Fig. 10Fig. 10
A Spherical Condenser.—Such a condenser as that shown byf, Fig 10, involves a method which may find application in a number of cases. The outer bulb is blown from a thick piece of tubing which has been inserted in asmaller piece (seed, Fig. 6); then the inner bulb by similar method. It is now necessary to introduce the smaller bulb into the larger, and for this purpose the larger bulb must be cut into halves. A small but deep cut is made with the file or glass-blowers' knife in the middle of the larger bulb, and at right angles to the axis of the tube on which it is blown. A minute bead of intensely heated glass is now brought in contact with the cut in order to start a crack. This crack may now be led round the bulb as described on page 30. If the work is carried out with care, it is possible to obtain the bulb in two halves as shown byd, and these two halves will correspond so exactly that when the cut edges are placed in contact they will be almost air-tight. The two tubes from the smaller bulb should be cut to such a length that they will just rest inside the larger, and the ends should be expanded. Place the inner bulb in position and fit the two halves of the outer bulb together, taking great care not to chip the edges. If the length of the tubes on the inner bulb has been adjusted properly, the inner bulb will be supported in position by their contact with the tubes on the outer bulb.Now rotate the cracked portion of the outer bulb in front of a blowpipe flame and press the halves together very gently as the glass softens. Expand slightly by blowing if necessary. If a small pin-hole develops at the joint it is sometimes possible to close this with a bead of hot glass; but if the bulb has been cut properly there should be no pin-holes formed. The condenser is finished by joining on the side tubes and sealing the inner tube through by the methods already given. In order to blow bulbs large enough to make a useful condenser, it will be convenient to employ the multiple-jet blowpipe described on page 4.
A Soxhlet-Tube or Extraction Apparatus.—This involves the construction of a re-entrant join where the syphon flows into the lower tube. It is of considerable value as an exercise and the complete apparatus is easy to make.
A large tube is sealed at the bottom and the top is lipped, as in making a test-tube. A smaller tube is then joined on by a method similar to that given on page 18, but without making a perforation in the bottom of the large tube. Heating and expanding by air pressure, first through the large tube, then through thesmaller tube and then again through the large tube, will give a satisfactory finish to this part of the work.
Fig. 11Fig. 11
The syphon tube is now joined on to the large tube as shown bya, Fig. 11, care being taken to seal the other end of the syphon tube before joining. The details of the final and re-entrant joint of the syphon tube are shown at the lower part ofa. This join is made by expanding the sealed end of the syphon tube into a small, thick-walled bulb, and the bottomof this bulb is burst out by local heating and blowing; the fragments of glass are removed and the edges made smooth by melting. A similar operation is carried out on the side of the tube to which the syphon tube is to be joined. This stage is shown bya. Now heat the syphon tube at the upper bend until it is flexible, and press the bulb at its end into the opening on the side of the other tube. Hold the glass thus until the syphon is no longer flexible. The final join is made by heating the two contacting surfaces, if necessary pressing the edges in contact with the end of a turn-pin, fusing together and expanding. The finished apparatus is shown byc.
Electrodes.—A thin platinum wire may be sealed into a capillary tube without any special precautions being necessary. The capillary tube may be drawn out from the side of a larger tube by heating a spot on the glass, touching with a glass rod and drawing the rod away; or the exhaustion branch described on page 18 may be used for the introduction of an electrode. It is convenient sometimes to carry out the exhaustion through the same tube that will afterwards serve for the electrode. Theelectrode wire is laid inside the branch before connecting to the exhaustion pump. When exhaustion is completed the tube is heated until the soft glass flows round the platinum and makes the seal air-tight. The branch is now cut off close to the seal on the pump side, a loop is made in the projecting end of the platinum wire, and the seal is finished by melting the cut-off end.
Platinum is usually employed for such work, but if care is taken to avoid oxidation it is not impossible to make fairly satisfactory seals with clean iron or nickel wire. Hard rods of fine graphite, such as are used in some pencils, may also be sealed into glass, but it seems probable that air would diffuse through the graphite in the course of time.
