In separating permanently elastic fluids or gases from the substances in which they are found, we are compelled to em ploy a distinct pneumatic apparatus ; and the gas may then be received either into vessels absolutely empty, or filled with some fluid, on which it exerts no action.
The first mode of collecting gases may be practised by means of a bladder, mois, teased s'tfficientiy to make it perfectly pliable, and then compressed so as to press out every particle of air from its cavity. In this state it may be easily filled with any gas. An oiled silk bag will answer the same purpose, and is more conve nient in some respects, as it may be made of any size or form. Glass or metallic vessels, such as balloons, may also be emptied for the purpose of receiving gases, by fitting them with a stop-cock, and exhausting the air from them by means of an air-pump.
But the second mode of collecting gases is the most convenient and com mon. In which case the vessels may be filled either with a fluid lighter, or hea vier, than the gas to be received into it.
The former method is seldom employ ed; but if we conduct a stream of any gas heavier than atmospheric air, such as carbonic acid gas, muriatic acid gas, &c. to the bottom of any vessel, it will gra dually displace the air, and fill the ves sel. On the contrary, a gas lighter than atmospheric air, such as hydrogen, may be collected in an inverted vessel by con ducting a stream of it to the top. But gases are most commonly collected by conducting the stream of gas into an in verted glass-jar, or any other vessel filled with water or mercury. The gas ascends to the upper part of the vessel, and dis places the fluid. In this way gas may be kept a very long time, provided a small quantity of the fluid be left in the vessels, which prevents both the escape of the gas, and the admission of atmospheric air.
The vessels may be made of various shapes ; but those most commonly em ployed are cylindrical. They may be either open only at one extremity, or furnished at the other with a stop-cock. The manner of filling them with fluid, is to immerse them completely in it, with the open extremity directed a little up wards, so that the whole air may escape from them, and then inverting them with their mouths downwards. For filling
them with convenience, a trough or cis tern is commonly used. This should either be hollowed out of a solid block of wood or marble ; or, if it be constructed of wood simply, be well painted or lined with lead or tinned copper. Its size may vary very much ; but it must contain a sufficient depth of fluid to cover the largest transverse diameter of the ves sels to be filled in it. At one end or side, there should be a shelf for holding the vessels after they are filled. This shelf should be placed about an inch and a half below the surface of the fluid, and should be perforated with several holes, forming the apices of corresponding conical exca vations on the lower side, through which, as through inverted funnels, gaseous fluids may be more easily introduced into the vessels placed over them. In general the vessels used with a mercurial appara tus should be stronger and smaller than those for a water cistern, and we must have a variety of glass and elastic tubes, for conveying the gases from the vessels in which they are formed to the funnels under the shelf.
The repeated distillation of any fluid is denominated rectification. When distil lation renders the fluid stronger, or abstracts water from it, it is termed de phlegmation. When a fluid is distilled off from any substance, it is called ab straction ; and if the product be redis tilled from the same substance, or a fresh quantity of the same substance, it is de nominated cohobation.
The difference between distillation and sublimation is only in the form of the product. When it is compact, it is term ed a sublimate ; when loose and spongy, it formerly had the appellation of flowers. Sublimation is sometimes performed in a crucible, and the vapours are condensed in a paper cone, or in another crucible inverted over it ; sometimes in the lower part of a glass flask, cucurbit, or phial, and the condensation is effected in the upper part or capital, and sometimes in a retort with a very short and wide neck, to which a conical receiver is fitted. The heat is most commonly applied through the medium of a sand-bath ; and the de gree of heat, and the depth to which the vessel is inserted in it, are regulated by the nature of the sublimation.