Standard Measure and RulerFig. 38.
Fig. 38.
Fig. 38represents the standard measure, and the ruler.
AtAAare the millimetre divisions on the edges of the measure, the longer transverse lines atBBare placed at intervals of five millimetres and of centimetres. The ruler is in the form of a right-angled triangle; it is shown, by the dotted lines, in position on the standard metre measure atI; and again, with its under surface upwards, in the smaller figure at 2. It consists of a perfectly flat sheet of metal, about ten centimetres in length fromCtoC, sufficiently thick to be rigid, and has a ledge,DDin each figure, which is pressed against the side of the measure when using it, to ensure that the successive positions of the edge (LL) shall be parallel to each other. AtGGare two small holes, into which fit small screws with fine points. These must be in a line parallel to the edge (LL), so that when the ruler is in position on the scale, the points of the two screws, which project slightly, shall fall into corresponding cuts on the divided scales (AA).
To graduate a strip of glass, or a glass tube (HH), the surface to be marked must first be coated with wax, which should be mixed with a little turpentine, and be applied to the surface of the glass, previously madewarmanddry, by means of a fine brush, so as to completely cover it with a thin, closely-adherent, and evenly-distributed coat of wax, which must be allowed to cool.
FixHHfirmly on a table, and fix the standard measure by the side ofHH. If the thickness ofHHbe about equal to, but not greater than that of the standard measure, this may be done by large drawing-pins. If, however, a large tube or thick sheet of glass is to be graduated, fix it in position by two strips of wood screwed to the table on each side of it. One of these wooden strips, on which the measure may be placed, may be about as broad as the standard measure, and of such thickness that when the measure lies upon it beside the tube to be graduated, the ruler, when moved alongthe measure, will move freely above the tube, but will not be elevated more than is necessary to secure free movement. The second strip of wood may be narrower, and of the same thickness as the broader piece on which the standard measure rests. In any case, let the standard measure and the object to be graduated be very firmly secured in their places. Bring the ruler into position at any desired part of the tube by placing the points of the screws (GG) in corresponding divisions of the scales (AA). With the style, which may be a needle mounted in a handle, make a scratch in the wax along the edge of the ruler atF, move the ruler so that the screws rest in the next divisions, and repeat the operation till the required number of lines has been ruled. Longer marks may be made at intervals of five and ten millimetres. Great care must be taken to hold the needle perpendicularly, and to press it steadily against the edge (LL) of the ruler in scratching the divisions.[19]The length of the lines marking the millimetre divisions should not be too long; about 1 mm. is a good length. If they are longer than this, theapparentdistance between them is diminished, and it is less easy to read fractions of millimetres. Before removing the scale to etch the glass, carefully examine it to see that no mistakes have been made. If it is found that any lines have been omitted, or that long lines have been scratched in the place of short ones, remelt the wax by means of a heated wire, and make new marks. Finally, mark the numbers on the scale with a needle-point, or better, with a fine steel pen.
The marks on the wax should cut through it. When theyare satisfactory, they may be etched by one of the following processes:—
(1.) By moistening some cotton wool, tied to a stick, with solution of hydrofluoric acid, and gently rubbing this over the scratched surface for a minute or so; then washing away the acid with water, and cleaning off the wax. This is the simplest method, but the marks made are generally transparent, and therefore not very easy to read. The simplicity of this method is a great recommendation, however.
(2.) Expose the tube to the fumes of hydrofluoric acid generated from a mixture of powdered fluor-spar and strong sulphuric acid, in a leaden trough. The marks produced in this way are usually opaque, and are therefore very visible, and easily read.
After the above detailed account it will only be necessary to give an outline of the other process of graduating tubes.
Scale of Equal PartsFig. 39.
Fig. 39.
The standard scale to be copied,A, which may in this case be another graduated tube, or even a paper scale, and the object to be ruled,B, are securely fixed, end to end, a little distance apart, in a groove made in a board or in the top of a table. A stiff bar of wood,C, has a point fixed atD, and a knife edge atE,Dis placed in any division ofA,Cis held firmly atEandD, and a cut is made by the knife through the wax onB, the pointDis then moved into the next division, and the operation is repeated. To regulate the length and position of the cuts,Bis usually held in position by two sheets of brass projecting over the edges of the groove in which it lies; the metal sheets have notches cut into them at the intervals at which longer marks are to be made.
