110 Mr. W. B. Hardy. On tJie Conditions which 



"A Preliminary Investigation of the Conditions which determine 

 the Stability of Irreversible Hydrosols." By W. B. Hardy, 

 Fellow of Gonville and Cains College, Cambridge. Com- 

 municated by F. H. ^N'eville, F.E.S. Eeceived January 12, — 

 Bead January 25, 1900. 



It has long been held that a large number of colloidal solutions are 

 related to or identical with suspensions of solid matter in a fluid in 

 which the particles of solid are so small as to settle at an infinitely 

 slow rate. Such solutions are the colloidal solutions of metals and of 

 sulphides such as those of antimony, arsenic, and cadmium. Such 

 solutions belong to the class of irreversible colloidal mixtures. A rise 

 of temperature assists the process of coagulation or precipitation;* 

 but neither a further rise nor a fall of .temperature will cause the 

 reformation of the hydrosol. On this ground they may provisionally 

 be classed with such colloidal solutions as those of silica, ferric hydrate, 

 alumina, &c., and with the modification of the albumen of white of 

 egg which is produced by heating an aqueous solution to the boiling 

 point. I also add to the class, for reasons to be developed in the 

 following pages, the suspension of mastic in water which is produced 

 by adding a dilute alcoholic solution of the gum to water. 



Looked at from the point of view of the phase rule, the equiHbrium 

 in these hydrosols, if they really consist of minute solid particles dis- 

 persed in a fluid, is not necessarily between the solid particle and water, 

 but between the solid particle and a solution of the particular solid in 

 water. The hydrosol of gum mastic gives off" a vapour of the gum of 

 a density sufficient to afi'ect the olfactory organs, and, therefore, the 

 water must contain a definite quantity in solution. Similarly, as it is 

 probable that no substance is completely insoluble, we may assume that 

 in all the examples a portion of the solid is in true solution in the fluid. 

 As the solid which is not in true solution is dispersed in particles 

 whose diameter is, as a rule, very much smaller than the mean wave 

 length of light, it follows that the surface of contact between soUd and 

 fluid is very great for unit mass of the former. The opportunity for 

 evaporation and condensation of the solid matter of the particles 

 afforded by the immense surface of contact is so very great that, 

 although only an immeasurably minute quantity of the solid may be 

 in true solution at any one time, this quantity, minute though it be, 

 is probably an important factor in determining the equilibrium between 

 solid and fluid. 



* Elsewhere (' Journal of Physiology,' vol. 24, 1899, p, 172) I have shown that 

 precipitation and coagulation are not discontinuous processes. Coagulation gives 

 way to precipitation when the concentration of the solid phase falls below a 

 certain amount. 



