PHARMACY. 



in nests. The Hessian crucibles are com- 

 posed of clay and sand, and vhen good, 

 will support an intense heat for many 

 hours, without softening 1 or melting ; but 

 they are disposed to crack when sudden- 

 ly heated or cooled. This inconvenience 

 may be on many occasions avoided, by 

 using a double crucible, and filling up the 

 interstice with sand, or by covering the 

 crucible with a lute of clay and sand, by 

 which means the heat is transmitted 

 more gradually and equally. Those 

 which ring clearly when struck, and are 

 of an uniform thickness, and have a red- 

 dish brown colour, without black spots, 

 are reckoned the best. Wedgewood's 

 crucibles are made of clay mixed with 

 baked clay finely pounded, and are in 

 every respect superior to the Hessian, 

 but they are very expensive. The black 

 lead crucibles, formed of clay and plum- 

 bago, are very durable, resist sudden 

 changes of temperature, and may be re- 

 peatedly used, but they are destroyed 

 when saline substances are melted in 

 them, and suffer combustion when ex- 

 posed red hot to a current of air. 



When placed in a furnace, Crucibles 

 should never be set upon the bars of the 

 grate, but always upon a support. Dr. 

 Kennedy found the hottest part of a fur- 

 nace to be about an inch above the grate. 

 They may be covered, 10 prevent the 

 fuel or ashes from falling into them, with 

 a lid of the same materials, or with ano- 

 ther crucible inverted over them. When 

 the fusion is completed, the substance 

 may be either permitted to cool in the 

 crucible, or may be poured into a heated 

 mould anointed with tallow, never with 

 oil, or what is still better, covered with 

 a thin coating of chalk, which is applied 

 by laying it over with a mixture of chalk 

 diffused in water, and then evaporating 

 the water completely by heat. To pre- 

 vent the crucible from being broken by 

 cooling too rapidly, it is to be either re- 

 placed in the furnace, to cool gradually 

 with it, or covered with some vessel to 

 prevent its being exposed immediately to 

 the air. 



Fusion is performed with the intention 

 of weakening the attraction of aggrega- 

 tion ; or of separating substances of dif- 

 ferent degrees of fusibility from each 

 other. 



Vaporization is the conversion of a solid 

 or fluid into vapour by the agency of ca- 

 loric. Although vaporability be merely 

 a relative term, substances are said to be 

 permanently elastic, volatile or fixed. 

 The permanently elastic fluids or gases 



are those which cannot be condensed 

 into a fluid or solid form by any abstrac- 

 tion of caloric we are capable of produc- 

 ing. Fixed substances, on the contrary, 

 are those which cannot be converted into 

 vapour by great increase of tempera- 

 ture. The pressure of the atmosphere 

 has very considerable effect in varying 

 the degree at which substances are con- 

 verted into vapour. Some solids, unless 

 subjected to very great pressure, are at 

 once converted into vapour, although 

 most of them pass through the interme- 

 diate state of fluidity. 



Vaporization is employed to separate 

 substances differing in volatility ; and to 

 promote chemical action, by disaggregat- 

 ing them. 



When employed with either of these 

 views, no regard is paid to the substances 

 volatilized, whether from solids, as in us- 

 tulation and charring; or from fluids, as in 

 evaporation ; or whether the substances 

 vaporized are condensed in proper ves- 

 sels : for example, in a liquid form, as in 

 distillation ; or in a solid form, as in sub- 

 limation. Or whether the substances va- 

 porized are permanently elastic, and are 

 collected in their gaseous form, in a pneu- 

 matic apparatus. 



Ustulation is almost entirely a metal- 

 lurgic operation, and is employed to ex- 

 pel the sulphur and arsenic contained in 

 some metallic ores. It is performed on 

 small quantities in tests placed within a 

 muffle. Tests are shallow vessels made 

 of bone ashes or baked clay. Muf- 

 fles are vessels of baked clay, of a semi- 

 cylindrical form, the fiat side forming the 

 floor, and the arched portion the roof 

 and sides. The end and sides are per- 

 forated with holes for the free transmis- 

 sion of air, and the open extremity is plac- 

 ed at the door of the furnace, for the in- 

 spection and manipulation of the process. 

 The reverb eratory furnace is commonly 

 employed for roasting, and the heat is at 

 first very gentle, and slowly raised to red- 

 ness. It is accelerated by exposing as 

 large a surface of the substance to be 

 roasted as possible, and bv stirring it fre- 

 quently, so as to prevent any agglutina- 

 tion, and to bring every part in succession 

 to the surface. 



Charring may be performed on any of 

 the compound oxides, by subjecting them 

 to a degree of heat sufficient to expel all 

 their hydrogen, nitrogen, and supera- 

 abundant oxvgen, while the carbon, be- 

 ing a fixed principle, remains behind in 

 the state of charroal. The temperature 

 necessary for the operation may be pro- 