Another method for the introduction of an electrode is illustrated byd,e,fandg, Fig. 11. In this case the bulb or thin-walled tube into which the electrode is to be sealed is perforated by a quick stab with an intensely heated wire—preferably of platinum—which is then withdrawn before the glass has had time to harden, and thus a minute circular hole is made. The electrode is coated with a layer of similar glass,or of the specially made enamel which is sold for this purpose, inserted into the bulb or tube by any convenient opening, and adjusted by careful shaking until the platinum wire projects through the small hole. The bulb or tube is then fused to the coating of the electrode and the whole spot expanded slightly by blowing. The appearance of the finished seal is shown byg. It is well to anneal slightly by smoking.
Thermometers.—Apart from the notes on page 20 with respect to the blowing of a suitable bulb on capillary tubing there is little to say in connection with the glass working needed in making a plain thermometer. The size desirable for the bulb will be determined by the bore of the capillary tube, the coefficient of expansion of the liquid used for filling, and the range of temperature for which the thermometer is intended.
Filling may be carried out as follows:—Fit a small funnel to the open end of the capillary by means of a rubber tube, and pour into the funnel rather more than enough of the liquid to be used than is required to fill the bulb. Mercury or alcohol will be used in practice, most probably. Warm the bulb until a few airbubbles have escaped through the liquid and then allow to cool. This will suck a certain amount of liquid into the bulb. Now heat the bulb again, and at the same time heat the capillary tube over a second burner. The liquid will boil and sweep out the residual air, but it is necessary to heat the capillary tube as well in order to prevent condensation. Allow the bulb and tube to cool, then repeat the heating once more. By this time the bulb and tube should be free from air, and cooling should give a completely filled thermometer. Remove the funnel and heat the thermometer to a few degrees above the maximum temperature for which it is to be used; the mercury or other filling liquid will overflow from the top, and, as the temperature falls, will recede, thus allowing the end of the capillary to be drawn out. Reheat again until the liquid rises to the top of the tube, then seal by means of the blowpipe flame. The thermometer is now finished except for graduation; this is dealt with on page 75.
An Alarm Thermometer.—A thermometer which will complete an electric circuit when a certain temperature is reached may be made bysealing an electrode in the bulb and introducing a wire into the top, which in this case is not sealed. Naturally, this thermometer will be filled with mercury. There is considerable difficulty in filling such a bulb without causing it to crack.
Several elaborations of this form are made, in which electrodes are sealed through the walls of the capillary tube, thus making it possible to detect electrically the variation of temperature when it exceeds any given limits.
An Enclosed or Floating Thermometer.—The construction of this type of thermometer is shown byhandi, Fig 11. It is made in the following stages:—A bulb is blown on the drawn-out end of a thin-walled tube as shown byh. A small bulb is blown on the end of a capillary tube, burst, and turned out to form a lip which will rest in the drawn-out part of the thin-walled tube but is just too large to enter the bulb. The capillary tube is introduced and sealed in position, care being taken to expand the joint a little. The thermometer is filled and the top of the capillary tube closed by the use of a small blowpipe flame. A paper scale having the necessary graduations is inserted,and the top of the outer tube is closed as shown byi.
A Maximum and Minimum Thermometer.—If a small dumb-bell-shaped rod of glass or metal is introduced into the capillary tube of a horizontally placed, mercury-filled thermometer in such a position that the rising mercury column will come in contact with it, the rod will be pushed forward. When the mercury falls again the rod will be left behind and thus indicate the maximum temperature attained. If a similar dumb-bell-shaped rod is introduced into an alcohol-filled thermometer and pushed down until it is within the alcohol column, it will be drawn down by surface tension as the column falls; but the rising column will flow passed it without causing any displacement; thus the minimum temperature will be recorded.
Six's combined maximum and minimum thermometer is shown byb, Fig. 11. In this case both maximum and minimum records are obtained from a mercury column, although the thermometer bulb is filled with alcohol. It is an advantage to make the dumb-bell-shaped rods of iron, as the thermometer can then be reset by the use of a small magnet, anotheradvantage consequent on the use of metal being that the rods can be easily adjusted, by slight bending, so as to remain stationary in the tubes when the thermometer is hanging vertically, and yet to move with sufficient freedom to yield to the pressure of the recording column.
