CHAPTER IV.

_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]

[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.

=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.

[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.

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]

[13] For a method of joining soda glass to lead glass, see p. 81.

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]

[14] If the india-rubber tube _C_ be secured by wires, iron wire, not copper wire, should be employed.

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_.

[15] From _Proceedings of Royal Society_, vol. XXV. p. 396.

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.

[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.

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.