8 PRACTICAL AGRICULTURAL CHEMISTRY [13—15 



Care must be taken that the flame is lessened, as the 

 liquid in the dish becomes reduced, or the liquid will spirt, 

 i.e. jump out of the dish. 



13. A very convenient form of apparatus is the steam- 

 bath shown in fig. 11. It consists of a copper bath, on the 

 upper side of which are cut a number of holes to fit various- 

 sized evaporating-dishes. The bath is three parts filled 

 with water, and heated until the water just boils. The 

 dishes containing the liquids to be evaporated can thus 

 be kept at a temperature of nearly ioo° C, and loss of the 

 liquid by spirting is avoided. The water which is driven off 

 from the bath when in use should be replaced from time 

 to time, so as to keep the level of the water constant, or better 

 a bath may be used which is fitted with an arrangement 

 for maintaining the water at a constant level. See fig. 14. 



Fig. 12 



14. Crystallisation. — Take the solution (paragraph 

 ii a) and heat over the Bunsen flame until about one-quarter 

 of the liquid is left, allow to cool, when crystals of alum 

 will form. If the solution is too dilute, and no crystals 

 form, further evaporation will be requisite. Most salts are 

 more soluble in hot than in cold water; hence the salt 

 usually crystallises out on cooling. 



15. Precipitation. — Two clear solutions when added 

 together frequently give rise to an insoluble solid, which 

 gives a turbid appearance to the mixture. A body so formed 

 is termed a precipitate. 



Experiment. — To a little calcium chloride solution in a 

 test-tube add a little ammonium carbonate solution ; a 



