CHAPTER IV 

 THE COLLOIDAL STATE 



IF we take a piece of metallic gold, immerse it in water, and divide it up into 

 smaller and smaller parts, it is obvious that in the end, supposing that our 

 powers of manipulation were adequate, we should arrive at the molecular condition. 

 But, before this state is reached, we should have passed through a state in which the 

 particles were so fine as to be invisible, as such, by ordinary means of illumina- 

 tion ; and they would remain in permanent suspension, so as to simulate very 

 closely a true solution, in which the substance dissolved is in the molecular, or 

 even ionic state. In the course of this process of division, the larger fragments 

 of gold of the early stages sink at once, after being stirred up, but as smaller 

 and smaller particles are formed, the time taken to fall becomes longer and 

 longer, until, when less than a certain size, they do not appear to sink at all. 

 They are now in what is called the " colloidal state." Their dimensions at this 

 stage are enormously greater than those of molecules of gold, but it is clear that 

 we can draw no definite lines of demarcation between the visible solid lump, from 

 which we started, the colloidal state and the final molecular state. 



We cannot, of course, actually perform the operation in the manner described. 

 In an indirect way, however, it was done by Faraday (1858, p. 159), who found 

 that, by acting on solutions of gold salts by reducing agents, beautiful red or 

 purple solutions were obtained. He also showed that these solutions, although 

 permanent, were, in reality, suspensions of minute particles of metallic gold 

 (p. 160 of the above paper). It is interesting to note that one of Faraday's gold 

 preparations is still preserved in the Royal Institution. 



Since these gold solutions have served as the foundation for much subsequent work, the 

 method of preparing them is worth description. The ruby-red solution is made thus, in the 

 words of Faraday himself (1858, p. 159) : " If a pint or two of the weak solution of gold before 

 described" (i.e., about 2 grains of gold chloride in two or three pints of water) "be put into 

 a very clean glass bottle, a drop of the solution of phosphorus in sulphide of carbon added, and 

 the whole well shaken together, it immediately changes in appearance, becomes red, and being 

 left for six to twelve hours, forms the ruby fluid required ; too much sulphide and phosphorus 

 should not be added, for the reduced gold then tends to clot about the portions which sink to 

 the bottom." Zsigmondy (1905, pp. 97-101) finds that the method is improved by the addition 

 of potassium carbonate, in order to neutralise the free acid produced in the reaction ; he also 

 gives other useful hints, pointing out the importance of pure water and Jena glass vessels ; 

 the absence of colloidal matter from the water used appears to be especially necessary if 

 uniform results are to be obtained. The necessity of cleanliness was well known to Faraday 

 himself, although at that time the properties of colloids were unknown. 



A beautiful deep blue solution of gold can be made by reduction with hydrazine hydrate 

 (Gutbier, quoted by Svedberg, 1909, i. p. 10). Gold chloride O'l per cent, is neutralised by 

 sodium carbonate and very dilute hydrazine hydrate (one part in 4,000 of water) added drop 

 by drop, carefully avoiding excess. 



How do we know that we have to do with suspended solid particles in these 

 preparations ? They are quite transparent to light of ordinary intensity, although 

 this does not apply to all colloidal solutions ; where the particles are larger the 

 solutions are turbid, and their appearance suggests their nature. Even the most 

 transparent gold preparations, however, were found by Faraday to show turbidity 

 in the track of a powerful beam of light. This observation forms the foundation 

 of the ultra-microscope, to be described later. It is frequently called the " Tyndall- 

 phenomenon," but its discovery was really made by Faraday (1858, p. 160). 

 Tyndall pointed out that the light reflected, or rather diffracted, from the path 



