36 INTRODUCTION TO GENERAL PHYSIOLOGY 



rapidly, a point of some importance in connection with the inter- 

 pretation of certain physiological phenomena, such as that of 

 muscular contraction (E., p. 179). 



When electrolytes are added to colloidal solutions, if more is 

 added than necessary to neutralise the charges, the particles may 

 have conferred upon them a charge of the opposite sign to their 

 original one and be re-suspended. It is somewhat difficult to give 

 a satisfactory explanation of this fact. The probable reason is 

 that the excess ions are adsorbed, owing to their effect on the 

 mechanical surface tension. If an ion is adsorbed owing to an 

 effect of this kind, independent of the sign of its charge, the surface 

 must obtain a charge of the sign of that of the ion in question 

 (E., p. 179). _ 



The addition of a colloid of opposite electrical sign to another 

 colloid has the effect of precipitating both (E., p. 179). Excess of 

 either causes re-suspension, owing to the excess charge of one sign 

 or the other. 



The precipitate in this last case is evidently composed of both 

 colloids, although not in chemical combination. It is a representa- 

 tive of a large class of substances, sometimes called " adsorption 

 compounds." The components of these are present in no relation 

 to chemical combining proportions, but to certain physical properties, 

 which, it may be pointed out, although less simple to determine in 

 any particular case, follow laws as definite as the purely chemical 

 ones. Much confusion of thought would be avoided if the expression 

 "chemical combination," or even "combination," were strictly 

 confined to those cases where the chemical properties of the atoms 

 or molecules are changed where the internal structure and energy 

 of the atom is altered. The name "adsorption compound" is not 

 to be recommended ; a better name is "colloidal complex." Such 

 complexes may be formed between colloids and crystalloids, as 

 when charcoal takes up iodine, as well as between two or more 

 colloids (P., p. 64). There are other cases, such as those of mixed 

 crystals and that of the water of crystallisation, where physical, 

 rather than chemical forces appear to be concerned. 



From the general properties of contact surfaces, as outlined 

 above, we see that complexes between colloids and substances that 

 lower surface energy are very apt to occur. They often cause 

 difficulty in the separation and purification of the compounds 

 present in cells and secretions, as will be more obvious later. Many 

 errors in interpretation have been made on this account. 



Electrical Adsorption and Histological Staining 

 The dyes used for the purpose of making evident various con- 



