On Laboratory Arts

by Richard Threlfall

The thistle funnels at the head of the fall tubes are made simply by blowing bulbs and then blowing the heads of the bulbs into wider ones, and finally blowing the heads of the wider bulbs off by vigorous blowing. The stoppers are ground in on the lathe before the tubes are attached to the fall tubes. The stoppers require to be at least half an inch long where they fit the necks, and must be really well ground in. The stoppers must first be turned up nicely and the necks ground out by a copper or iron cone and emery. The stoppers are rotated on a lathe at quite a slow speed, say 30 or 40 feet per minute, and the necks are held against them, as described in the section dealing with this art. The stoppers must in this case be finished with "two seconds" emery, and lastly with pumice dust and water (see chapter on glass-grinding).

Unless the stoppers fit exceedingly well trouble will arise from the mercury (which is poured into the thistle heads to form a seal) being forced downwards into the pump by atmospheric pressure.

The joints between the three fall tubes and the single exhaust main are easily made when the tubes are finally mounted, the hooked nozzle of the oxygas blow-pipe being expressly made for such work.

It is, on the whole, advisable to make the pump of flint glass, or at all events the air-trap tube and the fall tubes. A brush flame from the larger gas tube of the single blowpipe table is most suitable for the work of bending the tubes. The jointing of the long, narrow bore fall tubes is best accomplished by the oxygas flame, for in this way the minimum of irregularity is produced; the blowing tubes will of course be required for the job, and the narrow tubes must be well cleaned to begin with.

The air trap is an important though simple part of the pump. Its shoulder or fall should stand rather higher than the shoulders of the fall tubes, so that the mercury may run in a thin stream through a good Torricellian vacuum before it passes down to the fall tubes. This is easily attained by regulating the main mercury supply at the pinch cock situated between the tube from the upper reservoir and the air-trap tube, the other cocks being almost wide open.

It might be thought that the mercury would tend to pick up air in passing through the rubber connections to the fall tubes, but I have not found this to be the case in practice. There is, of course, no difficulty in eliminating the rubber connections between the fall tubes and the mercury supply from the air trap, but it impresses a greater rigidity on the structure and, as I say, is not in general necessary. It must not be forgotten that the mercury always exercises considerable pressure on the rubber joints, and so there is little tendency for gas to come out of the rubber.

The thistle funnels at the head of the fall tubes provide a simple and excellent means of cleaning the fall tubes. For this purpose some "pure" sulphuric acid which has been boiled with pure ammonium sulphate is placed in each thistle funnel, and when the fall tube is dirty the connection to the mercury supply is cut off at the pinch cock so as to leave the tube between this entry and the head of the fall tube quite full of mercury, and the sulphuric acid is allowed to run down the fall tube by raising the stopper. The fall tube should be allowed to stand full of acid for an hour or so, after which it will be found to be fairly clean.

Of course the mercury reservoir thus obtains a layer of acid above the mercury, and as it is better not to run the risk of any acid getting into the pump except in the fall tubes, the reservoir is best emptied from the bottom, by a syphon, if a suitable vessel cannot be procured, so that clean mercury only is withdrawn.

The phosphorus pentoxide tube is best made as shown simply from a bit of wide tube, with two side connections fused to the rest of the pump. It is no more trouble to cut the tube and fuse it up again when the drying material is renewed than to adjust the drying tube to two fixed stoppers, which is the alternative. The practice here recommended is rendered possible only by the oxygas blow-pipe with hooked nozzle. The connection between the pump and tube to be exhausted is made simply by a short bit of rubber tube immersed in mercury.

The phosphorus pentoxide should be pure, or rather free from phosphorus and lower oxides; unless this be the case, the vapour arising from it is apt to soil the mercury in the pump. The phosphorus pentoxide is purified by distilling with oxygen over red-hot platinum black; if this cannot be done, the pentoxide should at least be strongly heated in a tube, in a current of dry air or oxygen, before it is placed in the drying tube.