To blow a Bulb between two Points (Fig 21).—Select a piece of suitable tube, seal or cork one end, gather together a mass of glass at the desired part, as directed for blowing a bulb at the end of a tube; when a mass of glass has been collected of sufficient thickness, blow it into shape from the open end of the tube by a rapid succession of short blasts of air, till the expanding glass attains the desired dimensions. The tube must be held horizontally, and must be rotated steadily during the process. By slightly pressing together the glass while blowing, the bulb will be flattened; by slightly drawing apart the two ends of the tube, it will be elongated.
A pear-shaped bulb may be obtained by gently re-heating an elongated bulb, say from a to a, and drawing it out. It is easiest to perform this operation on a bulb which is rather thick in the glass.
If the tubes bb are to be small, and a globe of considerable size is wanted, contract a tube as shown in Fig. 22, taking care that the narrow portions of the tube are about the same axis as the wider portions, for if this be not the case, the mouths of the bulb will not be symmetrically placed; seal at C, cut off the wider tube at B, and make the bulb, as previously described, from the glass between AA. If, as probably will be the case, the contracted portions of the tube be not very regular, they may be cut off, one at a time, near the bulb, and replaced by pieces of tube of the size desired.
When a bulb has to be blown upon a very fine tube, for example upon thermometer tubing, the mouth should not be employed, for the moisture introduced by the breath is extremely difficult to remove afterwards. A small india-rubber bottle or reservoir, such as those which are used in spray-producers, Galton’s whistles, etc., securely attached to the open end of the tube, should be used. With the help of these bottles bulbs can be blown at the closed ends of fine tubes with ease, though some care is necessary to produce them of good shape, as it is difficult to rotate the hot glass properly when working in this way.
Making and Grinding Stoppers.—Apparatus which is to contain chemicals that are likely to be affected by the free admission of air, needs to have stoppers fitted to it. Making a good stopper is a much less tedious process than is commonly supposed.
Suppose that the tube I. of Fig. 23 is to be stoppered at A, it must be slightly enlarged by softening the end and opening it with a pointed cone of charcoal; or a conical mouth for the stopper may be made by slightly contracting the tube near one end, as at B, cutting off the cylindrical end of the tube at the dotted line C, and then very slightly expanding the end at C with a charcoal cone after its edges have been softened by heat. In either case the conical mouth should be as long and regular as possible.
For the stopper take a piece of rather thick tube, of such size that it will pass easily, but not too easily, into A or B. Expand this tube at D, as shown in II., by softening the glass and gently compressing it. The configuration of the enlarged tube as shown at D may be obtained by heating and compressing two or more zones of the tube that are adjacent, one zone being less expanded than the other, so as to give the sides of the imperfect stopper as nearly as possible the form shown at D, which, however, is much less regular than may easily be obtained. Seal off the head of the tube at H, and heat the glass till it runs together into a nearly solid mass; compress this with a pair of iron tongs to the flattened head E. In making D, aim at giving it a form which will as nearly as possible correspond to that of the tube into which it is to be ground, and make it slightly too large, so that only the lower part at D can be introduced into the mouth of A or B. Before it is ground, the stopper must be heated nearly to its softening-point and annealed.
Moisten D with a solution of camphor in recently distilled turpentine, and dust the wet surface with finely-ground emery, then gently grind it into its place till it fits properly. In this operation the tail G, which should fit loosely into the tube A, will be of assistance by preventing D from unduly pressing in any direction on A in consequence of irregular movements. The stopper should be completely rotated in grinding it. It must not be worked backwards and forwards, or a well-fitting stopper will not be produced. Renew the emery and camphorated turpentine frequently during the earlier part of the grinding; when the stopper almost fits, avoid using fresh emery, but continue to remove the stopper frequently at all stages of the operation. That added at the earlier stages will be reduced to a state of very fine division, and will therefore leave the stopper and mouth of A with smoother surfaces than fresh emery.[10]
Note.—The addition of camphor to the turpentine used for grinding glass is very important. Notwithstanding its brittle nature, glass will work under a file moistened with this solution almost as well as the metals. Small quantities should be made at a time, and the solution should be kept in a well-closed vessel, for after long exposure to the air it is not equally valuable.
If the stopper is to fit a tube contracted like B, it must be constructed from a piece of tube that will pass through the contraction at B. The tail GF will not do such good service as it does in the case of a tube which has been opened out to receive its stopper, but it will help to guide the stopper, and should be retained.
