached a copper can with a rubber diaphragm top. This diaphragm, which is, as a matter of fact, a ladies' pure rubber bathing-cap, allows for an expansion or contraction of air in the system of 2 to 3 liters. The apparatus shown in position is to be seen in fig. 25, in which the tin can I is covered with the rubber diaphragm J. If there is any change in volume, therefore, the rubber diaphragm rises or falls with it and under ordinary conditions of an experiment this arrangement results in a pressure in the chamber approximately that of the atmosphere. It was found, however, that even the slight resistance of the piping from the tension-equalizer to the chamber, a pipe some 26 millimeters in diameter and 60 centimeters long, was sufficient to cause a slightly diminished pressure inside the calorimeter, inasmuch as the air was sucked out by the blower with a little greater speed than it was forced in by the pressure at the diaphragm. Accordingly the apparatus has been modified so that at present the tension-equalizer is attached directly to the wall of the calorimeter independent of the air-pipe.
In most of the experiments made thus far it has been our custom to conduct the supply of fresh oxygen through pet-cock K on the side of the tension-equalizer. This is shown more in detail in fig. 32, in which, also, is shown the interior construction of the can. Owing to the fact that the air inside of this can is much dryer than the room air, we have followed the custom with the earlier apparatus of placing a vessel containing sulphuric acid inside the tension-equalizer, so that any moisture absorbed by the dry air inside the diaphragm may be taken up by the acid and not be carried into the chamber. The air passing through the pipe to the calorimeter is, it must be remembered, absolutely dry and hence there are the best conditions for the passage of moisture from the outside air through the diaphragm to this dry air. Attaching the tension-equalizer directly to the calorimeter obviates the necessity for this drying process and hence the sulphuric-acid vessel has been discarded.
The valve H (fig. 25) is used to cut off the tension-equalizer completely from the rest of the system at the exact moment of the end of the experimental period. After the motor has been stopped and the slight amount of air partly compressed in the blower has leaked back into the system, and the whole system is momentarily at equal tension, a process occupying some 3 or 4 seconds, the gate-valve H is closed. Oxygen is then admitted from the pet-cock K until there is a definite volume in J as measured by the height to which the diaphragm can rise or a second pet-cock is connected to the can I and a delicate petroleum manometer attached in such a manner that the diaphragm can be filled to exactly the same tension each time. Under these conditions, therefore, the apparent volume of air in the system, exclusive of the tension-equalizer, is always the same, since it is confined by the rigid walls of the calorimeter and the piping. Furthermore, the apparent volume of air in the tension-equalizer is arbitrarily adjusted to be the same amount at the end of each period by closing the valve and introducing oxygen until the tension is the same.
Recognizing the importance of measuring very accurately the barometric pressure, or at least its fluctuations, we have installed an accurate barometer of the Fortin type, made by Henry J. Green. This is attached to the inner wall of the calorimeter laboratory, and since the calorimeter laboratory is held at a constant temperature, temperature corrections are unnecessary, for we have here to deal not so much with the accurate measurement of the actual pressure as with the accurate measurement of differences in pressure. For convenience in reading, the ivory needle at the base of the instrument and the meniscus are well illuminated with electric lamps behind a white screen, and a small lamp illuminates the vernier. The barometer can be read to 0.05 millimeter.
The carbon-dioxide production, water-vapor elimination, and oxygen absorption of the subject during 1 or 2 hour periods are recorded in a general way by the amounts of carbon dioxide and water-vapor absorbed by the purifying vessels and the loss of weight of the oxygen cylinder; but, as a matter of fact, there may be considerable fluctuations in the amounts of carbon dioxide and water-vapor and particularly oxygen in the large volume of residual air inside the chamber. With carbon dioxide and water-vapor this is not as noticeable as with oxygen, for in the 1,300 liters of air in the chamber there are some 250 liters of oxygen, and slight changes in the composition of this air indicate considerable changes in the amount of oxygen. Great changes may also take place in the amounts of carbon dioxide and water-vapor under certain conditions. In some experiments, particularly where there are variations in muscular activity from period to period, there may be a considerable amount of carbon dioxide in the residual air and during the next period, when the muscular activity is decreased, for example, the percentage composition of the air may vary so much as to indicate a distinct fall in the amount of carbon dioxide present. Under ordinary conditions of ventilation during rest experiments the quantity of carbon dioxide present in the residual air is not far from 8 to 10 grams. There are usually present in the air not far from 6 to 9 grams of water-vapor, and hence this residual amount can undergo considerable fluctuations. When it is considered that an attempt is made to measure the total amount of carbon dioxide expired in one hour to the fraction of a gram, it is obvious that fluctuations in the composition of residual air must be taken into consideration.