When the scale is completed, the equality of the divisions in various parts of it may be, to some extent, verified as follows:—Adjust a compass so that its points fall into two divisions 5, 10, or 20 mm. apart. Then apply the points of the compass to various parts of the scale. In every part the length of a given number of divisions should be exactly the same. The individual divisions should also be carefully inspected by the eye; they should be sensibly equal. If badly ruled, long and short divisions will be found on the scale. Very often a long and a short division will be adjacent, and will be the more easily observed in consequence.
To Divide a Given Line into Equal Parts.—Occasionally it is necessary to divide a line of given length intoxequal parts. For instance, to divide the stem of a thermometer from the freezing-point to the boiling-point into one hundred degrees.
Dividing a LineFig. 40.
Fig. 40.
The following outline will explain how a line may be so divided. Suppose the lineAB(Fig. 40) is to be divided into nine equal parts. Adjust a hinged rule so that the pointsAandBcoincide with the inside edges of the limbs, one of them,A, being at the ninth division (e.g.the ninth inch) ofCE. Then if lines parallel toEDbe drawn from each division of the scale to meetAB,ABwill be divided into nine equal parts.
A very convenient and simple arrangement on thisprinciple for dividing a line into any number of equal parts with considerable accuracy, is described by Miss S. Marks in theProceedings of the Physical Society, July 1885.[20]One limb of a hinged ruleDis made to slide upon a plain rule fixed to it; the plain rule carries needles on its under surface which hold the paper in position. The position of the divided rule and line to be divided being adjusted, the hinged rule is gently pushed forwards, as indicated by the arrow inFig. 40, till division eight coincides with the lineAB. A mark is made at the point of coincidence, and division seven on the scale is similarly brought to the lineAB, and so on. The inner edge ofECshould have the divisions marked upon it, that their coincidence withABmaybe more accurately noted. The jointEmust be a very stiff one.
A line drawn of given length or a piece of paper may be divided into any given number of equal parts, and will then serve as the scaleAofFig. 39,p. 74, the thermometer or other object to be graduated taking the place ofB.
Scales carefully divided according to any of the methods described will be fairly accurateif trustworthy instruments have been employed as standards.
It will be found possible when observing the volume of a gas over mercury, or the height of a column of mercury in a tube, to measure differences of one-sixth to one-eighth of a millimetre with a considerable degree of accuracy. To obtain more delicate measurements a vernier[21]must be employed.
To Calibrate Apparatus.—The glass tubes of which graduated apparatus is made are, as already stated, veryrarely truly cylindrical throughout their entire lengths. It follows that the capacities of equal lengths of a tube will usually be unequal, and therefore it is necessary to ascertain by experiment the true values of equal linear divisions of a tube at various parts of it.
A burette may be calibrated by filling it with distilled water, drawing off portions, say of 5 c.c. in succession, into a weighing bottle of known weight, and weighing them.
Great care must be taken in reading the level of the liquid at each observation. The best plan is to hold a piece of white paper behind the burette, and to read from the lower edge of the black line that will be seen. Each operation should be repeated two or three times, and the mean of the results, which should differ but slightly, may be taken as the value of the portion of the tube under examination.
If the weights of water delivered from equal divisions of the tube are found to be equal, the burette is an accurate one, but if, as is more likely, different values are obtained, a table of results should be drawn up in the laboratory book showing the volume of liquid delivered from each portion of the tube examined. And subsequently when the burette is used, the volumes read from the scale on the burette must be corrected. Suppose, for example, that a burette delivered the following weights of water from each division of 5 c.c. respectively:—
and that in two experiments 20 c.c. and 45 c.c. respectively of a liquid re-agent were employed. The true volumes calculated from the table would be as 19·66 to 44·46.
If the temperature remained constant throughout the above series of experiments, and if the temperature selected were 4° C., the weights of water found, taken in grams, give the volumes in cubic centimetres, for one gram of water at 4° C. has a volume of one cubic centimetre. If the temperature at which the experiments were made was other than 4° C., and if great accuracy be desired, a table of densities must be consulted, with the help of which the volume of any weight of water at a known temperature can be readily calculated.