When the stopper has been ground into its place, melt off the tail at F. The flame must be applied very cautiously, as glass which has been ground is particularly apt to crack on heating. To avoid all risk of this, the tail may simply be cut off, and its edges filed smooth with a file moistened freely with camphorated turpentine.
The stoppers of bottles are not made exactly in the manner described above, though, on occasion, a new stopper may be made for a bottle by following those directions. Ill-fitting stoppers, which are very common, can be very easily re-ground with emery and camphorated turpentine.
[6] Remember that when the lead glass is heated to the proper temperature it will present an appearance which may be described as a greenish phosphorescence. At higher temperatures it assumes an orange-red appearance. If it loses its transparency and assumes a dull appearance, it must be moved further into the oxidising parts of the flame.
[7] Some glass-blowers at once work on the glass as next described, without this preliminary treatment. I find that some glass, usually soda glass, will not always bear the necessary movements without breaking unless first heated all round.
[8] If such an opening be observed, it may usually be closed by touching its edges with a fused point of glass at the end of a drawn out tube.
[9] This can be obtained from Messrs. Powells, Whitefriars Glassworks.
[10] Mr. Gimmingham recommends giving stoppers a final polish with rotten-stone (Proceedings of the Royal Society, p. 396, 1876).
MAKING THISTLE FUNNELS, U-TUBES, ETC.—COMBINING THE PARTS OF COMPLICATED APPARATUS—MERCURY, AND OTHER AIR-TIGHT JOINTS—VACUUM TAPS—SAFETY TAPS—AIR-TRAPS.
In Chapter III. the simpler operations used in making the separate parts of which apparatus is composed have been described. In this Chapter finished apparatus will be described, and the combination of the separate parts into the more or less complicated arrangements used in experiments will be so far explained as to enable the student to set up such apparatus as he is likely to require. I have thought it would be useful that I should add a short account of various contrivances that have come much into use of late years for experimenting under reduced pressure, such as safety taps, air-traps, vacuum joints, etc.
Electrodes.—On page 38 (Fig. 13) is shown a simple form of electrode sealed into a glass tube, which for many purposes answers very well. But frequently, in order that there may be less risk of leakage between the glass and the metal, the latter is covered for a considerable part of its length with solid glass, which at one extremity is united to the apparatus. In Fig. 24 W is the metal core of the electrode, and G the glass covering around it. The wire is fused into the glass, and the glass is then united to the apparatus; a little white enamel should be applied at one end and combined with the glass by fusion.
U-Tubes.—A U-tube is but a particular case of a bent glass tube. It is scarcely possible when bending very large tubes in the manner described on p. 29 to produce regular curves of sufficient strength.
To make a U-tube, or to bend a large tube, close one end of the tube selected with a cork, soften and compress the glass in the flame at the part where it is to be bent till a sufficient mass of glass for the bend is collected, then remove the mass of glass from the flame, let it cool a little, and simultaneously draw out the thickened glass, bend it to the proper form, and blow the bend into shape from the open end of the tube. Small irregularities may be partly corrected afterwards.
To make a good U-tube of large size, and of uniform diameter from end to end, requires much practice, but to make a tolerably presentable piece of apparatus in which the two limbs are bent round till they are parallel, without any considerable constriction at the bend, can be accomplished without much difficulty.[11]
Spiral Tubes.—These may be made by twisting a tube gradually softened by heat round a metal cylinder. Spiral tubes made of small thin tubes possess considerable elasticity, and have been used by Mr. Crookes for making air-tight connections between separate pieces of apparatus when a rigid connection would have been unnecessary and inconvenient. By the use of such spiral tubes it is possible to combine comparatively free movement with all the advantages attached to hermetically-sealed joints.
To make a flexible spiral tube, mount a copper cylinder on a screw, so that the cylinder will travel in the direction of its axis when it is rotated. Fix a fine glass tube to the cylinder, and direct a flame towards the cylinder so as to heat and soften the glass, which will then bend to the form of the cylinder. Gradually rotate the cylinder before the source of heat, so that fresh portions of tube are successively brought into position, softened, and bent. Useful spirals may also be made by hand without a cylinder. As each length of tube is bent, a fresh length may be united to it until the spiral is completed. The fine tubes employed are prepared by heating and drawing out larger tubes.
Thistle Funnels (Fig. 25).—Seal a moderately thick piece of small glass tube at A, then heat a wide zone of it a little below A by rotating it horizontally in the blow-pipe flame till the glass softens, and expand the glass to a bulb, as shown at B of 1; during the operation of blowing this bulb, the end A must be directed to the ground.