It is extremely difficult to get a fair sample of air from the chamber. The air entering the chamber is free from water-vapor and carbon dioxide. In the immediate vicinity of the entering air-tube there is air which has a much lower percentage of carbon dioxide and water-vapor than the average, and on the other hand close to the nose and mouth of the subject there is air of a much higher percentage of carbon dioxide and water-vapor than the average. It has been assumed that the composition of the air leaving the chamber represents the average composition of the air in the chamber. This assumption is only in part true, but in rest experiments (and by far the largest number of experiments are rest experiments) the changes in the composition of the residual air are so slow and so small that this assumption is safe for all practical purposes.
Another difficulty presents itself in the matter of determining the amount of carbon dioxide and water-vapor; that is, to make a satisfactory analysis of air without withdrawing too great a volume from the chamber. The difficulty in analysis is almost wholly confined to the determination ofwater-vapor, for while there are a large number of methods for determining small amounts of carbon dioxide with great accuracy, the method for determining water-vapor to be accurate calls for the use of rather large quantities of air. From preliminary experiments with a sling psychrometer it was found that its use was precluded by the space required to successfully use this instrument, the addition of an unknown amount of water to the chamber from the wet bulb, and the difficulties of reading the instrument from without the chamber. Recourse was had to the determination of moisture by the absolute method, in that a definite amount of air is caused to pass over pumice-stone saturated with sulphuric acid. It is of interest here to record that at the moment of writing a series of experiments are in progress in which an attempt is being made to use a hair hygrometer for this purpose.
The method of determining the water-vapor and carbon dioxide in the residual air is extremely simple, in that a definite volume of air is caused to pass over sulphuric acid and soda-lime contained inU-tubes. In other words, a small amount of air is caused to pass through a small absorbing-system constructed ofU-tubes rather than of porcelain vessels and silver-plated cans. Formerly a very elaborate apparatus was employed for aspirating the air from the chamber throughU-tubes and then returning the aspirated air to the chamber. This involved the use of a suction-pump and called for a special installation for maintaining the pressure of water constant. More recently a much simpler device has been employed, in that we have taken advantage of the pressure in the ventilating air-system developed by the passage of air through the blower. After forcing a definite quantity of air through the reagents in theU-tubes, it is then conducted back to the system after having been measured in a gas-meter.
This procedure is best noted from fig. 30. The connected series of threeU-tubes on the rack on the table is joined on one end by well-fitting rubber connections to the tube leading from the mercurial manometer and on the other end to the rubber tube A leading to the gas-meter. On lowering the mercury reservoir E, the mercury is drained out of the tube D and air passes through both arms of the tube and then through the threeU-tubes. In the first of these it is deprived of moisture, and in the last two of carbon dioxide. The air then enters the meter, where it is measured and leaves the meter through the tube B, saturated with water-vapor at the room temperature. To remove this water-vapor the air is passed through a tower filled with pumice-stone drenched with sulphuric acid. It leaves the tower through the tube C and enters the ventilating air-pipe on its way to the calorimeter.
The method of manipulation is very simple. After connecting theU-tubes the pet-cock connecting the tube C with the pipe is opened, the mercury reservoir E is lowered, and air is allowed to pass through untilthe meter registers 10 liters. By raising the reservoir E the air supply is shut off, and after closing the stop-cock at C the tubes are disconnected, a second set is put in place, and the operation repeated. TheU-tubes are of a size having a total length of the glass portion equal to 270 millimeters and an internal diameter of 16 millimeters. They permit the passage of 3 liters of air per minute through them without a noticeable escape of water-vapor or carbon dioxide. TheU-tubes filled with pumice-stone and sulphuric acid weigh 90 grams. They are always weighed on the balance with a counterpoise, but no attempt is made to weigh them closer than to 0.5 milligram.