The thermometer may be filled by the following method:—When the straight tube has been made the first dumb-bell is introduced and shaken down well towards the lower bulb, the tube is now bent to its final shape and the whole thermometer filled with alcohol as described on page 44. Now heat the thermometer to a little above the maximum temperature that it is intended to record, and pour clean mercury into the open bulb while holding the thermometer vertically. Allow to cool, and the mercury will be sucked down. The second dumb-bell is now introduced, sufficient alcohol being allowed to remain in the open bulb to about half fill it, and the alcohol in this bulb is boiled to expel air. The tube through which the bulb was filled in now sealed.
Clinical Thermometers.—The clinical thermometer is a maximum thermometer of adifferent type. In this case there is a constriction of the bore at a point just above the bulb. When the mercury in the bulb commences to contract, the mercury column breaks at the constriction and remains stationary in the tube, thus showing the maximum temperature to which it has risen.
Vacuum Tubes.—There are so many forms of these that it is scarcely practicable or desirable to give detailed instructions for making them; but an application of the various methods of glass-working which have already been explained should enable the student to construct most of the simpler varieties. An interesting vacuum tube is made which has no electrodes, but contains a quantity of mercury. When the tube is rocked so as to cause friction between the mercury and the glass sufficient charge is produced to cause the tube to glow.
A Sprengel Pump.—This, in its simplest form, is illustrated bya, Fig. 12. Such a form, although highly satisfactory in action, needs constant watching while in action, as should the mercury funnel become empty air will enter the exhausted vessel. Obviously, the fall-tube must be made not less than thirtyinches long; the measurement being taken from the junction of the exhaustion branch with the fall-tube to the top of the turned-up end.
Fig. 12Fig. 12
A Macleod Pump.—One form of this is illustrated byb, Fig. 12. It has the advantagethat the mercury reservoir may be allowed to become empty without affecting the vacuum in the vessel being exhausted.
"Spinning" Glass.—By the use of suitable appliances, it is quite possible to draw out a continuous thread of glass, which is so thin as to have almost the flexibility and apparent softness of woollen fibre; a mass of such threads constitutes the "glass wool" of commerce.
The appliances necessary are:—a blowpipe capable of giving a well-formed flame of about six or eight inches in length, a wheel of from eighteen inches to three feet in diameter and having a flat rim of about three inches wide, and a device for rotating the wheel at a speed of about three hundred revolutions per minute.
A very satisfactory arrangement may be made from an old bicycle; the back wheel having the tyre removed and a flat rim of tin fastened on in its place. The chain drive should be retained, but one of the cranks removed and a handle substituted for the remaining pedal. The whole device is shown by Fig. 13.
Fig. 13Fig. 13
The procedure in "spinning" glass is asfollows:—First melt the end of a glass rod and obtain a large mass of thoroughly softened glass, now spin the wheel at such a speed that its own momentum will keep it spinning for several seconds. Touch the end of the melted rod with another piece of glass and, without withdrawing the original rod from the blowpipe flame, draw out a thread of molten glass and twist it round the spinning wheel. If this is done properly, the thread of glass will gripon the flat rim, and by continuing to turn the wheel by hand it is possible to draw out a continuous thread from the melted rod, which must be advanced in the blowpipe flame as it is drawn on the wheel. If the rod is not advanced sufficiently the thread will melt off, if it is advanced too much, so as to heat the thick part and allow the glass to become too cool at the point of drawing out, then the thread will become too thick, but it is easy after a little practice to obtain the right conditions. Practice is necessary also in order to find the right speed for the wheel.
When sufficient glass has been "spun," the whole "hank" of thin thread may be removed by drawing the thumb-nail across the wheel at any point on its flat rim, thus breaking the threads, and allowing the "hank" to open.
Brushes for Use with Strong Acids.—Glass wool, if of fine enough texture to be highly flexible, can be used to make acid-resisting brushes. A convenient method for mounting the spun glass is to melt the ends of the threads together into a bead, and then to fuse the bead on to a rod; thus giving a brush. If a pointed brush is necessary, the point may be ground onan ordinary grindstone or carborundum wheel by pressing the loose end of the spun glass against the grinding wheel with a thin piece of cardboard.
When using brushes of this description, it is well to bear in mind the fact that there is always a liability of a few threads of glass breaking off during use.
