that Salts are more sohible in hot than in cold Wafer. 2^ 



tween the base and water when in the state of a solid hydrate, 

 we might expect the striking power of heat in weakening the 

 affinity or attraction, to affect the solubihty of the salt at dif- 

 ferent temperatures. Even supposing that the solvent power 

 of water increased to a certain extent with rise in temperature, 

 yet this rapid diminution of the attraction of the salt for water 

 as the temperature rose, might counteract and eventually over- 

 come the increasing power of the solvent, in salts so efflo- 

 rescent as phosphate of magnesia or sulphate of soda. Hence 

 the solubility of such salts might begin to lessen, when the 

 temperature was raised beyond a certain point. 



As the hydrates of all the salts, whether they may be efflo- 

 rescent at the temperature of the atmosphere or not, are de- 

 composed by heat, the cause assigned, as counteracting the 

 increase of the solvent power of water with temperature, if it 

 exists, must be general, and influence to a greater or less extent 

 the solubility at different temperatures of all salts whatever. 

 In fact, the consequence necessarily follows from it, that for 

 every salt there is a point in the scale of temperature above 

 which it ceases to become more soluble in water, and dimi- 

 nishes in solubility. In the case of the efflorescent salts, whose 

 affinity for water, when in the state of hydrate, is much im- 

 paired by slight elevation of temperature, this point of tem- 

 perature appears to be low — in some cases under 212° ; in the 

 case of hydrates which retain their water with more force it 

 will be higher, and in hydrates which require a considerable 

 heat to decompose them, the maximum point of solubility will 

 be proportionally high, and such as would require the reten- 

 tion of the solvent in the liquid state by vast pressure, in order 

 to be exhibited. 



In that extensive class of salts which do not form solid com- 

 binations with water, we do not possess such a clue to their so- 

 lubility at different temperatures. They may, therefore, be 

 subject in some cases to this anomaly in solubility as well as 

 the efflorescent salts. Indeed the theory is not applicable even 

 to all the hydrates without distinction. There is a class of hy- 

 drates, in which the combination between the base and water 

 appears to differ essentially from that of the ordinary hydrates 

 of the salts. This class comprehends the hydrates of the al- 

 kalies, the earths and metallic oxides, and these appear not to 

 be subject to the law. 



Many salts, oxides and earths of this class are known to 

 be deprived of solubihty by exposure to a considerable heat. 

 This arises from the loss of the water with whicli they were 

 previously combined, and not, as it is often supposed, from 

 the action of heat in hardening and increasing the cohesion 



Ntrw Series. Vol. 2. No. 7. Jn/jj 1827. K of 