The gas-meter is made by the Dansk Maalerfabrik in Copenhagen, and is of the type used by Bohr in many of his investigations. It has the advantage of showing the water-level, and the volume may be read directly. The dial is graduated so as to be read within 50 cubic centimeters.
The Elster meter formerly used for this purpose was much smaller than the meter of the Dansk Maalerfabrik we are now using. The volume of water was much smaller and consequently the temperature fluctuations much more rapid. While the residual analyses for which the meter is used are of value in interpolating the results for the long experiments, and consequently errors in the meter would be more or less constant, affecting all results alike, we have nevertheless carefully calibrated the meter by means of the method of admitting oxygen from a weighed cylinder.[23]The test showed that the meter measured 1.4 per cent too much, and consequently this correction must be applied to all measurements made with it.
With an apparatus as elaborate as is the respiration calorimeter and its accessories, the calculation of results presents many difficulties, but the experience of the past few years has enabled us to lessen materially the intricacies of the calculations formerly thought necessary.
The total amount of water-vapor leaving the chamber is determined by noting the increase in weight of the first sulphuric-acid vessel in the absorber system. This vessel is weighed with a counterpoise and hence only the increment in weight is recorded. A slight correction may be necessary here, as frequently the absorber is considerably warmer at the end of the period than at the beginning and if weighed while warm there may be an error of 0.1 to 0.2 gram. If the absorbers are weighed at the same temperature at the beginning and end, this correction is avoided.
The amount of carbon dioxide absorbed from the ventilating air-current is found by noting the changes in weight of the potash-lime can and the last sulphuric-acid vessel. As shown by the weights of this latter vessel, it is very rare that sufficient water is carried over from the potash-lime to the sulphuric acid to cause a perceptible change in temperature, and no temperature corrections are necessary. It may occasionally happen that the amount of carbon dioxide absorbed is actually somewhat less than the amount of water-vapor abstracted from the reagent by the dry air-current as it passes through the can. The conditions will then be such that there will be a loss in weight of the potash-lime can and a large gain in weight of the sulphuric-acid vessel. Obviously, the algebraic sum of these amounts will give the true weight of the carbon dioxide absorbed.
The amount of oxygen admitted is approximately measured by noting the loss in weight of the oxygen cylinder. Since, however, in admitting the oxygen from the cylinder there is a simultaneous admission of a small amount of nitrogen, a correction is necessary. This correction can be computed either by the elaborate formulas described in the publication of Atwater and Benedict[24]or by the more abbreviated method of calculation which has been used very successfully in all short experiments in this laboratory. In either case it is necessary to know the approximate percentage of nitrogen in the oxygen.
With the modified method of computation discussed in detail on page 88 it is seen that such exceedingly exact analyses of oxygen as were formerly made are unnecessary, and further calculation is consequently very simple if we know the percentage of nitrogen to within a fraction of 1 per cent. We have used a Haldane gas-analysis apparatus for analyzing the oxygen, although the construction of the apparatus is such that this presents somelittle difficulty. It is necessary, for example, to accurately measure about 16 cubic centimeters of pure nitrogen, pass it into the potassium pyrogallate pipette, and then (having taken a definite sample of oxygen) gradually absorb the oxygen in the potassium pyrogallate and measure subsequently the accumulated nitrogen. The analysis is tedious and not particularly satisfactory. Having checked the manufacturer's analysis of a number of cylinders of oxygen and invariably found them to agree with our results, we are at present using the manufacturer's guaranteed analysis. If there was a very considerable error in the gas analysis, amounting even to 1 per cent, the results during short experiments would hardly be affected.