Pipettes which are to be used as measuring instruments should also have the relation one to another of the volumes of liquid which they deliver determined, and also the proportions these bear to the values found for the divisions of the burettes in conjunction with which they will be employed.
To Calibrate Tubes for Measuring Gases.—Prepare a small glass tube sealed at one end and ground at the other to a plate of glass. The tube should hold about as much mercury as will fill 10 mm. divisions of the graduated tube. Fill this tube with mercury, removing all bubbles of air that adhere to the sides by closing the open end of the tube with the thumb, and washing them away with a large air-bubble left for the purpose. If any persistently remain, remove them by means of a fine piece of bone or wood. Then completely fill the tube with mercury, removing any bubbles that may be introduced in the operation, and remove the excess of mercury by placing the ground-glass plate onthe mouth of the tube, and pressing it so as to force out all excess of mercury between the two surfaces. Clean the outside of the tube, and place it on a small stand (this may be a small wide-mouthed glass bottle), with which it has been previously weighed when empty, and re-weigh. Repeat this operation several times. From the mean of the results, which should differ one from another but very slightly, the capacity of the tube can be calculated.
The purest mercury obtainable should be used. Since the density of pure mercury at 0° C. is 13·596, the weight of mercury required to fill the tube at 0° C., taken in grams, when divided by 13·596, will give the capacity of the tube at 0° C. in cubic centimetres. If the experiment be not made at 0° C., and if a very exact determination of the capacity of the tube be required, the density of mercury must be corrected for expansion or contraction.
Having now a vessel of known capacity, it can be employed for ascertaining the capacities of the divisions of a graduated tube in the following manner:—The graduated tube is fixed perpendicularly, mouth upwards, in a secure position. The small tube of known capacity is filled with mercury as previously described, and its contents are transferred to the divided tube. The number of divisions which the known volume of mercury occupies is noted after all air-bubbles have been removed. This process is repeated until the divided tube is filled. A table of results is prepared, showing the number of divisions occupied by each known volume of mercury introduced.
In subsequently using the tube the volumes of the gases measured in it must be ascertained from the table of values thus prepared.
In observing the level of the mercury, unless a cathetometer is available, a slip of mirror should be held behind the mercury close to the tube, in such a position that the pupilwhich is visible on the looking-glass is divided into two parts by the surface of the mercury.
A correction must be introduced for the error caused by the meniscus of the mercury. As the closed end of the tube was downwards when each measured volume of mercury was introduced, and as the surface of mercury is convex, the volume of mercury in the tube when it is filled to any divisionl(Fig. 41) is represented byAof 1. But in subsequently measuring a gas over mercury in the same tube, when the mercury stands at the same divisionl, the volume of the gas will be as represented byBof 2, which is evidently somewhat greater thanA. This will be seen still more clearly in 3, wherearepresents the boundary of the mercury, andbthe boundary of the air, when the tube is filled to the marklwith mercury or a gas over mercury respectively.
Correction for MeniscusFig. 41.
Fig. 41.
It is plain that when the level of the mercury in measuring a gas is read atl, the volume of the gas is greater than the volume of the mercury recorded, by twice the difference between the volumeAof mercury measured, and that which would fill the tube to the levell, if its surface were plane.
The usual mode of finding the true volume of a gas collected over mercury is as follows:—
Place the graduated tube mouth upwards, introduce some mercury, and, after removing all bubbles, note the division atwhich it stands. Then add a few drops of solution of mercuric chloride; the surface of the mercury will become level, read and record its new position. Then, in any measurement, having observed that the mercury stands atndivisions of the tube, add twice the difference between the two positions of the mercury ton, and ascertain the volume which corresponds to this reading from the table of capacities.
To Calibrate the Tube of a Thermometer.—Detach a thread of mercury from half an inch to one inch in length from the body of the mercury. Move it from point to point throughout the length of the tube, and note its length in each position. If in one part it occupies a length of tube corresponding to eight degrees, and at another only seven degrees, then at the former point the value of each division is only seven-eighths of those at the latter position.
From the results obtained, a table of corrections for the thermometer should be prepared.
It is sometimes necessary to join soda glass to lead glass. In this case the edge of the lead glass tube may be bordered with white enamel before making the joint. Enough enamel must be used to prevent the lead and soda glasses from mingling at any point. The enamel is easily reduced, and must be heated in the oxidising flame. Dr. Ebert recommendsVerre d’uranefor this purpose. It is supplied by Herr Götze of Leipzig (Liebigstrasse).