Glass, Its Composition and Characteristics. Annealing. Drilling, Grinding, and Shaping Glass by methods other than Fusion. Stopcocks. Marking Glass. Calibration and Graduation of Apparatus. Thermometers. Exhaustion of Apparatus. Joining Glass and Metal. Silvering Glass.
Glass, Its Composition and Characteristics. Annealing. Drilling, Grinding, and Shaping Glass by methods other than Fusion. Stopcocks. Marking Glass. Calibration and Graduation of Apparatus. Thermometers. Exhaustion of Apparatus. Joining Glass and Metal. Silvering Glass.
There are three kinds of glass rod and tubing which are easily obtainable; these are soda-glass, which is that usually supplied by chemical apparatus dealers when no particular glass is specified; combustion-glass, which is supplied for work requiring a glass that does not so easily soften or fuse as soda-glass; and lead-glass, which is less common. There are also resistance-glass, made for use where very slight solubility in water or other solutions is desirable, and a number of other special glasses; but of these soda-glass, combustion-glass, lead-glass, and resistance-glass are the most important to the glass-blower whose work is connected with laboratory needs.
Soda-Glass.—Consists chiefly of sodiumsilicate, but contains smaller quantities of aluminum silicate, and often of calcium silicate; there may also be traces of several other compounds.
The ordinary soda-glass tubing melts easily in the blowpipe flame, it has not a long intermediate or viscous stage during fusion, but becomes highly fluid rather suddenly; it does not blacken in the reducing flame. Bad soda-glass or that which has been kept for many years, tends to devitrify when worked. That is to say the glass becomes more or less crystalline and infusible while it is in the flame; and in this case it is often impossible to do good work with that particular sample of glass; although the devitrification may sometimes be remedied by heating the devitrified glass to a higher temperature. The presence of aluminum compounds appears to have some influence on the tendency of the glass to resist devitrification. Soda-glass, as a rule, is more liable to crack by sudden heating than lead-glass, and articles made from soda-glass often tend to crack spontaneously if badly made or, in the case of heavier and thicker articles, if insufficiently annealed.
Combustion-Glass.—Is usually a glass containing more calcium silicate and potassium silicate than the ordinary "soft" soda-glass. It is much less fusible than ordinary soda-glass, and passes through a longer intermediate or viscous stage when heated. Such a glass is not very suitable for use with the blowpipe owing to the difficulty experienced in obtaining a sufficiently high temperature. If, however, a certain amount of oxygen is mixed with the air used in producing the blowpipe flame this difficulty is minimised.
Resistance-Glass.—May contain zinc, magnesium, and other substances. As a rule it is harder than ordinary soda-glass, and less suitable for working in the blowpipe flame. It should have very little tendency to dissolve in water, and hence is used when traces of alkali or silicates would prove injurious in the solutions for which the glass vessels are to be used.
Lead-Glass.—This, or "flint" glass as it is often called from the fact that silica in the form of crushed and calcined flint was often used in making the English lead-glasses, contains a considerable proportion of lead silicate. Sucha glass has, usually, a particularly bright appearance, a high refractive index, and is specially suitable for the production of the heavy "cut-glass" ware.
Lead-glass tubing is easy to work in the blowpipe flame, melts easily, but does not become fluid quite so suddenly as most soda-glasses; articles made from it are remarkably stable and free from tendency to spontaneous cracking, although, as is essential for all the heavy or "glass-house" work, the massive articles need annealing in the oven.
The two chief disadvantages of lead-glass for laboratory work are that it is blackened by the reducing gases if held too near to the blue cone of the blowpipe flame, and that it is rather easily attacked by chemical reagents; thus ammonium sulphide will cause blackening.
The effect of the reducing flame on lead is not altogether a disadvantage, however; because a little care in adjusting the blowpipe and a little care in holding the glass in the right position will enable the student to work lead-glass without producing the faintest trace of blackening. This, in addition to being a valuable exercise in manipulation, will teachhim to keep his blowpipe in good order, and prove a useful aid in his early efforts to judge as to the condition of the flame. It prevents discouragement if the student does his preliminary work with the soda-glass, but he should certainly make experiments with lead-glass as soon as he has acquired reasonable dexterity with soda-glass.