A careful inspection of the elaborate method of calculation required for use with the calorimeter formerly at Wesleyan University shows that a large proportion of it can be eliminated owing to the fact that we are here able to work in a room of constant temperature. It has been pointed out that the fluctuations in the temperature of the gas-meter affect not only the volume of the gas passing through the meter, but likewise the tension of aqueous vapor. The corrections formerly made for temperature on the barometer are now unnecessary; finally (and perhaps still more important) it is no longer necessary to subdivide the volume of the system into portions of air existing under different temperatures, depending upon whether they were in the upper or lower part of the laboratory. In other words, the temperature of the whole ventilating circuit and chamber, with the single exception of the air above the acid in the first sulphuric-acid absorber, may be said to be constant. During rest experiments this assumption can be made without introducing any material error, but during work experiments it is highly probable that some consideration must be given to the possibility of the development of a considerable temperature rise in the air of the potash-lime absorbers, due to the reaction between the carbon dioxide and the solid absorbent. It is thus apparent that the constant-temperature conditions maintained in the calorimeter laboratory not only facilitate calorimetric measurements, but also simplify considerably the elaborate calculations of the respiratory exchange formerly required.
In the earlier form of apparatus the largest variation in the apparent volume of air was due to the fluctuations in the height of the large rubber diaphragms used on the tension equalizer. In the present form of apparatus there is but one rubber diaphragm, and this is small, containing not more than 3 to 4 liters as compared to about 30 liters in the earlier double rubber diaphragms. As now arranged, all fluctuations due to the varyingpositions of the tension-equalizer are eliminated as each experimental period is ended with the diaphragm in exactly the same position,i. e., filled to a definite tension.
In its passage through the purifiers the air is subjected to more or less pressure, and it is obvious that if these absorbers were coupled to the ventilating system under atmospheric pressure, and then air caused to pass through them, there would be compression in a portion of the purifier system. Thus there would be a contraction in the volume, and air thus compressed would subsequently be released into the open air when the absorbers were uncoupled. The method of testing the system outlined on page 100 equalizes this error, however, in that the system is tested under the same pressure used during an actual experiment, and hence between the surface of the sulphuric acid in the first porcelain vessel and the sulphuric acid in the second porcelain vessel there is a confined volume of air which at the beginning of an experimental period is under identically the same pressure as it is at the end. There is, then, no correction necessary for the rejection of air with the changes in the absorber system.
As the water-vapor is absorbed by the sulphuric acid, there is a slight increase in volume of the acid. This naturally results in the diminution of the apparent volume of air and likewise again affects the amount of oxygen admitted to produce constant apparent volume at the end of each experimental period. The amount of increase which thus takes place for each experimental period is very small. It has been found that an increase in weight of 25 grams of water-vapor results in an increase in volume of the acid of some 15 cubic centimeters. Formerly this correction was made, but it is now deemed unnecessary and unwise to introduce a refinement that is hardly justified in other parts of the apparatus. Similarly, there is theoretically at least an increase in volume of the potash-lime by reason of the absorption of the carbon dioxide. This was formerly taken into consideration, but the correction is no longer applied.
With experiments on man, there is a constant transformation of solid body material into gaseous products which are carried out into the air-current and absorbed. Particularly where no food is taken, this solid material becomes smaller in volume and consequently additional oxygen is required to take the place of the decrease in volume of body substance. But this so-called respiratory loss is more theoretical than practical in importance, and in the experiments made at present the correction is not considered necessary.
The ventilating air-circuit may be said to consist of several portions of air. The largest portion is that in the respiration chamber itself and consists of air containing oxygen, nitrogen, carbon dioxide, and water-vapor. This air is assumed to have the same composition up to the moment when it begins to bubble through the sulphuric acid in the first acid-absorber. The air in this absorber above the acid, amounting to about 14 liters, has a different composition in that the water-vapor has been completely removed. The same 14 liters of air may then be said to contain carbon dioxide, nitrogen, and oxygen. This composition is immediately disturbed the moment the air enters the potash-lime can, when the carbon dioxide is absorbed and the volume of air in the last sulphuric-acid absorber, in the sodium-bicarbonate can, and in the piping back to the calorimeter may be said to consist only of nitrogen and oxygen. The air then between the surface of the sulphuric acid in the last porcelain absorber and the point where the ingoing air is delivered to the calorimeter consists of air free from carbon dioxide and free from water. Formerly this section also included the tension-equalizer, but very recently we have in both of the calorimeters attached the tension-equalizer directly to the respiration chamber.