[17]Originally suggested by Bunsen.
[18]Such measures can be obtained of steel for aboutfifteen shillingseach. They are made by Mr. Chesterman of Sheffield. They can be obtained also from other makers of philosophical instruments, at prices depending upon their delicacy. Those of the greatest accuracy are somewhat costly.
[19]To avoid variations of the position in which the needle is held when marking the divisions, the edge (LL) should not be bevelled; and an upright support may be placed upon the ruler, with a ring through which the handle of the needle passes, thereby securing that the angle formed by the needle and surface of the ruler is constant, and that equal divisions are marked.
[20]Since this was printed I have observed that the above method is not identical with that described by Miss Marks, but for ordinary purposes I do not think it will be found to be inferior.
[21]For the nature and use of the vernier, a treatise on Physics or Physical Measurements may be consulted.
GLASS TUBING.
The diagrams given below show the sizes and thickness of the glass tubes most frequently required. In ordering, the numbers of these diagrams may be quoted, or the exact dimensions desired may be stated.
Glass tubes are usually sold by weight, and therefore the weight of tube of each size that is wished for should be indicated, and also whether it is to be of lead or soda glass.
Glass Tubing 1
VITREOUS SILICA.
Introductory.—Vitreous Silica was made in fine threads by M. Gaudin in 1839,[22]and small tubes of it were made in 1869 by M. A. Gautier, but its remarkable qualities were not really recognised till 1889, when Professor C. V. Boys rediscovered the process of making small pieces of apparatus of this substance, and used the torsion of “quartz fibres” for measuring small forces. More recently the author of this book has devised a process for preventing the “splintering” of quartz which gave so much trouble to the earlier workers, and jointly with Mr. H. G. Lacell, has produced a variety of apparatus of much larger dimensions than had been attemptedpreviously. At the time of writing we can produce by the processes described in the following pages tubes 1 to 1·5 cm. in diameter and about 750 cm. in length, globes or flasks capable of containing about 50 c.c., masses of vitreous silica weighing 100 grams or more, and a variety of other apparatus.
Properties of Vitreous Silica.—For the convenience of those who are not familiar with the literature of thissubject, I may commence this chapter with a brief account of the properties and applications of vitreous silica, as far as they are at present ascertained. Vitreous silica is less hard than chalcedony, but harder than felspar. Tubes and rods of it can be cut with a file or with a piece of sharpened and hardened steel, and can afterwards be broken like similar articles of glass. Its conducting power is low, and Mr. Boys has shown that fine fibres of silica insulate remarkably well, even in an atmosphere saturated with moisture. The insulating qualities of tubes or rods of large cross sections have not yet been fully tested; one would expect them to give good results provided that they are kept scrupulously clean. A silica rod which had been much handled would probably insulate no better than one of glass in a similar condition. The density of vitreous silica is very near to that of ordinary amorphous silica. In the case of a small rod not absolutely free from minute bubbles it was found to be 2·21.
Vitreous silica is optically inactive, when homogeneous, and is highly transparent to ultraviolet radiations.
The melting point of vitreous silica cannot be definitely stated. It is plastic over a considerable range of temperature. Professor Callendar has succeeded in measuring the rate of contraction of fine rods in cooling from 1200° to 1500° C., so that its plasticity must be very slight below the latter temperature. If a platinum wire embedded in a thick silica tube be heated from without by an oxy-hydrogen flame the metal may be melted at temperatures at which the silica tube will retain its form for a moderate length of time, but silica softens to a marked extent at temperatures a little above the melting point of platinum.
It has been observed by Boys, Callendar, and others that fine rods of silica, and also the so-called “quartz fibres,” are apt to become brittle after they have been heated to redness.But I have not observed this defect in the case of more massive objects, such as thick rods or tubes; and as I have repeatedly observed that mere traces of basic matter, such as may be conveyed by contact with the hand, seriously injure the surface of silica, and have found that silica quickly becomes rotten when it is heated to about 1000° in contact with an infusible base such as lime, I am disposed to ascribe the above-mentioned phenomenon to chemical rather than to purely physical causes.[23]It is certain, however, that silica apparatus must never be too strongly heated in contact with basic substances. Silica is easily attacked by alkalis and by lime, less readily by copper oxide, and still less by iron oxide.
