IN PROTEIN SOLUTIONS n 



molecules in a stoichiometric proportion. In some instances 

 this hydra tion is sharply denned (as for sulphuric acid and ferric 

 chloride) , but in other cases the hydra tion is variable and shows 

 a continuous alteration with dilution and with change of tem- 

 perature. We can describe such phenomena with more accu- 

 racy as addition of water (or formation of envelopes) by the 

 particles, rather than as combinations in the more restricted 

 sense of the word. 



Such an addition of the medium occurs on the particles of 

 many colloids, and is described as hydration in the wider sense, 

 and as imbibition when the adding of water occurs together with 

 penetration of water into a large aggregate. The indications 

 of hydration of a disperse phase are a disproportionately great 

 viscosity of the solution, the decreased activity of the move- 

 ment of the particles as displayed in diffusion or in an electric 

 field, and an alteration in volume or density in the direction of 

 compression of the medium. The latter is made obvious by a 

 comparison of the volume with that calculated from the volume 

 of the particles plus that of the medium, or by the increase in 

 the effective concentration of a substance in solution, as also 

 by anomalous behaviour when the solvent is removed by 

 freezing or evaporation, owing to a stronger partial combination. 

 In some cases there is an evolution of heat on solution in the 

 medium. 



We shall see that hydration of particles of albumin solutions 

 has been demonstrated, and, consequently, dissolved proteins 

 are classed among hydrated (solvated) colloids. These are 

 called hydrophile by J. Perrin, and in general lyophile by H. 

 Freundlich. Colloids with non-solvated particles are named 

 hydrophobe (lyophobe), and although the definition of sus- 

 pensoids and emulsoids is on a different basis, in practice the 

 two classifications approximately coincide. The embarrassing 

 use of the double classification according to the state of aggrega- 

 tion or solvation of the disperse phase is strongly to be depre- 

 cated. With water as continuous phase the obvious notation 

 hydro- and anhydro-colloicfc^is useful. Hydrated colloids are 

 usually more stable in solutioh\than anhydro-colloids. 



The role of solvation in the stability of various colloid 