Soften the end A and a small portion of B as before, and, holding the tube horizontally from the mouth, blow out the end C as at 2. Heat the end of C gradually, till the glass softens and collapses to the dotted line dd, and at once blow a steady stream of air into the open end of the tube, rotating it steadily, till it is about to burst; finally clean off the thin glass from round the edges of the funnel, which should have the form shown at 3, and round them. An inspection of a purchased thistle funnel will generally show that the head B has been formed from a larger tube sealed to E at f.
Closing Tubes containing Chemicals for experiments at high temperatures.—Tubes of the hard glass used for organic analyses answer best for this purpose; the operation of drawing out the end of such a tube is practically identical with what has been described under the head of choking, p. 35. A well-sealed tube presents the appearance of that shown by Fig. 26.
In order to secure a thick end to the point of the tube a, about an inch or so of the tube near the contracted part should be warmed a little, if it is not already warm, at the moment of finally sealing it; the contraction of the air in the tube, in consequence of the cooling of the warm tube, will then ensure the glass at a running together to a solid end when it is melted in the flame.
If it will be necessary to collect a gas produced during a chemical action from such a tube, make the contracted end several inches long, and bend it into the form of a delivery tube. It will then be possible to break the tip of this under a cylinder in a trough of liquid.
In order to explain the construction of apparatus consisting of several parts, it will be sufficient to take as examples, two very well-known instruments, and to describe their construction in detail. From what is learned in studying these, the student will gather the information that is wanted.
1. To make Hofman’s Apparatus for the electrolysis of water (Fig. 27).
Take two tubes about 35 cm. in length, and 14 mm. in diameter for AA, join taps TT to the end B of each of them, draw out the other end, as shown at D, after sheets of platinum foil with wires attached to them[12] have been introduced into the tubes, and moved by shaking to BB. Then allow the platinum wires to pass through the opening D left for the purpose, and seal the glass at D round the platinum as at E. Pierce the tubes at JJ, and join them by a short piece of tube K, about 14 mm. in diameter, to which the tube T, carrying the reservoir R, has been previously united. R may be made by blowing a bulb from a larger piece of tube attached to the end of T. The mouth M of the reservoir being formed from the other end of the wide tube afterwards. One of the taps can be used for blowing through at the later stages. Each joint, especially those at JJ, must be annealed after it is blown. Some operators might prefer to join AA by the tube K in the first instance, then to introduce the electrodes at E and D. In some respects this plan would be rather easier than the other, but, on the whole, it is better to make the joints at JJ last in order, as they are more apt to be broken than the others during the subsequent manipulations.
2. I have before me the vacuum tube shown by Fig. 28, in which the dotted lines relate to details of manipulation only.
It is usually possible to detect the parts of which a piece of apparatus has been built up, for even the best-made joints exhibit evidence of their existence. Thus, although I did not make the tube that is before me, and cannot therefore pretend to say precisely in what order its parts were made and put together, the evidence which it exhibits of joints at the dotted lines A, B, C, D, E, F, enables me to give a general idea of the processes employed in its construction, and to explain how a similar tube might be constructed. I should advise proceeding as follows:—
Join a piece of tube somewhat larger than M to its end A, draw out the other end of the larger tube, and blow a bulb L as directed on p. 47. Then seal the electrode R into the bulb L (p. 55).
Blow a similar but larger bulb N from a large piece of tube sealed between two tubes of similar size to M, as described at p. 50. Cut off one of the tubes at B, and join the bulb N to M at B. Form the bulb Q in the same manner as in the case of L, seal into it the electrode R, and add the tube marked by the dotted lines at F.
Seal a narrow tube P to the end of a larger tube, and blow out the tube at the joint till the glass is thin and regular. Take a tube O, of similar size to M, slightly longer than P, contract its mouth slightly to meet the wide end of P at D, and after loosely supporting P inside O with a cork, or otherwise, close the end N of O by sealing or corking it, and join P to O at D. Cut off O just above D at E, and join it to the bulb Q, closing either O or F for the purpose. Cut off the end of O at C parallel to the end of P, and connect O to N, using F for blowing the joint at C. F may be used subsequently for introducing any gas into the tube, and, when a vacuum has been established, may be sealed before the blow-pipe.