The rate of expansion of vitreous silica has been studied by H. le Chatelier, and more recently by Callendar. The former found its mean coefficient of expansion to be 0·0000007 between 0° and 10000°,[24]and that it contracted when heated above 700°.
Professor Callendar used rods of silica prepared by the author from “Brazil crystal”; these were drawn in the oxy-gas flame and had never been heated in contact with solid foreign matter, so that they consisted, presumably, of very pure silica. His results differ in some respects from those obtained by Le Chatelier, for he finds the mean coefficient of expansion to be only 0·00000059,i.e.about one seventeenth as great as that of platinum. Callendar found the rods of silica expanded very regularly up to 1000° but less regularly above that temperature. Above 1200° they contracted when heated.
The behaviour of vitreous silica under sudden changes of temperature is most remarkable. Large masses of it may be plunged suddenly when cold into the oxy-gas flame, and tubes or rods at a white heat may be thrust into cold water, or even into liquid air, with impunity. As a consequence of this, it is in one respect much more easily worked in the flame than any form of glass. Difficult joints can be thrust suddenly into the flame, or removed from it, at any stage, and they may be heated unequally in different parts with impunity. It is safe to say that joints, etc., in silica never crack whilst one is making them nor during the subsequent cooling. They may be set aside in an unfinished state and taken up again without any precautions. Therefore it is possible for an amateur to construct apparatus in silica which he would be quite unable to produce from glass.
The behaviour of vitreous silica with solvents has not yet been fully investigated, but Mr. H. G. Lacell has this subject in hand. If it behaves like the other forms of anhydrous silica it will withstand the action of all acids except hydrofluoric acid. It is, of course, very readily acted upon by solutions of alkalis and alkaline salts.
As regards the use of silica in experiments with gases, it must be remarked that vitreous silica, like platinum, is slightly permeable to hydrogen when strongly heated. One consequence of this is that traces of moisture are almost always to be found inside recently-made silica tubes and bulbs, however carefully we may have dried the air forced into them during the process of construction. Owing to the very low coefficient of expansion of silica, it is not possible to seal platinum wires into silica tubes. Nor can platinum be cemented into the silica by means of arsenic enamel, nor by any of the softer glasses used for such purposes. I have come near to success by using kaolin, but the results withthis material do not afford a real solution of the problem, though they may perhaps point to a hopeful line of attack. Possibly platinum wires might be soldered into the tubes (seeLaboratory Arts, R. Threlfall), but this also is uncertain.
The process of preparing silica tubes, etc., from Lumps of Brazil Crystal may be described conveniently under the following headings. I describe the various processes fully in these pages, as those who are interested in the matter will probably wish to try every part of the process in the first instance. But I may say that in practice I think almost every one will find it advantageous to start with purchased silica tubes, just as a glass-worker starts with a supply of purchased glass tubes. The manufacturer can obtain his oxygen at a lower price than the retail purchaser, and a workman who gives much time to such work can turn out silica tube so much more quickly than an amateur, that I think it will be found that both time and money can be saved by purchasing the tube. At the same time the beginner will find it worth while to learn and practise each stage of the process at first, as every part of the work described may be useful in the production of finished apparatus from silica tubes.
This being so, I am glad to be able to add that a leading firm of dealers in apparatus[25]has commenced making silica goods on a commercial scale, so that the new material is now available for all those who need it or wish to examine its properties.
Preparing non-splintering Silica from Brazil Pebble.—The best variety of native Silica is Brazil Pebble, which may be obtained in chips or larger masses. Theseshould be thoroughly cleaned, heated in boiling water, and dropped into cold water, the treatment being repeated till the masses have cracked to such an extent that they may be broken easily by blows from a clean steel pestle or hammer.
The fragments thus produced must be hand-picked, and those which are not perfectly free from foreign matter should be rejected. The pure and transparent pieces must then be heated to a yellow-red heat in a covered platinum dish in a muffle or reverberatory furnace and quickly plunged into a deep clean vessel containing clean distilled water; this process being repeated, if necessary, till the product consists of semi-opaque friable masses, very much like a white enamel in appearance. After these have been washed with distilled water, well drained and dried, they may be brought into the hottest part of an oxy-gas flame safely, or pressed suddenly against masses of white hot silica without any preliminary heating, such as is necessary in the case of natural quartz. Quartz which has not been submitted to the above preparatory process, splinters on contact with the flame to such an extent that very few would care to face the trouble and expense of working with so refractory a material. But after the above treatment, which really gives little trouble, all the difficulties which hampered the pioneer workers in silica disappear as if by magic.
