108 Chemistry. [Lecture 28. 



for this purpose are crucibles of a conical form ; 

 they should have covers to keep out the fuel and 

 air. Lead is commonly fused in iron, and salts 

 in glass. 



II. VESSELS for EVAPORATION; these may 

 be divided into three classes: 1. for evapora- 

 tion merely; 2. Distillation, or Sublimation ; 3. 

 Cementation. 



1. EVAPORATION. Flat vessels may be used 

 when the heat is gentle, but in a strong heat 

 crucibles must be employed. See PI. V. fig. 1 1 

 and 12. 



2. DISTILLATION. Distillation is of three 

 kinds: 1. Distillation per descensum ; 2. per 

 ascension ; 3. per latus. 



The first is when the heat is applied above, 

 and the vapour falls directly downward into a 

 condensing vessel. In this manner oil of cloves 

 used to be distilled, but it is now laid aside. The 

 tar manufacture is, however, still carried on by 

 this mode. The wood that produces the tar is 

 put into an oven, which is heated from above, 

 and this causes the oily and resinous parts to 

 sweat out and fall into water, which is placed 

 below to receive it. Zinc is also prepared in 

 this way from lapis calammaris, one of its ores. 



2. By the common still per ascensum. For 

 corrosive substances we use CUCURBITS, or glass 

 vessels in the shape of an egg, which are called 

 by some practical chemists matrasses or bodies. 

 See fig. J 3. The common still used for the 