Modes of combining the Parts of Heavy Apparatus.—It is often necessary to connect pieces of apparatus which are too heavy to be freely handled before the blow-pipe, and which, therefore, cannot be welded together as described on p. 39, by some more effective method than the ordinary one of connecting by india-rubber tubing. For example, apparatus which is to be exhausted by a Sprengel air-pump must be attached to the pump by a joint as perfectly air-tight as can be obtained. This, indeed, often may be done by welding the apparatus to be exhausted to the air-pump before the blow-pipe. But such a method is open to the obvious objection that it is very troublesome to connect and disconnect the parts as often as may be necessary, and that there is some risk of accidental breakages. Nevertheless it may be done on occasion, especially if there be no objection to the use of the flexible spiral tubes already alluded to. When the use of a spiral connecting-tube is not admissible the difficulty is considerably increased. For example, the author has lately required to attach an ozone generator, of the form shown by Fig. 19, which previously had been cemented into a heavy copper jacket, to a pressure-gauge rigidly fixed to a support, and of considerable size. The employment of a flexible spiral connection was prohibited by the fact that it was necessary that the volume of the connecting-tube should be but a small fraction of that of the ozone generator, a condition which compelled the use of a tube of almost capillary bore, and of inconsiderable length. At the same time the frailness of such a connection made it necessary to fix the generator and pressure-gauge rigidly to their supports, in order to avoid the possibility of breakage by slight accidental movements of either of them, and it was obviously necessary to fix the pieces of apparatus in their final positions before joining them, lest the fine tube which connected them should be fractured during adjustment. The possibility of a strain being caused by the contraction that would occur during the cooling down of the joint last made had to be provided for also. The desired object was effected as follows. In Fig. 29 A represents a section of the ozone generator at the point where the tube to connect it to the gauge was fixed. B represents the top of the gauge, with the side tube C, which was to be connected with that from A, viz. D. The ends of C and D were expanded as shown at D (by melting them and blowing them out), so that one of them, made rather smaller than the other, could be overlapped by the larger one. A and B being rigidly fixed in their final positions, with C and D in contact, as shown in the figure, all openings in the apparatus were closed, except one, to which was attached an india-rubber blowing-bottle by means of a tube of india-rubber long enough to be held in the hand of the operator, and to allow him to observe the operation of joining the tubes at D. When everything was in readiness, a very small-pointed flame from a moveable blow-pipe held in the hand was directed upon the glass at D till it melted and the two tubes united. To prevent the fine tube when melted from running into a solid mass of glass, and so becoming closed, a slight excess of pressure was maintained inside the apparatus during the operation by forcing air into it with the india-rubber blower from the moment at which C and D united. A point of charcoal was kept in readiness to support the softened glass at D in case it showed any tendency to fall out of shape.
The V-tube at C served to prevent the subsequent fracture of the joint in consequence of any strain caused by the contraction of the glass in cooling.[13]
It is not difficult to connect several pieces of apparatus successively in this manner, nor is this method only useful in such cases as that just described. Pieces of apparatus of great length and weight may be joined in a similar manner, irrespective of the size of the tubes to be united.
The ends to be joined, prepared as before, so that one slightly overlaps the other, must be held firmly in contact by clamps, and heated in successive portions by a blow-pipe held in the hand of the operator, each patch of glass being re-heated and gently blown, after a rough joint has been made. Finally, a larger flame may be used to heat up the whole joint for its final blowing. It is important to place the apparatus so that the operator has free access to it on all sides. A revolving table might be employed. An assistant to work the bellows is necessary. Or, better still, air may be admitted to the blow-pipe from a large gas-bag placed in some convenient position.
But in most cases one or other of the following air-tight joints can be employed, and will be found to be very convenient:—
Mercury Joints.—The simplest form of mercury joint is shown at Fig. 30. A and B are the two tubes which are to be connected. A larger tube or cup F is attached to A by the india-rubber tube E, and placed on A so that the end of B may be brought into contact with A at C, and connected to it by a well-fitting piece of india-rubber tube C. The cup E is then brought into the position shown in Fig. 30, and mercury is introduced till the india-rubber tube at C is covered. As mercury and glass do not come into true contact, however, such a joint, though said to give good results in practice, is not theoretically air-tight, for air might gradually find its way between the liquid and the glass. By covering the mercury with a little sulphuric acid or glycerine the risk of this occurring may be removed. The same result may be attained by the use of glycerine in place of the mercury in the cup F; but glycerine is less pleasant to work with than mercury.[14]
When sulphuric acid is to be employed in such a joint, or when for any other reason the use of an india-rubber tube is undesirable, the joint may consist of a hollow stopper B (Fig. 31), made of glass tube, and ground to fit the neck of a thistle funnel A. A and B are joined respectively to the pieces of apparatus to be connected, and connection is made by placing B in position in the neck of A; the joint is made air-tight by introducing mercury with strong sulphuric acid above it into the cup A. The joint may be rendered air-tight by introducing sulphuric acid only into the cup. But this plan must not be adopted if the interior of the apparatus is to be exhausted, as sulphuric acid is easily forced between the ground glass surfaces by external pressure. Mercury, however, will not pass between well-ground glass surfaces, and is therefore to be employed for connecting apparatus which is to be exhausted, and, if necessary, protected by a layer of strong sulphuric acid to completely exclude air.
