PHARMACY. 



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 sume purpose, and is more conve- 

 nient in some respects, as it may be made 

 of any size or form. Glass or metallic 

 vessel's, such as balloons, may also be 

 emptied for the purpose of receiving 1 

 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 salid block of 

 wood or marble ; or, if it be constructed 

 of wood simplv, 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 necl: . 

 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. 



Congelation is the reduction of a fluid 

 to a solid form, in consequence of the ab- 

 straction of caloric. The means employ- 

 ed for abstracting the caloric, are the 

 evaporation of volatile fluids, the solution 

 of solids, and the contact of cold bodies. 



Coagulation is the conversion of a fluid 

 into a solid of greater or less consistence, 

 merely in consequence of a new arrange- 

 ment of its particles, as during 1 the pro- 

 cess there is no separation of caloric or 

 any other substance. The means of pro- 

 ducing coagulation are, increase of tem- 

 perature, and the addition of certain 

 substances, as acids and rennets. 



Chemical Combination, is the intimate 

 union of the particles of at least two he- 

 terogeneous bodies. It is the effect re- 

 sulting 1 from the exertion of the attraction 



