'Salts. H3 



and requires more heat to expel the water than 

 soda does. The vapour expands very violently, 

 and we must be careful that it does not boil over. 

 It is generally evaporated to dryness, and has 

 been commonly supposed not to assume a cry- 

 stalline appearance. Saturated, however, with 

 carbonic acid, it readily shoots into four-sided 

 prisms with dihedial summits. The potass used 

 in commerce is a subcarbonat of potass. This 

 combination with carbonic acid deprives potass 

 of its caustic quality. When treated with quick 

 lime, the lime has a stronger attraction for the 

 carbonic acid, and consequently takes it from 

 the carbonat of potass, which by that means is 

 rendered exceedingly caustic. The same hap- 

 pens to soda. Potass is frequently found in a 

 state of combination with nitric acid, forming 

 nitrate of potass (nitre), from which, by a par- 

 ticular process, it may be obtained. It is, how- 

 ever, procured in large quantities by infusion and 

 evaporation from the ashes of land vegetables, 

 particularly from those of wood, in those coun- 

 tries where wood is the common fuel. It is con- 

 verted into what is called pearl-ash, by extracting 

 first a strong tincture from the ashes by water, 

 which is called a lye. The watery parts being 

 evaporated, a mass of a brownish colour remains, 

 which is calcined to make it whiter. Some use 

 another method, in order to render it more fixed 

 and less deliquescent, which is done by steeping 

 fresh vegetables, particularly beanstraw, in the 



