ON WATER AND ITS COMPOUNDS 78 



larly instructive example of the case in question. If this salt be taken 

 in an ignited state (deprived of its water of crystallisation), then its 

 solubility in 100 parts of water varies with the temperature in the 

 following manner : at 0, 5 parts of the salt form a saturated solution ; 

 at 20, 20 parts of the salt, at 33 more than 50 parts. As will be 

 seen, the solubility increases with the temperature, as is the case 

 with nearly all salts ; but starting from 33 it suddenly diminishes, 

 and at a temperature of 40, there dissolves less than 50 parts of 

 the salt, at 60 only 45 parts of the salt, and at 100 about 43 

 parts of the salt in, 100 parts of water. This phenomenon may be 

 traced to the following facts : Firstly, that this salt forms various 

 compounds with water, as will be afterwards explained ; secondly, 

 that at 33 the compound Na 2 SO 4 + 10H. 2 formed from the solu- 

 tion at lower temperatures, melts ; and thirdly, that on evaporation 

 at a temperature above 33 there separates out an anhydrous salt, 

 Na 2 S0 4 . It will be seen from this example how complicated such a 

 seemingly simple phenomenon as solution really is ; and all data con- 

 cerning solutions lead to the same conclusion. This complexity becomes 

 evident in investigating the heat of solution. If solution consisted of 

 a physical change only, then in the solution of gases there would be 

 evolved and in the solution of solids, there would be absorbed so 

 much heat as answers to the change of state ; but in reality a large 

 amount of heat is always evolved in solution, depending on the fact 

 that in the process of solution there is accomplished an act of chemical 

 combination, accompanied by an evolution of heat. Seventeen grams of 

 ammonia (this weight corresponds with its formula NH 3 ), in passing 

 from a gaseous into a liquid state, evolve 4,400 units of heat (latent 

 heat) ; that is, the quantity of heat necessary to raise the temperature 

 of 4,400 grams of water 1. The same quantity of ammonia, in dissolv- 

 ing in an excess of water, evolves twice as much heat namely 8,800 

 units showing that the combination with water is accompanied by the 

 evolution of 4,400 units of heat. Further, the chief part of this -heat 

 is separated in dissolving in small quantities of water, so that 17 grams 

 of ammonia, in dissolving in 18 grams of water (this weight corre- 

 sponds with its composition H 2 O), evolve 7,535 units of heat, and there- 

 fore the formation of the solution NH 3 + H 2 O evolves 3,135 units of 

 heat beyond that due to the change of state. As in the solution of 

 gases, the heat of liquefaction (of physical change of state) and of chemi- 

 cal combination with water are both positive ( + ), therefore in the 

 solution of gases in water a heat effect is alwa}*s observed. This pheno- 

 menon is different in the solution of solid substances, because their 

 passage from a solid to a liquid state is accompanied by an absorption 