Annealing.—Annealing is a process by which any condition of strain which has been set up in a glass article, either by rapid cooling of one part while another part still remains hot, or by the application of mechanical stress after cooling is relieved. Annealing is carried out by subjecting the article to a temperature just below the softening point of the glass, maintaining that temperature until the whole article has become heated through the thicker part, and then reducing the temperature very gradually; thus avoiding any marked cooling of the thinner and outer parts first.
For thin glass apparatus of the lamp-blown or blowpipe-made variety in which there are no marked difference of thickness, such as joins on tubes, ordinary seals, bulbs, etc., there is little need for annealing; and even those havingrather marked changes of thickness, such as filter pumps, can be annealed sufficiently by taking care that the last step in making is heating to just below visible redness in the blowpipe flame and then rotating in a sooty gas flame until covered with a deposit of carbon. The article should then be allowed to cool in a place free from draughts and where the hot glass will not come in contact with anything.
A few of the blowpipe-made articles, such, for example, as glass stopcocks, need more careful annealing, and for this purpose a small sheet-iron oven which can be heated to dull redness over a collection of gas burners will serve. Better still, a small clay muffle can be used. In either case, the article to be annealed should be laid on a clean, smooth, fireclay surface, the temperature should be maintained at a very dull red for two or three hours and then reduced steadily until the oven is cold. This cooling should take anything from three to twelve hours, according to the nature of the article to be annealed. A thick article, or one having great irregularities in thickness will need much longer annealing than one thinneror more regular. As a rule, soda-glass will need more annealing than lead-glass.
Drilling Glass.—Small holes may be drilled in glass by means of a rod of hard steel which has been broken off, thus giving a more or less irregular and crystalline end.
There are several conditions necessary to enable the drilling of small holes to be carried out successfully:—the first of these is that the "drill" should be driven at a high speed. This may be done by means of a geared hand-drill such as the American pattern drill, although a somewhat higher speed than this will give is even more satisfactory. The second condition is that the pressure on the drill is neither too light nor too heavy; this is conveniently regulated by hand. The third condition is that the drill be prevented from "straying" over the surface of the glass; for this purpose a small metal guide is useful. The fourth condition is that a suitable lubricant be used; a strong solution of camphor in oil of turpentine is perhaps the most suitable. For commercial work, a diamond drill is often used, but this is scarcely necessary for the occasional work of a laboratory.
Larger Holes in Glass.—The method of drilling with a hard steel rod is not highly satisfactory for anything but small holes. When a larger hole, say one of an eighth of an inch or more, is needed it is better to use a copper or brass tube. This tube may be held in an American hand-drill, but a mixture of carborundum or emery and water is supplied to the rotating end. Tube or drill must be lifted at frequent intervals in order to allow a fresh supply of the grinding material to reach the end. In this case, also, a guide is quite essential in the early stages of drilling; otherwise the end of the tube will stray. The speed of cutting may be increased slightly by making a number of radial slots in the end of the tube; these serve to hold a supply of the grinding material.
Grinding Lenses.—This is scarcely within the scope of a book on glass-blowing for laboratory purposes, but it may be said that the lens may be ground by means of a permutating mould of hard lead or type-metal. The rough shaping is done with coarse carborundum or emery, and successive stages are carried on with finer and finer material. The last polishing is by theuse of jewellers' rouge on the mould, now lined with a fine textile.
Filing Glass.—If a new file, thoroughly lubricated with a solution of camphor in oil of turpentine, is used, there is but little difficulty in filing the softer glasses. A slow movement of the file, without excessive pressure but without allowing the file to slip, is desirable. After a time the cutting edges of the file teeth will wear down and it will be necessary to replace the file by another.
Grinding Stoppers.—This is, perhaps, the most common form of grinding that the laboratory worker will need to perform, and for that reason, rather full details of the procedure are desirable.
A very crude form of ground-in stopper may be made by drawing out the neck and the mass of glass which is intended to form the stopper to approximately corresponding angles, wetting the surfaces with a mixture of the abrasive material and water, and grinding the stopper in by hand. Frequent lifting of the stopper is necessary during grinding, in order to allow fresh supplies of abrasive material to reach the contacts. When an approximate fit is obtained,the coarse abrasive should be washed off, care being taken that the washing is complete, and a finer abrasive substituted. After a while, this is replaced in its turn by a still finer grinding material.