In the Middletown apparatus, these portions of air of varying composition were likewise subject to considerable variations in temperature, in that the temperature of the laboratory often differed materially from that of the calorimeter chamber itself, especially as regards the apparatus in the upper part of the laboratory room. It is important, however, to know the total volume of the air inclosed in the whole system. This is obtained by direct measurement. The cubic contents of the calorimeter has been carefully measured and computed; the volumes of air in the pipes, valve systems, absorbing vessels, and tension-equalizer have been computed from dimensions, and it has been found that the total volume in the apparatus is, deducting the volume of the permanent fixtures in the calorimeter, 1,347 liters. The corresponding volume for the bed calorimeter is 875. These values are altered by the subject and extra articles taken into the chamber.
From a series of careful measurements and special tests the following apparent volumes for different parts of the system have been calculated:
Liters.Volume of the chair calorimeter chamber (without fixtures)1360.0Permanent fixtures (5); chair and supports (8)13.0———Apparent volume of air inside chamber1347.0Air in pipes, blower, and valves to surface of acid in first acid vessel4.5———Apparent volume of air containing water-vapor1351.5Air above surface of acid in first sulphuric-acid vessel and potash-lime can16.0———Apparent volume of air containing carbon dioxide1367.5Air in potash-lime can, second sulphuric-acid vessel and connections, sodium-bicarbonate cans, and pipes to calorimeter chamber23.5———Apparent volume of air containing carbon dioxide, water, oxygen, and nitrogen1391.0
These volumes represent conditions existing inside the chamber without the subject,i. e., conditions under which an alcohol check-test would be conducted. In an experiment with man it would be necessary to deduct the volume of the man, books, urine bottles, and all supplemental apparatus and accessories. Under these circumstances the apparent volume of the air in the chamber may at times be diminished by nearly 90 to 100 liters. At the beginning of each experiment the apparent volume of air is calculated.
The increment in weight of the absorbers for water and carbon dioxide and the loss in weight of the oxygen cylinder give only an approximate idea of the amounts of carbon dioxide and water-vapor produced and oxygen absorbed during the period, and it is necessary to make correction for change in the composition of the air as shown by the residual analyses and for fluctuations in the actual volume. In order to compute from the analyses the total carbon-dioxide content of the residual air, it is necessary to know the relation of the air used for the sample to the total volume, and thus we must know accurately the volume of air passing through the gas-meter.
In the earlier apparatus 10-liter samples were used, and the volume of the respiration chamber was so large that it was necessary to multiply the values found in the residual sample by a very large factor, 500. Hence, the utmost caution was taken to procure an accurate measurement of the sample, the exact amounts of carbon dioxide absorbed, and water-vapor absorbed. To this end a large number of corrections were made, which are not necessary with the present type of apparatus with a volume of residual air of but about 1,300 liters, and accordingly the manipulation and calculations have been very greatly simplified.
While formerly pains were taken to obtain the exact temperature of the air leaving the gas-meter, with this apparatus it is unnecessary. When the earlier type of apparatus was in use there were marked changes in the temperature of the calorimeter laboratory and in the water in the meter which were naturally prejudicial to the accurate measurement of the volume of samples, but with the present control of temperature in this laboratory it has been found by repeated tests that the temperature of the water in the meter does not vary a sufficient amount to justify this painstaking measurement and calculation. Obviously, this observation also pertains to the corrections for the tension of aqueous vapor. It has been found possible to assume an average laboratory temperature and reduce the volume as read on the meter by means of a constant factor.
The quantity of air passing through the meter is so adjusted that exactly 10 liters as measured on the dial pass through it for one analysis. The air as measured in the meter is, however, under markedly different conditions from the air inside the respiration chamber. While there is the same temperature, there is a material difference in the water-vapor present, and hence the moisture content as expressed in terms of tension of aqueous vapor must be considered. This obviously tends to diminish the true volume of air in the meter.
Formerly we made accurate correction for the tension of aqueous vapor based upon the barometer and the temperature of the meter at the end of the period, but it has now been found that the reduction of the meter readings to conditions inside of the chamber can be made with a sufficient degree of accuracy by multiplying the volume of air passing through the meter by a fraction,(h-t)/h, in whichhrepresents the barometer andtthe tension of aqueous vapor at the temperature of the laboratory, 20° C. Since the tension of aqueous vapor at the laboratory temperature is not far from 15 mm., a simple calculation will show that there may be considerable variations in the value ofhwithout affecting the fraction materially, and we have accordingly assumed a value ofhas normally 760 mm., and the correction thus obtained is (760 - 15)/760 = 0.98, and all readings on the meter should be multiplied by this fraction.
