96 DISPERSE SYSTEMS 



tion of the proteins, and is determined only by their concentration 

 and charge. Such stains as Van Gieson's depend on this reaction. 



Action of Radiant Energy. (See also Chap, XIII.) 



The intimate connection between coagulation and the charge 

 carried by the particles is shown by the action of the ^ rays of 

 radium. As these rays are negative charges of electricity, they 

 should stabilise negative colloids by increasing their charge, and 

 precipitate positive colloids by neutralising their charge. Hardy 

 found that positively charged acid -globulin was reduced to a 

 state of jelly in three minutes, while the particles of negatively 

 charged alkaline-globulin were rendered more mobile by exposure 

 to 13 radiation. Schanz found that ultra-violet light by its power 

 of ionising water could decrease the solubility of albumin so that it 

 was precipitated along with the globulins. He attributed the 

 production of sclerosis and lack of elasticity of the lens of the eye 

 to light of short wave-length acting in this way on the mixture of 

 albumin and globulin composing it. 



Heating and Cooling, which alter viscosity directly and also 

 indirectly by altering the amount of water distributed between 

 the two phases, also cause coagulation. Heating certain sols- 

 changes them into the more rigid gels. Various native proteins, 

 for instance, those of egg white, serum, muscle, coagulate to a 

 gel on heating to a temperature specific for each protein. This 

 process is irreversible and takes place in the presence of electro- 

 lytes. On the other hand, gelatin forms a sol on heating and a 

 gel on cooling — a reversible reaction which is profoundly modified 

 by the presence of electrolytes. 



3. PROPERTIES DEPENDING ON SIZE, CHARGE 

 AND STRUCTURE 



Adsorption. Adsorption to a surface was considered in Chap. VI. 

 Colloids are characterised by their large specific surface, by the 

 development of surface charges due to this surface, or, in some, to 

 their amphoteric nature, and in the case of gels, by an internal 

 structure offering a surface to the dispersion fluid. If the colloidal 

 particles are free to move, i.e., if the colloid is in sol form, adsorption 

 may take place in either of two ways. The colloid may be adsorbed 

 to a surface or matter may be adsorbed to the colloid. Dyeing 

 with colloidal dyes offers an example of the first kind, while the 

 fact that natural colloids are always impure on account of adsorbed 

 mineral matter indicates the truth of the latter statement. The 

 gels with their internal structure present peculiar surface properties. 

 They have a special propensity for adsorbing their dispersion 



