Page:EB1911 - Volume 22.djvu/663

Rh H. Geissler of Bonn, and first described in 1858 by W. H. Theo. Meyer in a pamphlet Ueber das geschichtete electrische Licht surpassed all previous forms in both simplicity and efficiency.

The general scheme of Geisler's pump is shown in fig. 3. A and B are pear-shaped glass vessels connected by a long narrow india-rubber tube, which must be sufficiently strong in the body (or strengthened by a linen coating) to stand an outward pressure of 1 to 1$1⁄2$ atmospheres. A terminates below in a narrow vertical tube c which is a few inches longer than the height of the barometer, and to the lower end of this tube the india-rubber tube is attached which connects A with B. At the upper end of A is a glass two-way stop-cock, by turning which the vessel A can either be made to communicate with the vessel to be exhausted, or with the atmosphere, or can be shut off from both when the cock holds an intermediate position. The apparatus, after having been carefully cleaned and dried, is charged with pure and dry mercury which must next be worked backwards and forwards between A and B to remove all the air-bells. The air is then driven out of A by lifting B to a sufficient level, turning the cock so as to communicate with the atmosphere and letting the mercury flow into A until it gets to the other side of the stop-cock, which is then placed in the intermediate position. Supposing the vessel to be exhausted to have already been securely connected to the pump, we now lower the reservoir B so as to reduce the pressure in A sufficiently below the tension in the gas to be sucked in, and, by turning the cock so as to connect A with the vessels to be exhausted, cause the gas to expand into and almost fill A. The cock is now shut against both communications, the reservoir lifted, the gas contents of A discharged and so on, until, when after an exhaustion mercury is let into A, the metal strikes against the top without interposition of a gas-bell. In a well-made apparatus the pressure in the exhausted vessel is now reduced to, $1⁄10$ or $1⁄20$, of a millimetre, or even less. An absolute vacuum cannot be produced on account of the unavoidable air-film between the mercury and the walls of the apparatus.

. 3. Geisler's Air-Pump.

As it takes a height of about 30 in. of mercury to balance the pressure of the atmosphere, a Geisler pump necessarily is a somewhat long-legged and unwieldy instrument; in addition, the long tube is liable to breakage. It can be considerably shortened, the two vessels A and B brought more closely together, and the somewhat objectionable india-rubber tube be dispensed with, if we connect the air-space in B with an ordinary air pump, and by means of it do the greater part of the sucking and the whole of the lifting work. An instrument thus modified was constructed by Poggendorff in 1865.

Even a Geisler's stop-cock requires to be lubricated to be absolutely gas-tight, and this occasionally proves a nuisance. Hence a number of attempts have been made to do without stop-cocks altogether. In the pump generally attributed to Töpler, but which was previously devised by J. Mile of Warsaw in 1828, who termed it a "hydrostatic air-pump without cylinders, taps, lids or stoppers," this is attained by using, both for the inlet and the outlet, vertical capillary glass tubes, soldered, the former to somewhere near the bottom, the latter to the top of the vessel. These tubes, being more than 30 in. high, obviously act as efficient mercury-traps; but the already considerable height of the pump is thus multiplied by two. This consideration led Alexander Mitscherlich, F. Neisen and others to introduce glass valves in lieu of stop-cocks. A pump similar to Töpler's construction was devised by Mendeléeff, and the original device has been much improved by Wiedemann, Bessel-Hagen and others.

The Dynamic Pump.

The best-known pump of this type was invented in 1865 by H. Sprengel, although the idea had been previously conceived by Magnus and Buff. The instrument, in its original (simplest) form (fig. 4), consists of a vertical capillary glass tube a of about 1 mm. bore, provided with a lateral branch b near its upper end, which latter, by an india-rubber joint governable by a screw-clamp, communicates with a funnel. The lower end is bent into the shape of a hook, and dips into a pneumatic trough. The vessel to be exhausted is attached to b, and, in order to extract its gas contents, a properly regulated stream of mercury is allowed to fall through the vertical tube. Every drop of mercury, as it enters from the funnel, entirely closes the narrow tube like a piston, and in going past the place where the side tube enters entraps a portion of air and carries it down to the trough, where it can be collected. If the vertical tube, measuring from the point where the branch comes in, is a few inches greater than the height of the barometer, and the glass and mercury are perfectly clean, the apparatus slowly but surely produces an almost absolute vacuum.

The great advantages of Sprengel's pump lie in the simplicity of its construction and in the readiness with which it adapts itself to the collecting of the gas. It did excellent service in the hands of Graham for the extraction of gases occluded in metals. Many improvements upon the original construction have been proposed.

. 4. Sprengel's Air-Pump.

Many other devices have been introduced for facilitating the production of vacua. For example Raps in 1893 described an automatic arrangement to be used in Connexion with a Töpler pump; whilst in 1893 Schulze-Berge devised a rotary form. For the description of these forms see Winkelmann, Handbuch der Physik (1906), i. 1316. The history of mercurial pumps is treated by S. P. Thompson, The Development of the Mercurial Air Pump (1888). For the production of high vacua, see

PUMPKIN, the fruit of the gourd Cucurbita Pepo, well-known in English cottage gardens, and largely cultivated in continental Europe and North America. The pumpkin varies much in form, being sometimes nearly globular, but more generally oblong or ovoid in shape; the rind is smooth and very variable in colour. It is a useful plant to the American backwoods farmer, yielding, both in the ripe and the unripe condition, a valuable fodder for his cattle and pigs, being frequently planted at intervals among the maize that constitutes his chief crop. The largest kinds acquire a weight of 40 to 80 ℔ but smaller varieties are in more esteem for garden culture. When ripe, the pumpkin is boiled or baked, or made into various kinds of pie, alone or mixed with other fruit; while small and green it may be eaten like the vegetable marrow. The name squash is applied in America to this and other species of the genus Cucurbita. The name is adapted from an American Indian word (see L. H. Bailey, Cyclopaedia of American Horticulture, where is a fuller account of the squashes). Summer squashes are mostly varieties of C. Pepo; winter squashes are either C. maxima or C. moschata, chiefly the former. The varieties of pumpkins and squashes are numerous and of great variety in size and shape; it is difficult to keep them pure if various kinds are grown together, but the true squashes (C. maxima) do not hybridize with the true pumpkin species. If carefully handled to avoid cracking of the skin, and kept dry and fairly warm, winter squashes may be kept for months.

 PUN, a play upon words, particularly the use of a word in two or more different applications or of two or more words similar in sound but with different meanings by which a humorous or ludicrous effect is produced; thus Charles I.'s Court Jester is said to have made the punning grace “great praise be to God and little Laud to the devil” for which the archbishop dismissed him from his service. Another famous pun was that upon The Beggar's Opera, which “made Gay rich and Rich gay.” Thomas Hood was the king of pun-makers. “They went and told the sexton, and the Sexton toll'd the bell” (“Sally Brown”) is one example among the innumerable puns with which his poems are filled. The derivation of the word is not known. It first appears in the second half of the 17th century. Skeat