CRYSTALLOID IN LIVING CELLS 309 



When this feeble attachment has once occurred it may become 

 altered in different ways. First, if the growing protein aggregation 

 has not yet reached its full size, there may be a swing into tru 

 chemical union with the elimination of a water molecule. 



If chemical union does not take place, a diminution of pressure 

 of one of the constituents may occur, causing dissociation or dis- 

 ruption, or conversely an increased pressure may lead to firmer 

 attachment, increasing association at the expense of dissociation, 

 and favouring chemical combination. 



The form of union described above as " feeble union " or union 

 by " residual affinities " is usually spoken of as adsorption, although 

 often a number of processes which may be dissimilar in nature are 

 placed together under this term. 



Thus, the invisible layer of moisture that collects on the surface 

 of glass ; the adhesion of gases on the inner surface of glass vessels 

 which are in process of exhaustion; the moisture taken up by 

 textile fabrics; the gases occluded by certain metals such as 

 platinum, palladium, and iron; the concentration of dyes upon 

 the surfaces of fibres and tissues being dyed ; the union or adhesion 

 between inorganic salts or other crystalloids and colloids of various 

 kinds these and a great many other phenomena are variously 

 given as examples of adsorption, and it is maintained that this 

 process is physical in character and essentially different from 

 chemical combination. 



If extreme cases of adsorption on the one hand and of chemical 

 combination on the other be taken for comparison, it becomes 

 obvious at once that there exist great differences between them 

 such, for example, as the hygroscopic absorption of water by a 

 glass surface compared with the combination of hydrogen and 

 oxygen to form water. In the former case the glass remains 

 unaltered, and by heating the water molecules can be removed 

 unaltered from the surface; while in the latter case the water is 

 quite different in all its physical and chemical properties from 

 either of the two gases which have united with great evolution of 

 energy to form it. 



But if the comparison be made between chemical reactions 



Other examples are the dyeing of tissues and fabrics by dyes, where a 

 saturated dye combines with a saturated colloid substratum. In all such 

 cases the best effects are obtained where the chemical sign of dye and sub- 

 stratum are opposite. 



