SOLUTIONS 171 



nitrate or chloride of calcium, containing also a certain pro- 

 portion of water. The case of sugar in water differs from that of 

 marble in acid, therefore, in the fundamental fact that the 

 sugar can be recovered unchanged in properties, while the 

 marble cannot be recovered from the liquid because a chemical 

 change has taken place, and part of its components has been lost. 



Why does the sugar dissolve in water while the marble does 

 not dissolve except on condition of undergoing chemical change ? 

 These are questions to which the physicist and chemist can give 

 as yet only partial and imperfect answers. We may try to 

 follow in imagination the change in the sugar. First we must 

 recall the fact that the molecules of gases are free from each 

 other and every one, according to the kinetic theory, moves 

 about rapidly and independently of the rest, only knocking up 

 against them and continually altering the direction of its course. 

 In liquids we must believe, from the phenomena of diffusion, 

 that something of the same kind is continually going on with 

 this important difference, that there are relatively few separate 

 and independent single molecules. A large proportion of the 

 molecules move together in clusters or companies, which are 

 larger at low temperatures and smaller if the temperature is 

 raised. Thus the molecule of water in the state of gas, that is 

 superheated steam, consists of two atoms of hydrogen and one 

 atom of oxygen, or expressed in symbols H 2 0. In the liquid 

 state at the common temperature of the air these join together 

 or at any rate move together in parties of two or more molecules, 

 such as (H 2 0) 2 , (H 2 0) 3 , etc. 



A minute quantity of the compound is also probably in a 

 state of dissociation, being resolved into ions H and HO. This 

 will be explained later and does not concern for the moment the 

 consideration of the question relating to sugar. We must 

 suppose then that a crystal of sugar immersed in water is exposed 

 to a shower of blows from the moving molecules of the water 

 which are sufficiently strong to detach separate molecules of the 

 sugar from the surface and cause them to move about in the 

 liquid in the same manner as molecules of the solvent itself. 

 They are thus made to behave as they would do if converted 

 into gas by the application oi heat. Sugar cannot be gasified 

 in this way because its atoms separate from each other and form 

 new combinations, that is chemical decomposition takes place, 

 before the necessary temperature is reached. 