Apparatus.—Very little special apparatus need be provided for working with silica, but it is absolutely essential to protect the eyes with very dark glasses. These should be so dark as to render it a little difficult to work with them at first. If long spells of work are undertaken, two pairs of spectacles should be provided, for the glasses quickly become hot enough to cause great inconvenience and even injury to the eyes.
Almost any of the available oxy-gas burners may be used, but they vary considerably in efficiency, and it is economical to obtain a very efficient burner. The ’blow-through’ burners are least satisfactory, and I have long since abandoned the use of them. Some of the safety ’mixed-gas jets’ have an inconvenient trick of burning-back, with sharp explosions, which are highly disconcerting, if the work be brought too near the nozzle of the burner. I have found the patent burner of Mr. Jackson (Brin’s Oxygen Company, Manchester) most satisfactory, and it offers the advantage that several jets can be combined in a group easily and inexpensively for work on large apparatus. The large roaring flames such as are used, I understand, for welding steel are very expensive, and not very efficient for the work here described.
The method of making Silica Tubes.—Before commencing to make a tube a supply of vitreous silica in rods about one or two millimetres in diameter must be prepared. To make one of these, hold a fragment of the non-splintering silica described above in the oxy-gas flame by means of forceps tipped with platinum so as to melt one of its corners, press a small fragment of the same material against the melted part till the two adhere and heat it from below upwards,[26]till it becomes clear and vitreous, add a third fragment in a similar manner, then a fourth, and so on till an irregular rod has been formed. Finally re-heat this rod in sections and draw it out whilst plastic into rods or coarse threads of the desired dimensions. If one works carefully the forceps do not suffer much. I have had one pair in almost constant use for several years; they have been used in the training of five beginners and are still practically uninjured.
The beginner should work with a gauge and regulator on the bottle of oxygen, and should watch the consumption of oxygen closely. A large expenditure of oxygen does not by any means necessarily imply a corresponding output of silica, even by one who has mastered the initial difficulties.
When a supply of the small rods of vitreous silica has been provided, bind a few of them round a rod of platinum (diameter say, 1 mm.) by means of platinum wires at the two ends and heat the silica gradually, beginning at one end after slightly withdrawing the platinum core from that end, till a rough tube about four or five centimetres in length has been formed. Close one end of this, expand it, by blowing, into a small bulb, attach a silica rod to the remote end of the bulb, re-heat the bulb and draw it out into a fine tube. Blow a fresh bulb on one end of this and again draw it out, proceeding in this way till you have a tube about six or eight centimetres in length. All larger tubes and vessels are produced by developing this fine tube suitably.
Precautions.—The following points must be carefully kept in mind, both during the making of the first tube and afterwards:—
(1) The hottest spot in the oxy-gas flame is at a point very near the tip of the inner cone of the flame, and silica can be softened best at this hot spot. The excellence of a burner does not depend on the size of its flame, so much as on the temperature of its “hot spot,” and the success of the worker depends on his skill in bringing his work exactly to this part of the flame. Comparatively large masses of silica may be softened in a comparatively small jet if the hot spot is properly utilised.
(2) Silica is very apt to exhibit a phenomenon resemblingdevitrification during working. It becomes covered with a white incrustation, which seems to be comparatively rich in alkali.[27]This incrustation is very easily removed by re-heating the whitened surface, provided that the material has been kept scrupulously clean. If the silica has been brought into the flame when dusty, or even after much contact with the hands of the operator, its surface is very apt to be permanently injured.Too much attention cannot be given to cleanliness by the workman.
(3) When a heated tube or bulb of silica is to be expanded by blowing, it is best not to remove it from the flame, for if that is done it will lose its plasticity quickly unless it be large. The better plan is to move it slightly from the “hot spot” into the surrounding parts of the flame at the moment of blowing.
