10 THE EQUILIBKIUM OF COLLOID AND 



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 true 

 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 com- 

 pared with the combination of hydrogen and oxygen to form 

 water. In the former case the glass remains unaltered, and by 

 heating or by drying agencies 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 



solution of free acids and caustic alkalies in water. Other examples are the union 

 of anaesthetics such as chloroform with proteins, where the chloroform or other 

 anaesthetic is in all cases a chemically saturated body, yet the proofs of union 

 with protein are indisputable, there being finally obtained with sufficient pressure 

 of chloroform actual precipitation, the precipitate containing chloroform in high 

 percentage. 



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 when the chemical sign of dye and substratum are opposite. 