Tubes placed horizontally may be joined by a glycerine or mercury joint such as is shown in Fig. 32. The two tubes A and B are joined as before by an india-rubber connection C, or one may be ground to fit the other, and the joint is then enclosed within a larger jacketing-tube D, with a mouth at F, which is filled with glycerine or mercury. D is easily made by drawing out both ends of a piece of tube, leaving them large enough to pass over the connection at C, however, and piercing one side at F.
Vacuum Taps.—It is not necessary to enter into a description of the construction of ordinary glass taps, which can be purchased at very reasonable prices. It may be remarked here, however, as a great many of them are very imperfectly ground by the makers, that they may easily be made air-tight by hand-grinding with camphorated turpentine and fine emery, finishing with rotten-stone. A well-ground tap, which is well lubricated, should be practically air-tight under greatly reduced pressure for a short period; but when it is necessary to have a tap which absolutely forbids the entrance of air into apparatus, one of the following may be employed:—
(1.) Mr. Cetti’s Vacuum Tap (Fig. 34): This tap is cupped at A and sealed at B, and the cup A is filled with mercury when the tap is in use, so that if, for example, the end C be attached to a flask, and D to an apparatus for exhausting the flask, it will be possible to close the flask by turning off the tap E, and if no air be allowed access through D, the vacuum produced in the flask at C cannot be affected by air leaking through the tap at A or B.
A passage F must be drilled from the bottom of the plug E to meet G, in order that when the plug is in position no residue of air shall be confined within B, whence it might gradually leak into any apparatus connected to it.
It is obvious, however, that this tap does not protect a flask sealed to C from the entrance of air through D, which, in fact, is the direction in which air is most likely to effect an entrance. When using one of these taps as part of an apparatus for supplying pure oxygen, I have guarded against this by attaching a trap (Fig. 33) to the end D, C being joined to the delivery tube from the gas-holder. The structure and mode of action of the trap are as follows:—
A narrow tube G is joined to D of Fig. 34, and terminates in the wide tube I, which is connected above to H, and below to the air-trap J. J is connected at K, by a piece of flexible tube, to a reservoir of mercury, from which mercury enters the air-trap, and passing thence to I, can be employed for filling the V-trap HLG. The air-trap J is in the first instance filled with mercury, and then serves to intercept any stray bubbles of air that the mercury may carry with it. The particular form of the trap shown at HLG was adopted because with it the arm LG is more readily emptied of mercury than with any other form of trap made of small tube that I have tried. It has been used in my apparatus in the following manner:—H was connected with a vessel to be filled with pure oxygen, the tap E closed, and the rise of mercury above L prevented by a clamp on the flexible tube; the vessel to be filled and the trap were then exhausted by a Sprengel pump, and oxygen allowed to flow into the exhausted space by opening E, the operation of exhausting the tubes and admitting oxygen being repeated as often as necessary.
To prevent access of air to E on disconnecting the vessel at H, the mercury was allowed to flow into the trap till it reached to MM. E was then closed, and H exposed without danger of air reaching E, the length of the arms of the trap being sufficient to provide against the effects of any changes of temperature and pressure that could occur.
A delivery tube may be connected to H and filled with mercury, by closing E and raising the mercury reservoir. All air being in that way expelled from the delivery tube, and the supply of mercury cut off by clamping the tube from the reservoir, oxygen can be delivered from the tube by opening E, when it will send forward the mercury, and pass into a tube placed to receive it without any risk of air being derived from the delivery tube.
