"N WATEB AND ITS COMPOUNDS 75 



most cases solid substances in dissolving in water evolve heat, notwith- 

 standing the passage into a liquid state, which indicates so considerable 

 an evolution of ( + ) heat in the act of combination with water that it 

 exceeds the absorption of ( ) heat dependent on the passage into a 

 liquid state. Thus, for instance, calcium chloride, CaCl,, magnesium 

 sulphate, ^IgSO,, and many other salts in dissolving evolve heat ; for 

 example, 60 grams of magnesium sulphate evolves about 10,000 units 

 of heat. Therefore, in the solution of solid bodies there is produced 

 either a cooling 2G or a heating 27 effect, according to the difference of 

 the reacting affinities. When they are considerable that is, when 

 water is with difficulty separated from the resultant solution, and only 

 with a rise of temperature (such substances absorb water) then 

 much heat is evolved in the process of solution, just as in many 

 reactions of direct combination, and therefore a considerable heating of 

 the solution is observed. "Of such a kind, for instance, is the solution 



matioii (in the sense of a mechanical and not a chemical representation of solutions) that 

 substances in solutions preserve their physical states (as gases, liquids, or solids), it is 

 very doubtful, for it would necessitate admitting the presence of ice in water or its 

 vapour. His theory starts from an unsupported hypothesis which is, however, held by 

 many that the sizes (weights) of the molecules of one and the same substance are very 

 different in different physical states. At present the weight of gaseous molecules is 

 determined from the freezing of solutions (see later), and therefore it must either be 

 admitted that solutions contain gaseous molecules or else that the weight of liquid 

 molecules is the same as that of gaseous molecules, which is far simpler and more 

 probable. 



From what has been said above, it will be clear that even in so very simple a case as 

 solution, it is impossible to calculate the heat emitted by chemical action alone, and that 

 the chemical process cannot be separated from the physical and mechanical. 



16 The cooling effect produced in the solution of solids (and also in the expansion of 

 gases and in evaporation) is applied to the production of low temperatures. Ammo- 

 nium nitrate is very often used for this purpose ; in dissolving in water it absorbs 77 

 units of heat per each part by weight. On evaporating the solution thus formed, the 

 solid salt is re-obtained. The application of the various freezing mixtures is based on 

 the same principle. Snow or broken ice frequently enters into the composition of these 

 mi. rin res, advantage being taken of its latent heat of fusion in order to obtain the 

 lowest possible temperature (without altering the pressure or employing heat, as in other 

 methods of obtaining a low temperature). For laboratory work recourse is most often 

 had to a mixture of three parts of snow and one part of common salt, which causes the 

 temperature to fall from to - 21 C. Potassium thiocyanate, KCNS, mixed with water 

 (f by weight of the salt) gives a still lower temperature. By mixing ten parts of crystal- 

 line calcium chloride, CaCl 2 ,6H 2 O, with seven parts of water, the temperature may even 

 fall from to - 55. 



27 The heat which is evolved in solution, or even iu the dilution of solutions, is also 

 sometimes made use of in practice. Thus caustic soda (NaHO), in dissolving or on the 

 addition of water to a strong solution of it, evolves so much heat that it can replace fuel. 

 In a steam boiler, which has been previously heated to the boiling point, another boiler 

 is placed containing caustic soda, and the exhaust steam is made to pass through the 

 latter ; the formation of steam then goes on for a somewhat long period of time without 

 any other heating. Norton makes use of this for smokeless street boilers. 