On the one hand, then, there is the correction on the meter itself, which correction is +1.4 per cent (see page 75); and on the other hand the correction on the sample for the tension of aqueous vapor, which is -2.0 per cent, and consequently the resultant correction is -0.6 per cent. From the conditions under which the experiments are made, however, it is rarely possible to read the meter closer than ±0.05 liter, as the graduations on the meter correspond to 50 cubic centimeters. It will be seen, then, that this final correction is really inside the limit of error of the instrument, and consequently with this particular meter now in use no correction whatever is necessary for the reduction of the volume. The matter of temperature corrections has been taken up in great detail in an earlier publication, and where there are noticeable differences in temperature between the meter and the calorimeter chamber the calculation is very much more complicated.
For practical purposes, therefore, we may assume that the quantity of air passed through the meter, as now in use, represents exactly 10 liters measured under the conditions obtaining inside of the respiration chamber, and in order to find the total amount of water-vapor present in the chamber it is necessary only to multiply the weight of water found in the 10-liter sample by one-tenth of the total volume of air containing water-vapor.
The total volume of air which contains water-vapor is not far from 1,360 liters; consequently multiplying the weight of water in the sample by 136 gives the total amount of water in the chamber and the piping. The volume of air containing carbon dioxide is that contained in the chamber and piping to the first sulphuric-acid vessel plus 16 liters of air above the sulphuric acid and connections in the first porcelain vessel, and in order to obtain the amount of carbon dioxide from the sample it is only necessary to multiply the weight of carbon dioxide in the sample by 137.6.
Since in the calculation of the total amount of residual oxygen volumes rather than weights of gases are used, it is our custom to convert the weights of carbon dioxide and water-vapor in the chamber to volumes by multiplying by the well-known factors. The determination of oxygen depends upon the knowledge of the true rather than the apparent volume of air in the system, and consequently the apparent volume must be reduced to standard conditions of temperature and pressure each time the calculation is made. To this end, the total volume of air in the inclosed circuit (including that in the tension-equalizer, amounting to 1,400 liters in all) is reduced to 0° and 760 millimeters by the usual methods of computation. The total volume of air (which may be designated asV) includes the volumes of carbon dioxide, water-vapor, oxygen, and nitrogen. From the calculations mentioned above, the volumes of water-vapor and carbon dioxide have been computed, and deducting the sum of these from the reduced volume of air gives the volume of oxygen plus nitrogen. If the volume of nitrogen is known, obviously the volume of oxygen can be found.
At the beginning of the experiment, it is assumed that the chamber is filled with ordinary air. By calculating the amount of nitrogen in the chamber at the start as four-fifths of the total amount, no great error is introduced. In many experiments actual analyses of the air have been made at the moment of the beginning of the experiment. The important thing to bear in mind is that having once sealed the chamber and closed it tightly, no nitrogen can enter other than that admitted with the oxygen, and hence the residual amount of nitrogen remains unaltered save for this single exception. If care is taken to keep an accurate record of the amount of nitrogen admitted with the oxygen, the nitrogen residual in the chamber at any given time is readily computed. While from an absolute mathematical standpoint the accuracy of this computation can be questioned, here again we are seeking an accurate record of differences rather than an absolute amount, and whether we assume the volume of the air in the chamber to contain 20.4 per cent of oxygen or 21.6 per cent is a matter of indifference. It is of importance only to note the increases in the amount of nitrogen, since these increases represent decrease in the residual oxygen and it is with the changes in the residual oxygen that we particularly have to do.
