PHARMACY. 



Clack employed two different coatings. 

 Next to the iron he applied a composition 

 of three parts by weight of charcoal, and 

 one of fine clay. These are first mixed 

 in the state of fine powder, and then 

 worked up with as much water as will 

 permit the mass to be formed into balls, 

 which are applied to the sides of the fur- 

 nace, and beat very firm and compact, 

 with the face of a broad hammer, to the 

 thickness of about one inch and a half in 

 general, but so as to give an elliptical 

 form to the cavity. Over this, another 

 lute, composed of six or seven parts of 

 sand, and one of clay, is to be applied in 

 the same manner, to the thickness of 

 about half an inch. These lutes must be 

 allowed to become perfectly dry before 

 the furnace is heated, which should at 

 first be done gradually. They may also 

 be lined with fire-bricks of a proper form, 

 accurately fitted and well cemented to- 

 gether before the top plate is screwed 

 on. 



The general fault of furnaces is, that 

 they admit too much air, which prevents 

 us from regulating the temperature. It 

 either becomes too violent and unma- 

 nageable, or when more cold air is admit- 

 ted than what is necessary for supporting 

 the combustion, it carries off heat, and 

 prevents us from raising the temperature 

 as high as we otherwise would. The su- 

 perior merit of Dr. Black's furnace con- 

 sists in the facility with which the admis- 

 sion of air is regulated ; and every at- 

 tempt hitherto made to improve it, by in- 

 creasing the number of its apertures, have 

 in reality injured it. 



Heat may be applied to vessels em- 

 ployed in chemical operations, directly, 

 as in the open fire and reverberatory fur- 

 nace : or through the medium of sand; 

 the sand-bath : of water ; the water-bath : 

 of steam ; the vapour-bath : of air ; as in 

 the muffle. 



Changes produced by chemical processes^ 

 These consist chiefly in a new mode of 

 aggregation, combination, and decompo- 

 sition. 



The form of aggregatioji may be alter- 

 ed by fusion, vaporization, condensation, 

 congelation, and coagulation. 



Fusion is the conversion of a solid 

 into a liquid by the sole agency of calo- 

 ric. Substances differ very much in the 

 degrees of their fusibility; some, as wa- 

 ter and mercury, existing as fluids 

 in the ordinary temperatures of the at- 

 mosphere ; while others, as the pure 

 earths, cannot be melted by any heat we 

 can produce. 



Liquefaction is commonly employed 

 to express the melting of substances, as 

 tallow, wax, resin, Sec. which pass through 

 intermediate states of softness before 

 they become fluid. Fusion is the melt- 

 ing of substances which pass immediate- 

 ly from the solid to the fluid state, as 

 the salts and metals, except iron and pla- 

 tinum. 



When, in consequence of fusion, the 

 substances operated on acquire a greater 

 or less degree of transparency, a dense 

 uniform texture, and great brittleness, 

 and exhibit a conchoidal fracture, with a 

 specular surface, and the edges of the 

 fragments very sharp, it is termed vitri- 

 faction. 



In general, simple substances are less 

 fusible than compounds; for example, the 

 simple earths cannot be melted singly, 

 but when mixed are easily fused. The ad- 

 ditions which are sometimes made to re- 

 fractory substances, to, promote their fu- 

 sion, are termed fluxes : which fluxes are 

 generally saline bodies. 



Thus, the alkalies potash and soda pro- 

 mote powerfully the fusion of silicious 

 stones ; but they are only used for accu- 

 rate experiments. The white flux is a 

 mixture of a little potash with carbonate 

 of potash, and is prepared by deflagrating 

 together equal parts of nitrate of potash 

 and super-tartrate of potash. When an ox- 

 ide is at the same time to be reduced, the 

 black flux is preferred, which is produc- 

 ed by the deflagration of two parts of 

 super-tartrate of potash, and one of ni- 

 trate of potash. It differs from the for- 

 mer only in containing a little charcoal. 

 Soap promotes fusion by bfing 1 converted 

 by the fire into carbonate of soda and 

 charcoal. 



Aluminous stones have their fusion 

 greatly promoted by the addition of sub- 

 borate of soda. 



Muriate of soda, the mixed phosphate 

 of soda and ammonia, and other salts, are 

 also occasionally employed for the same 

 purpose. 



An open fire is sufficient to melt some 

 substances, others require the heat of a 

 furnace. 



The vessels in which fusion is perform- 

 ed, must resist the heat necessary for the 

 operation. In some instances an iron or 

 copper ladle or pot may be used, but 

 most commonly crucibles are employed. 

 These are of various sizes. The large 

 crucibles are generally conical, with a 

 small spout for the convenience of pour- 

 ing out ; the small ones are truncated tri- 

 angular pyramids, and are commonly sold 



