(>.\ \VATKK AM) ITS m.MI>nrNI>S 59 



In this case desiccation takes place ; because sulphuric acid, for instance, 

 iirst dries the air in the bell by absorbing its moisture, the substance 

 to he dried then parts with its moisture to the dry air, from which it is 

 again absorbed by the sulphuric acid, Arc. Desiccation proceeds still 

 better under the receiver of an air pump, for then the aqueous vapour 

 is formed more quickly than in a bell full of air. 



From what has been said above, it is evident that the transference 

 of moisture to gases and the absorption of hygroscopic moisture present 

 un-at resemblance to, but still are not, chemical combinations with 

 water. Water, when combined as hygroscopic water, does not lose 

 its properties and does not form new substances. 15 



The attraction of water for substances which dissolve in it is of a 

 different character. In the solution of substances in water there pro- 

 ceeds a peculiar kind of indefinite combination ; there is formed a new 

 homogeneous substance from the two substances taken. But here also 

 the bond connecting the substances is very unstable. Water contain- 

 ing different substances in solution boils at a temperature near to its 

 usual boiling point, and acquires properties which are closely allied to 

 the properties of water itself and of the substances dissolved in it. 

 Thus, from the solution of substances which are lighter than water 

 itself, there are obtained solutions of a less density than water as, for 

 example, in the solution of alcohol in water ; whilst a heavier sub- 

 stance in dissolving in water gives it a higher specific gravity. Thus 

 salt water is heavier than fresh. 16 



We will consider aqueous solutions somewhat fully, because, among 

 other reasons, solutions are constantly being formed on the earth and 

 in the waters of the earth, in plants and in animals, in chemical prac- 

 tice and in the arts, and these solutions play an important part in 

 the chemical transformations which are everywhere taking place, not 

 only because water is everywhere met with, but chiefly because a sub- 

 stance in solution presents the most propitious conditions for the process 

 of chemical changes, which require a mobility of parts and an intimate 



1 ' Chapuy, however, determined that in wetting 1 gram of charcoal with water 7 units 

 of lieat are evolved, and on pouring carbon bisulphide over 1 gram of charcoal as much 

 as -Jl units of heat are evolved. Alumina (1 gram), wlien moistened with water, evolves 

 lories. This indicates that even in respect to evolution of heat moistening already 

 presents a transition towards exothermal combinations (those evolving heat in their 

 formation), like solutions. 



"' Strong acetic acid (CoH 4 O.j, whose specific gravity at 15 is T055, does not become 

 lighter on the addition of water (a lighter substance, sp. gr. = 0'99!)), but heavier, so that 

 a solution of so parts of acetic acid and 20 parts of water has a specific gravity of 1'074, 

 and even a solution of equal parts of acetic acid and water (50 p.c.) has a sp. gr. of T065, 

 which is still greater than that of acetic acid itself. This shows the high degree of con- 

 traction which takes place on solution. In fact, solutions and, in general, liquids on 

 mixing with water, decrease in volume. 