The air, being confined in a space with semi-rigid walls, is subjected naturally to variations in true volume, depending upon the temperature and barometric pressure. If the air inside of the chamber becomes considerably warmer there is naturally an expansion, and were it not for the tension-equalizer there would be pressure in the system. Also, if the barometer falls, there is an expansion of air which, again, in the absence of the tension-equalizer, would produce pressure in the system. It is necessary, therefore, in calculating the true volume of air, to take into account not only the apparent volume, which, as is shown above, is always a constant amount at the end of each period, but the changes in temperature and barometric pressure must also be noted. Since there is a volume of about 1,400 liters, a simple calculation will show that for each degree centigrade change in temperature there will be a change in volume of approximately 4.8 liters. In actual practice, however, this rarely occurs, as the temperature control is usually inside of 0.1° C. and for the most part within a few hundredths. A variation in barometric pressure of 1 millimeter will affect 1,400 liters by 1.8 liters.
In actual practice, therefore, it is seen that if the barometer falls there will be an expansion of air in the system. This will tend to increase the volume by raising the rubber diaphragm on the tension-equalizer, the ultimate result of which is that at the final filling with oxygen at the end of the period less is used than would be the case had there been no change in the barometer. In other words, for each liter expansion of air inside of the system, there is 1 liter less oxygen required to bring the apparent volume the same at the end of the period. Similarly, if there is an increase in temperature of the air, there is expansion, and a smaller amount of oxygen is required than would be the case had there been no change; and conversely, if the barometer rises or the temperature falls, more oxygen would be supplied than is needed for consumption. It is thus seen that the temperature and barometer changes affect the quantity of oxygen admitted to the chamber.
Any variations in the residual amount of carbon dioxide or water-vapor likewise affect the oxygen. Thus, if there is an increase of 1 gram in the amount of residual carbon dioxide, this corresponds to 0.51 liter, and consequently an equal volume of oxygen is not admitted to the chamber during the period, since its place has been taken by the increased volume of carbon dioxide. A similar reasoning will show that increase in the water-vapor content will have a similar effect, for each gram of water-vapor correspondsto 1.25 liters and therefore influences markedly the introduction of oxygen. All four of the factors, therefore (barometric pressure, temperature, residual carbon dioxide, and residual water-vapor), affect noticeably the oxygen determination.
Of the three factors to be determined in the residual air, the oxygen (which is most important from the standpoint of the relative weight to be placed upon the analysis) unfortunately can not be directly determined without great difficulty. Furthermore, any errors in the analysis may be very greatly multiplied by the known errors involved in the determination of the true volume of the air in the chamber as a result of the difficulties in obtaining the average temperature of the air. Believing that the method of analysis as outlined above should be controlled as far as possible by other independent methods, we were able to compare the carbon dioxide as determined by the soda-lime method with that obtained by the extremely accurate method used by Sondén and Pettersson. An apparatus for the determination of carbon dioxide and oxygen on the Pettersson principle has been devised by Sondén and constructed for us by Grave, of Stockholm.
In the control experiments, the air leaving the mercury valve D (fig. 30, page 66) was caused to pass through aT-tube, one arm of which connected directly with the sampling pipette of the Sondén gas-analysis apparatus, the other arm connecting with theU-tubes for residual analyses. By lowering and raising the mercury reservoir on the gas-analysis apparatus, a sample of air could be drawn into the apparatus for analysis. The results of the analysis were expressed on the basis of moist air in volume per cents rather than by weight, as is done with the soda-lime method. Hence in comparison it was necessary to convert the weights to volume, and during this process the errors due to not correcting for temperature and barometer are made manifest. However, the important point to be noted is that whatever fluctuations in composition of the residual air were noted by the soda-lime method, similar fluctuations of a corresponding size were recorded by the volumetric analysis with the Sondén apparatus. Under these conditions, therefore, we believe that the gravimetric method outlined above is sufficiently satisfactory, so far as the carbon-dioxide content is concerned, for ordinary work where there are no wide variations in the composition of the air from period to period.
It is impossible to obtain in the market absolutely chemically pure oxygen. All the oxygen that we have thus far been able to purchase contains nitrogen and, in some instances, measurable amounts of water-vapor and carbon dioxide. The better grade of oxygen, that prepared from liquid air, is practically free from carbon dioxide and water-vapor, but it still containsnitrogen, and hence with every liter of oxygen admitted there is a slight amount of nitrogen added. This amount can readily be found from the gasometric analysis of the oxygen and from the well-known relation between the weight and the volume of nitrogen the weight can be accurately found. This addition of nitrogen played a very important rôle in the calculation of the oxygen consumption as formerly employed. As is seen later, a much abbreviated form of calculation is now in use in which the nitrogen admitted with the oxygen does not influence the calculation of the residual oxygen.
