106 PRINCIPLES OF GENERAL PHYSIOLOGY 



greater, and, as we have seen in the case of the Bence-Jones' protein, the action 

 of salts as ions on particles becomes more marked. 



Silk is a protein and forms a convenient means of testing some of the relations of these 

 substances to electrolytes. In pure water, it has a slight negative charge, due, no doubt, 

 to its acidic function exceeding its basic one. As an electro-negative colloid, it is especially 

 sensitive to the action of cations, so far as concerns all properties depending on its charge. 

 I have recently tested its behaviour towards a colloidal acid, that of Congo-red, with which it, 

 as a potential base, is capable of forming a salt of the usual red colour, being dyed, in fact. 

 Now ooth the silk and the colloidal acid are negative, so that very little adsorption takes 

 place, unless we reverse the sign of the charge on the silk by the addition of cations. Calcium 

 sulphate, of very low concentration, was used for the purpose. The effect of the elrrtnilvtr 

 was the same as in the case of filter paper as described on page 58 above. The dye was adsorbed 

 by the silk, which was thus dyed blue, the colour of the free acid, like the adsorption com- 

 pound of thorium hydroxide with the same acid. On heating, chemical reaction occurred, 

 with the formation of a red salt of silk protein. The interest of this experiment is that it 

 shows the intervention of electric forces in addition to the purely chemical ones. Leucine, 

 suspended in its saturated solution, also forms a blue adsorption compound with the Congo-red 

 acid, which becomes a red salt on warming. 



The natural proteins are, as we have seen, comparatively insensitive to the 

 action of neutral salts. Certain of them, however, known as albumins and 

 globulins, are capable of a change, called " denaturation," by which they approxi- 

 mate to the suspensoid class, in so far as becoming more sensitive to the action of 

 salts, although their high viscosity and low surface tension shows them to be also 

 hydrophile. A familiar instance of "denaturation" is the effect of boiling water 

 on white of egg. What precisely happens, is as yet unknown, although the work 

 of Hardy (1899, i. p. 182) and of Chick and Martin (1912) has thrown much light 

 on the process. Hardy showed that in the coagulation of egg-white by heat there 

 are two distinct stages : (1) denaturation, by which the protein becomes precipitable 

 by salts, according to the same law of valency as the inorganic suspensoids, and (2) 

 the agglutination of the denaturated particles by electrolytes, if present. 



As we saw in the case of blood corpuscles acted on by cerium salt, if the 

 concentration of the Ce -<> ions be large, the sign of the charge is reversed on 

 all the corpuscles together, and redispersion takes place. Similarly, dispersion 

 of protein particles by salts can occur. When all particles are equally charged, 

 although of an opposite sign to their original one, mutual repulsion ensues, 

 while dispersion is also assisted by the lowering of surface tension which is 

 the result of the increased charge. Chick and Martin (1912, p. 293) call 

 attention to the relation of the facility with which weak acid or alkali causes 

 redispersion of the heat coagulum of a protein to the nature of the aggregated 

 precipitate. In a loosely agglutinated mass, each particle is sufficiently distinct 

 to carry its own charge, whereas when the particles are closely packed without 

 interspaces, the charge will be on the surface of the mass as a whole. A 

 small charge will readily produce breaking up in the former case, but can 

 only affect the most superficial particles in the latter. For further informa- 

 tion the reader is referred to the papers of the investigators named. 



Chick and Martin (1913) have also devoted a detailed investigation to the phenomena of 

 "salting out," which should be consulted. We may note that the effect of hydrogen ion 

 concentration shows that electrical charge plays a considerable part, as does also the effect 

 of the valency of the precipitating ion. 



From the preceding short account of the colloidal nature of proteins, it 

 will be obvious that the phenomena presented by them are of much complexity, 

 and are not yet altogether clear. Owing to their great variety in chemical 

 constitution and the corresponding variety in their behaviour, it becomes 

 almost a necessity to devote a special study to each one. There can be no 

 doubt that their manifold capabilities of change in state make them very 

 important in physiological processes. The effect of electrolytes, and especially 

 of H' and OH' ions, on this state may be emphasised. Adsorption of salts, 

 especially of those which are comparatively insoluble, is also to be remembered. 

 Since the degree of adsorption is proportional to the surface, it will be seen 

 how, by alterations of state of aggregation, electric charge, or surface tension, 



