Salts. 



If they differ in solubility, we may partially 

 evaporate the solution, and thus the less soluble 

 may be taken out in crystals. Saltpetre, for in- 

 stance, frequently contains much common salt. 

 Dissolve the salt in hot water; as the water 

 cools, the saltpetre falls separate from the com- 

 mon salt, for the heat of the water keeps much 

 more of the saltpetre dissolved than when cold 

 it can do. Another way is this: dissolve the 

 salt in water, and put the water on the fire till 

 much of it has evaporated ; the common salt will 

 then fall, whilst the saltpetre is dissolved ; for 

 the heat of the water will keep the saltpetre 

 suspended when it will not suspend common salt. 

 Common salt does not dissolve more easily in 



hot water than in cold. When the water cools, 

 the saitpeue m ^ vi^ r -.!.-.*. on.- r ~ i . 



water again, and so all the salts will be alter- 

 nately deposited. 



If one of the salts is deliquescent, and we are 

 indifferent about it, we may set them on a spongy 

 paper, which sucks up the deliquescent salt as it 

 liquefies. 



If these methods do not answer, the only re- 

 maining one is crystallization. The separation, 

 by crystallization, depends upon the different 

 form of the crystals, when we pick them out, 

 and separate such as differ in form, and assemble 

 such as agree. This means of separation is not 

 so perfect as the others. Few of these methods 

 of making a separation answer the first time of 