(2.) Gimmingham’s Vacuum Tap,[15] shown in Fig. 35, consists of three parts. A tube A is ground to fit the neck of B. B is closed at its lower end, and has a hole d drilled through it; when B is fitted to C, d can be made to coincide with the slit e. When A, B, C are fitted together, if d meet e, there is communication between any vessels attached to A and any other vessel attached to C, entrance of external air being prevented by mercury being placed in the cups of C and B. The tap may be opened and closed at pleasure by rotating B.
If A has to be removed, C may be converted into a mercury joint pro tem. by letting a little mercury from the upper cup fall into the tube and cover d, the tap being closed. This mercury must be removed by a fine pipette in order to use the tap again. It should be noted, however, that though external air cannot enter by way of the ground glass joints, there is no absolute protection against the passage of air between A and C, or vessels joined to A and C, even when the tap is closed. The passage of air from A to C depends upon the grinding and lubrication of the joint at C.
Lubricating Taps.—For general purposes resin cerate answers very well. In special cases burnt india-rubber, or a mixture of burnt india-rubber and vaseline will answer well, or vaseline may be used alone. Sulphuric acid and glycerine are too fluid. When a lubricant is wanted that will withstand the action of ether, the tap may be lubricated by sprinkling phosphorus pentoxide upon it, and exposing it to air till the oxide becomes gummy. The joint must then be protected from the further action of the air if possible. For example, if a safety tap be used the cup may be filled with mercury.
Air-Traps.—In Fig. 33, p. 66, an air-trap (J) is shown. An air-trap is a device for preventing the mercury supplied to Sprengel pumps, etc., from carrying air into spaces that are exhausted, or are for any reason to be kept free from air. Figs. 36 and 37 give examples of air-traps. In the simpler of the two (Fig. 36) mercury flowing upwards from C that may carry bubbles of air with it passes through the bulb A, which is filled with mercury before use.[16] Any air which accompanies the mercury will collect at a, the mercury will flow on through b. So long as the level of the mercury in A is above b, the trap remains effective.
In the trap shown by Fig. 37, the tube d, which corresponds to b in Fig. 36, is protected at its end by the cup E. E prevents the direct passage of minute bubbles of air through d. This trap, like the other, must be filled with mercury before it is used, and it will then remain effective for some time.
[11] Large tubes may also be bent by rotating a sufficient length of the tube in a large flame till it softens, and bending in the same manner as in the case of smaller tubes, and after filling them with sand, closing one end completely, and the other so that the sand cannot escape, though heated air can do so.
[12] Red-hot platinum welds very well. The wire may be joined to the sheet of foil by placing the latter on a small piece of fire-brick, holding the wire in contact with it at the place where they are to be united, directing a blow-pipe flame upon them till they are at an intense heat, and smartly striking the wire with a hammer. The blow should be several times repeated after re-heating the metal.
[13] For a method of joining soda glass to lead glass, see p. 81.
[14] If the india-rubber tube C be secured by wires, iron wire, not copper wire, should be employed.
[15] From Proceedings of Royal Society, vol. XXV. p. 396.
[16] This may be done by clamping the tube which supplies mercury below C, exhausting A, and then opening the clamped tube and allowing the mercury to rise.
GRADUATING AND CALIBRATING GLASS APPARATUS.
Although the subjects to which this concluding chapter is devoted do not, properly speaking, consist of operations in glass-blowing, they are so allied to the subject, and of such great importance, that I think a brief account of them may advantageously be included.
Graduating Tubes, etc.—It was formerly the custom to graduate the apparatus intended for use in quantitative work into parts of equal capacity; for example, into cubic centimetres and fractions of cubic centimetres. For the operations of volumetric analysis by liquids this is still done. But for most purposes it is better to employ a scale of equal divisions by length, usually of millimetres, and to determine the relative values of the divisions afterwards, as described under calibration. It rarely happens that the tube of which a burette or eudiometer is made has equal divisions of its length of exactly equal capacities throughout its entire length, and indeed, even for ordinary volumetric work, no burette should be employed before its accuracy has been verified. An excellent method for graduating glass tubes by hand[17] has been described in Watts’s Dictionary of Chemistry, and elsewhere. Another excellent plan, which I have permission to describe, has been employed by Professor W. Ramsay. It will be sufficient if I explain its application to the operation of graduating a tube or strip of glass in millimetre divisions.
The apparatus required consists of a standard metre measure,[18] divided into millimetres along each of its edges, with centimetre divisions between them, a ruler adapted to the standard metre, as subsequently explained, and a style with a fine point for marking waxed surfaces.