determine the Stability of Irreversible Hydrosols. Ill 



It is necessary to keep such considerations as these in mind in view 

 of the readiness with which these mixtures have been regarded as 

 simple suspensions'^ in which the only relation between solid and fluid 

 is a mechanical one. These hydrosols are, as a matter of fact, singularly 

 stable when pure. They can, for instance, be concentrated by boiling 

 to a remarkable extent, and their stability depends upon complex 

 relations between fluid and solid, which gives the former, so to speak, 

 a definite hold over the latter. 



Mode of Preparation of the Different Solutions. — The hydrosol of gold 

 was prepared by adding a couple of drops of a solution of phosphorus 

 in ether to about a litre of a very dilute solution of gold chloride. 

 The fine ruby-coloured fluid which was formed was dialysed against 

 distilled waterf for fourteen days, and then concentrated by boiling. 

 The hydrosol of silicic acid was prepared by acting on soluble glass 

 with excess of hydrochloric acid, and dialysing the product. A 

 hydrosol of ferric hydrate was prepared by prolonged dialysis of the 

 solution in ferric chloride. 



The hydrosol of gum mastic was prepared by adding a very dilute 

 solution of the gum in alcohol to distilled water. It was dialysed for 

 fourteen days against distilled water. The hydrosol of heat-modified 

 egg-white was prepared by dissolving white of egg in nine times its 

 volume of distilled water, filtering and boiling. The result should be a 

 brilliant fluid which scatters blue light. Surface action, however, 

 plays an extraordinary part. If the solution is boiled in a test-tube 

 a milky fluid is formed and a film of proteid is left on the glass ; a 

 second quantity boiled in the same test-tube comes out less milky, 

 until, when the proteid film is sufficiently thick to eliminate all action 

 by the glass, the solution after boiling contains the proteid dispersed 

 as particles so small that they scatter pure blue light. After pre- 

 paration the hydrosol was dialysed against distilled water for some 

 days. 



Behaviour of the Hydrosols in an Electric Field. — It has long been known 

 that the particles in these colloidal solutions move in an electric field. 

 Zgismondyl found that the gold in colloidal solutions moves against 

 the current. Picton and Linder§ established the important fact that 

 the direction of movement of the particles, as compared with the 

 direction of the current, depends upon their chemical nature. I have 

 shown that the heat-modified proteid is remarkable in that its direction 

 of movement is determined by the reaction acid, or alkaline of the fluid 



* Cf., for instance, Stoeckl and Yanino, * Zeits, f . plijs. Chem.,' vol. 30, 1899, 

 p. 98; also Ostwald, ' Lelirbucli.' 



t In working with these colloidal solutions it is very necessary to use distilled 

 water freed from dissolved carbonic acid. 



t * Lieb. Ann.,' vol. 301, p. 29. 



§ ' Journal of Cliem. Soc.,' vol. 70, 1897, p. 568. 



L 2 