It is best to blow the bulb through an india-rubber tube attached to the open end of the silica tube. At first one frequently bursts the bulbs when doing this, but holes are easily repaired by stopping them with plastic silica applied by the softened end of a fine rod of silica and expanding the lump, after re-heating it, by blowing. After a few hours’ practice these mishaps gradually become rare.
I find it a good plan to interpose a glass tube packed with granulated potash between the mouth and the silica tube. This prevents the interior of the tube from being soiled. The purifying material must not be packed so closely in the tube as to prevent air from passing freely through it under a very low pressure.
It may be mentioned here that a finished tube usually contains a little moisture, and a recognisable quantity of nitric peroxide. These may be removed by heating thetube and drawing filtered air through it, but not by washing, as it is difficult to obtain water which leaves no residue on the silica.
Making larger tubes and other apparatus of Silica.—In order to convert a small bulb of silica into a larger one or into a large tube, proceed as follows:—Heat one end of a fine rod of silica and apply it to the bulb so as to form a ring as shown in the figure. Then heat the ring and the end of the bulb till it softens, and expand the end by blowing. If this process is repeated, the bulb first becomes ovate and then forms a short tube which can be lengthened at will, but the most convenient way to obtain a very long tube is to make several shorter tubes of the required diameter, and say 200 to 250 mm. in length, and to join these end to end. It does not answer to add lumps of silica to the end of the bulb, for the sides of the tube made in this way become too thin, and blow-holes are constantly formed during the making of them. These can be mended, it is true, but they spoil the appearance of the work.
Large Apparatus
Tubes made in the manner described above are thickened by adding rings of silica and blowing them when hot to spread the silica. If a combination of several jets is employed, very large tubes can be constructed in this way. One of Messrs. Baird and Tatlock’s workmen lately blew a bulb about 5 cm. in diameter, and it was clear that he could have converted it into a long cylindrical tube of equal diameter had it been necessary to do so.
Very thin tubes of 1·5 cm. diameter, and tubes of considerable thickness and of equal size, are easily made after somepractice, and fine capilliaries and millimetre tube can be made with about equal readiness.
If a very fine tube of even bore is required, it may be drawn from a small thick cylinder after a little practice.
When a tube becomes so large that it cannot be heated uniformly on all sides by rotating it in the flame, it is convenient to place a sheet of silica in front of the flame a little beyond the object to be heated, in order that the former may throw back the flame on those parts of the tube which are most remote from the jet. A suitable plate may be made by sticking together small lumps of silica rendered plastic by heat.
The silica tubes thus made can be cut and broken like glass, they can be joined together before the flame, and they can also be drawn into smaller tubes when softened by heat.
In order to make a side connection as in a T piece, a ring of silica should be applied to the tube in the position fixed upon for the joint. This ring must then be slightly expanded, a new ring added, and so on, till a short side tube is formed. To this it is easy to seal a longer tube of the required dimensions. It is thus possible to produce Geissler tubes, small distilling flasks, etc. Solid rods of silica are easily made by pressing together the softened ends of the fine rods or threads previously mentioned. Such rods and small masses can be ground and polished without annealing them.
Quartz Fibres.—These were introduced into physical work by Mr. Boys in 1889. They may be made by attaching a fine rod of vitrified quartz to the tail of a small straw arrow provided with a needle-point; placing the arrow in position on a cross-bow, heating the rod of silica till it is thoroughly softened and then letting the arrow fly from the bow, when it will carry with it an extremely fine thread of silica. A little practice is necessary to ensure success, buta good operator can produce threads of great tenacity and great uniformity. Fuller accounts of the process and of the various properties and uses of quartz fibres will be found in Mr. Boys’ lectures (Roy. Inst. Proc. 1889, and Proc. Brit. Assn. 1890), and in Mr. Threlfall’s Laboratory Arts.
[22]A brief summary of the history of this subject will be found inNature, Vol. 62, and in the Proceedings of the Royal Institution, 1901.
[23]In a recent communication Professor Callendar tells me that the devitrification commences at the outside and is hastened by particles of foreign matter.
[24]The silica blocks used were prepared by fusion in an electric furnace; it is therefore probable that they were not quite pure.
[25]Messrs. Baird and Tatlock.
[26]This is to avoid bubbles in the finished glass.
[27]The rock crystal exhibits a yellow flame when first heated in the oxy-gas flame, and most samples contain spectroscopic quantities of lithium.