In long-continued experiments, where there is a possibility of a noticeable diminution in the percentage of oxygen in the chamber—a diminution caused either by a marked fall in barometer, which expands the air inside of the chamber and permits admission of less oxygen than would otherwise be required, or by the use of oxygen containing a high percentage of nitrogen, thus continually increasing the amount of nitrogen present in the system—it is highly probable that there may be such an accumulation of nitrogen as to render it advisable to provide for the admission of a large amount of oxygen to restore the air to approximately normal conditions. In rest experiments of short duration this is never necessary. The procedure by which such a restoration of oxygen percentage is accomplished has already been discussed elsewhere.[25]It involves the rejection of a definite amount of air by allowing it to pass into the room through the gas-meter and then making proper corrections for the composition of this air, deducting the volume of oxygen in it from the excess volume of oxygen introduced and correcting the nitrogen residual in order to determine the oxygen absorption during the period in which the air has been rejected.
The volume of air in the food-aperture between the two glass doors is approximately 5.3 liters. When the door on the inside is opened and the material placed in the food-aperture and the outer door is subsequently opened, there is by diffusion a passage outward of air of the composition of the air inside of the chamber, and the food-aperture is now filled with room air. When the inner door is again opened this room air enters the chamber and is replaced by air of the same composition as that in the chamber. It is seen, then, that there may theoretically be an interchange of air here which may have an influence on the results. In severe work experiments, where the amount of carbon dioxide in the air is enormously increased, such interchange doubtless does take place in measurable amounts and correction should undoubtedly be made. In ordinary rest experiments,where the composition of the air in the chamber is much more nearly normal, this correction is without special significance. Furthermore, in the two forms of calorimeter now in use, the experiments being of but short duration, provision is made to render it unnecessary to open the food-aperture during the experiment proper. Consequently at present no correction for interchange of air in the food-aperture is made, and for the same reason the slight alteration in volume resulting from the removal or addition of material has also not been considered here.
To facilitate the calculations and for the sake of uniformity in expressing the results, a special form of blank is used which permits the recording of the principal data regarding the analyses of air in the chamber at the end of each period. Thus at the head of the sheet are recorded the time, the number of the period, kind of experiment, the name or initials of the subject, and the statement as to which calorimeter is used. The barometer recorded in millimeters is indicated in the column at the left and immediately below the heading, together with the temperature of the calorimeter as expressed in degrees centigrade. The temperature of the calorimeter as recorded by the physical observer is usually expressed in the arbitrary scale of the Wheatstone bridge and must be transposed into the centigrade scale by means of a calibration table.
The apparent air-volumes in the subsections of the ventilating system are recorded under the headings I, which represents the volume of air containing water-vapor and therefore is the air in the chamber plus the air in the piping to the surface of the acid in the first sulphuric-acid absorber; I-II, which represents the air containing carbonic acid and includes volume I plus the volume of the air in the first sulphuric-acid vessel and the volume of air in the potash-lime absorber; I-III, which includes the total confined volume of the whole system, since this air contains both oxygen and nitrogen. These volumes change somewhat, depending upon the size of the body of the subject, the volume of the materials taken into the chamber, and the type of calorimeter.
The data for the residual analyses are recorded in the lower left-hand corner: first the weight of the water absorbed from 10 liters of air passing through the meter; to the logarithm of this is added the logarithm of volume I; the result is the logarithm of the total weight of water-vapor in the ventilating air-current. To convert this into liters the logarithmic factor 09462[26]is added to the logarithm of the weight of water and (a) is the logarithm of water expressed in liters. A similar treatment is accorded the weight of carbon dioxide absorbed from the air-sample, (b) being ultimately the logarithm of the volume of carbon dioxide.
In order to determine the total volume of air in the chamber under standard conditions of temperature and pressure, to the logarithm of volume I-III is added, first, a logarithmic factor for the temperature recorded for the calorimeter to correct the volume of air to standard temperature. As the temperature fluctuations are all within 1 degree, a table has been prepared giving the standard fluctuation represented by the formula