Such a method of grinding may give a satisfactory stoppering if the angles of the plug and socket correspond very closely before grinding is commenced; but if there is a wide difference in the original angles, then no amount of grinding by this method will produce a good result. The reason for this is that the plug will become so worn in the preliminary grinding as to assume the form of a highly truncated cone; the socket will assume a reverse form, and the end result will be a loose-fitting plug and socket.
Satisfactory grinding may be carried out by the use of copper or type-metal cones for the preliminary shaping. Such cones should be mounted on a mandrel which will fit into the chuck of the American hand-drill and turned on the lathe to the desirable angle for stoppering. A number of these cones will be necessary. A number of similar moulds, that is to say blocks of type-metal or hard lead in which is a hole corresponding in size and angleto the plug desired, should be made also. These must be rotated, either in the lathe or by other means, and are used for the preliminary shaping of the plug. If but few plugs are to be ground it is unnecessary to provide a means of rotating the moulds, as the plug may be held in the hand and ground into the mould in a manner similar to that used in the first method of stoppering.
Fig. 14Fig. 14
When the socket and plug have been ground, by the successive use of cones and moulds, to the desired angle, so that they correspond almost exactly, the plug is given its final fitting into the socket by grinding-in with a fine abrasive, in the manner first described.
Stopcocks.—Although it would be more strictly in keeping with the form of this book to divide the making of stopcocks into two parts; shaping by heat and grinding, we will consider the whole operation here, and take for our example a simple stopcock such as that illustrated by Fig. 14.
The "blank,"f, that is the socket before grinding, is made by drawing out a piece of fairly thick-walled tubing into the form shown bya. Two zones on this tube are then heated by means of a small, pointed flame, and the tube is compressed along its axis, thus producing two raised rings as shown byb. Two zones, slightly towards the outer sides of these two raised rings are heated and the tube is drawn while air pressure is maintained within. This produces two thin-walled bulbs or extensions similar to those shown byc. One of these extensions is now broken off by means of asharp blow with the edge of a file or other piece of metal, and the edges of the broken glass are rounded in the flame. The other extension is left to serve as a handle. We have now a piece of glass like that shown byd. Now heat a spot on the side of this, medially between the raised rings, until the glass is on the point of becoming deformed, and bring the intensely heated end of a smaller tube in contact with the heated spot. Without disturbing the relative positions of the two tubes, press the smaller tube down on a thin steel wire, so that the wire passes along the tube and enters the soft glass; thus forming a projection inside the sockets as shown bye. The wire must be withdrawn, again immediately. When the wire has been withdrawn, heat the place where it entered to dull redness, in order to relieve any strain; break off the thin extension, which up to the present has served as a handle, round off the broken edges in the flame, and join on and indent a similar piece of small tubing to the opposite side of the socket; the socket at this stage being shown byf. The "blank" for the socket is now completed, but it must be heated to dull redness in order to relieve strainand be placed in an annealing oven, where it should be annealed for some hours.
The "blank" for the plug offers no special difficulty; it is made by heating a glass rod and compressing it axially until a mass having the form shown byg, Fig. 14, is produced; the end of this is heated intensely and brought in contact with the rather less heated side of a glass tube which has been drawn to the shape desired for the handle; when contact is made a slight air pressure is maintained in the glass tube, thus producing a hollow join. The ends of the tube are sealed and the bottom of the plug is drawn off, thus giving the finished "blank" as shown byh. This blank is now held in a pair of asbestos-covered tongs, heated to dull redness all over, and transferred to the annealing oven.
When cold, the socket is ground out by the second method given under "Grinding Stoppers"; that is to say, by means of type-metal or copper cone, and the plug is ground to fit in a corresponding mould. When the fit is almost perfect, the transverse hole is drilled in the plug, and the final finishing is made with fine abrasive powder. Great care must be taken in thefinal grinding that there is no accumulation of abrasive material in the transverse hole of the plug; if this is allowed to occur there will be a ring ground out of the socket where the holes move, and the tightness of the finished stopcock will be lost.
