176 PROTOPLASM 



Adsorption Bonds. — We have indicated that adsorption may- 

 be a molecule for molecule reaction and thus involve primary 

 valence, or that it may be a surface phenomenon which is non- 

 stoichiometric and involve a less firm union. That adsorption 

 has to do with bonds differing in degree of firmness is evident 

 from the many kinds of surface reactions that are included under it. 



When smoke clings to a glass surface on being blown across it, 

 or when glass is wet by water, the bond is a loose one, and no 

 question of a true chemical union arises. The inert gases argon 

 and radon are readily adsorbed by charcoal; a union involving 

 primary valence is hardly possible here, for these gases rarely, 

 if ever, form compounds. Between such examples of loose 

 adsorption bonds, on the one hand, and others involving true 

 chemical (primary valence) bonds, there are many intermediate 

 types of union which the chemist has difficulty in distinguishing. 

 Thus, some dyes are held very firmly, and others loosely. Congo 

 red is taken out from its aqueous solution by filter paper. The 

 ease with which the dye is then set free again from the paper by 

 the addition of a little acid or alcohol is evidence of a loose union 

 between paper and dye. Water is held by substances in varying 

 degrees of firmness. Some water can be driven out of wet char- 

 coal by the application of slight heat, but all the water is removed 

 with difficulty. The force with which charcoal holds water by 

 adsorption is very great, reaching many thousands of atmospheric 

 pressures. Under such pressure, water is compressed to 75 per 

 cent of its original volume, and it is one of the least compressible 

 substances known. 



This leads us to the problem of the nature of water of crystal- 

 lization which was formerly regarded as adsorbed water and is 

 now looked upon as involving a true primary-valence bond 

 (page 422). Bragg believes the water molecules of hydrated 

 salt to possess, at least in a large number of cases, no special 

 clear identity; that is to say, they are not simply attached to 

 other molecules in the solid, but rather is the crystal with the 

 water a new compound altogether, with a new arrangement of the 

 atoms. It does not follow that because the water is readily 

 separable from the salt by heat and is given off as a molecule, 

 it is existent as such in the crystal. J. A. Wilson expresses the 

 view that adsorbed ions in general become an integral part of 

 the surface. 