Marking Glass.—As a preliminary to a consideration of the methods of graduating and calibrating glass apparatus, it is convenient to consider the various methods which are available for marking glass. Among these are, the writing diamond, the carborundum or abrasive pencil, the cutting-wheel, and etching by means of hydrofluoric acid. Each produces a different class of marking and each is worthy of independent consideration.
The Writing Diamond.—This is the name given to a small irregular fragment of "bort" which is usually mounted in a thin brass rod. Such a diamond, if properly selected, has none of the characteristics of a cutting diamond; although one occasionally finds so-called "writing diamonds" which will produce a definite cut. These should be rejected.
The writing diamond is used in much the same way as a pencil, but is held more perpendicularlyto the object, and a certain amount of pressure is necessary. The mark produced is a thin scratch which, although fairly definite, lacks breadth, and this is a disadvantage where the marking has to be read at a distance. This disadvantage may to some extent be overcome by making a number of parallel scratches.
The Abrasive Pencil.—A rod of carborundum composition may be ground or filed to a point, and this forms a very useful pencil for general work. The marking produced is rather less definite than that produced by a writing diamond, but has the advantage of being broader.
The Cutting Wheel.—"Cutting" in this case is scarcely the ideal expression, it should rather be "grinding," but "cutting" is more commonly used. Exceedingly good graduations may be made by the edge of a small, thin, abrasive wheel which is mounted on the end of a small mandrel and driven by a flexible shaft from an electric motor or any other convenient source of power. The depth of the mark can be controlled, and very light pressure will suffice.
Etching.—This is often the quickest andeasiest way of marking glass apparatus. The object to be marked should first be warmed and coated very thoroughly with a thin film of paraffin wax. When cold, the marking is made through the paraffin wax by means of a needle point, and the object is then exposed to the action of hydrofluoric acid. If a shallow but clearly visible marking is desired, it is well to use the vapour of the acid; this may be done by bending up a sheet-lead trough on which the object can rest with the marked surface downwards. A little of the commercial hydrofluoric acid, or a mixture of a fluoride and sulphuric acid, is distributed over the bottom of the trough, and the whole arrangement is allowed to stand for about an hour. The object is washed thoroughly and the paraffin wax removed, either by melting and wiping off or by the use of a solvent, and the marking is finished.
If a deep marking is desired, in order that it may afterwards be filled with some pigment, a better result is obtained by the use of liquid commercial hydrofluoric acid, which is a solution of hydrogen fluoride in water. The acid is mopped on to the object after themarkings have been made on the paraffin wax film, and allowed to remain in contact for a few minutes. It is advantageous to repeat the mopping-on process at intervals during the etching.
In all cases where hydrofluoric acid is used, or stored, it is of great importance to keep it well away from any optical instruments, as the most minute trace of vapour in the air will produce a highly destructive corrosion of any glass surfaces.
Methods of Calibration.—In the case of apparatus for volumetric work, this is usually carried out by weighing, although some of the smaller subdivisions are often made by measurement. When the subdivisions are made in this way it is of importance to see that the walls of the tube or vessel to be calibrated are parallel. Great errors arise in some of the commercial apparatus from neglect of this precaution. A convenient method of testing for parallelism, in the case of a wide tube, is to close one end and to weigh in successive quantities of mercury. An observation of the length occupied by each successive quantity will indicate any change in the bore. In the case of capillary tubes, it isconvenient to introduce an unweighed quantity of mercury, measure its length accurately, and then to move it along the tube in stages, either by tilting the tube or by the application of air pressure. A measurement of the length at each stage will indicate whether the bore is approximately parallel or not. Neither of these methods is to be relied on without a careful examination of the tube, as it may happen that there are local irregularities in the bore which compensate for each other, and do not, therefore, affect the volume of a given length. Obviously, the smaller the quantity of mercury with which the test is carried out and the greater the number of observations made, the less risk will there be of such an error. A liquid, such as water or alcohol, which wets the glass is not suitable for such a test, unless special precautions are taken.
When, however, a pipette or burette has to be calibrated to deliver a certain volume of water, the final calibration must be made with this liquid. Thus, the burette would first be calibrated by weighing in definite quantities of mercury of say 13.54 grammes (1 cc at 15°C.), each of the 1 cc divisions should be marked bysome temporary marking. The burette is now filled with a solution of potassium bichromate and sulphuric acid and allowed to soak for some time; the bichromate is washed out and distilled water is put in. Successive quantities of water are run out of the jet, a fixed time being allowed for draining, and the weights of the quantities delivered are noted. This procedure will give the necessary data for altering the marking so that it may correspond to 1 ccdelivered. Each 1 cc division is now divided into tenths by the method described below. A final verification of the markings should be made when the subdivision is completed.
Subdivision of Graduations.—Mark out the spaces to be subdivided on a sheet of paper. Take a reliable ruler on which any convenient length is divided into the desired number and place it across the lines at such an angle that the limits noted on the rule exactly bridge the gap. Now draw parallel lines through the markings.
Copying a Scale.—When a scale has been prepared on paper and it is necessary to copy that scale on the waxed-glass surface for etching, a convenient method is to employ along wooden bar having a sharp needle passing through it at either end. The scale and object to be marked are fastened in line with one another, and the caliper bar is used from step to step. The mark is made by moving the bar through a minute portion of a circle, which provided that the bar is two or three feet in length, will not introduce any perceptible error in a scale of say a quarter of an inch in width. The arrangement is shown by Fig. 15.
Fig. 15Fig. 15
Graduating a Thermometer.—Assuming that the thermometer has been made of carefully selected tubing in which the bore is parallel and free from any small irregularities, we have only to fix the freezing point and boiling point. The intervening space may then be divided into 100 (if the thermometer is to be Centigrade) or 180 (if Fahrenheit). This division may be carried out by the method given under"Subdivisions of Graduations." A thermometer should not be calibrated until some weeks after making, as the glass bulb tends to contract.
Joining Glass and Metal.—It sometimes happens that one needs to make a more permanent and less flexible joint between a glass and metal tube than can be obtained by means of a rubber tube. To this end, any one of three slightly different methods may be employed. In the method of Chatelier one first coats the glass with platinum or silver, which may be done by moistening the glass with platinum chloride or silver nitrate and then heating to redness; a layer of copper is then deposited electrolytically on the treated surface of the glass, and soldering is carried out in the usual manner.
McKelvy and Taylor call attention to two other methods in theJournal of the Chemical Societyfor September, 1920. In one of these methods the glass is coated with platinum by covering it with a suspension of platinum chloride in oil of lavender and heating until the oil is burnt off. The metal tube is then tinned on its inner side and soldered to the preparedglass, slightly acid zinc chloride being used as a flux.
In the second method, a joint is made by means of the Kraus flux, which consists of equal weights of zinc oxide, borax, and powdered soda-glass fused together. This is coated on the inner surface of the metal tube, and the hot glass tube, which has had the end slightly flanged to give support, is inserted. Fusion of the flux is completed by heating the outside of metal tube.
Silvering Glass.—In all cases where it is intended to deposit a silver mirror on a glass surface, thorough cleaning is essential. Prolonged soaking in a hot solution of potassium bichromate which has been acidified with sulphuric acid will often prove useful. The glass should then be washed thoroughly, rinsed in distilled water, and the solution should then be used.
There are many formulæ for the silvering solution, but that used in Martin's method may be given:—
A—Nitrate of Silver40grammesDistilled Water1000c. cm.B—Nitrate of Ammonium60grammesDistilled Water1000c. cm.C—Pure Caustic Potash100grammesDistilled Water1000c. cm.D—Pure Sugar Candy100grammesDistilled Water1000c. cm.Dissolve and add:—Tartaric Acid23grammesBoil for ten minutes, and when cool add:—Alcohol200c. cm.Distilled Water to2000c. cm.
For use take equal parts of A and B. Mix together also equal parts of C and D in another vessel. Then mix both liquids together in the silvering vessel and suspend the glass to be silvered face downwards in the solution. Or if a vessel has to be silvered on the inside, the solution is poured in. In this case, the deposition of silver may be hastened by immersing the vessel to be silvered in warm water.
In working with a silver solution containing ammonia or ammonium salts there is sometimes the possibility of forming an explosive silver compound. It is well, therefore, to avoid keeping such solutions longer than is necessary, and to bear in mind that any depositformed by solutions containing both silver and ammonia may have explosive properties, especially when dry.