THE PROPERTIES OF COLLOIDS 155 



sets with a large amount of a fluid, such as water, to form a jelly. 

 This solid form is known as a gel. The most familiar instance is the 

 jelly which is obtained on dissolving a little gelatin in hot water and 

 allowing the mixture to cool. Such a jelly is known as a hydrogel. 

 In many of these gels the water can be replaced by other fluids, such 

 as alcohol, without any alteration in the appearance of the solid, 

 which is then known as an alcogel. Another example of an alcogel 

 is the jelly which can be made by dissolving soap in warm alcohol and 

 allowing the mixture to cool. 



A number of these colloidal substances can be shown on purely 

 chemical grounds to consist of monstrous molecules. Thus the mole- 

 cular weight of haemoglobin is at least 16,000, and one must 

 ascribe similar high molecular weights to such substances as egg- 

 albumin and globulin. Still greater must be the molecular size of such 

 substances as the cell proteins, which may be made up of more than 

 one type of protein built up with various nucleins, with lecithin 

 and cholesterin, to form a gigantic complex, to which it would probably 

 not be an exaggeration to ascribe a molecular weight of over 

 100,000. This chemical complexity is not, however, a necessary 

 condition of the colloidal state, as is shown by the existence of 

 colloidal silica, of colloidal ferric hydrate and alumina, and even of 

 colloidal metals. On neutralising a weak solution of sodium silicate 

 or water-glass by means of HC1, we obtain a solution which contains 

 sodium chloride and silicic acid. On dialysing this mixture for some 

 days against distilled water, the whole of the NaCl passes out, and a 

 solution of silicic acid or colloidal silica is left in the dialyser. This 

 solution can be concentrated over sulphuric acid. When concentrated 

 to a syrupy consistence it becomes extremely unstable. The addition 

 of a minute trace of sodium chloride or other electrolyte to the solution 

 causes it to set at once to a solid jelly (gelatinous silica), the change 

 being accompanied by an appreciable rise of temperature. The change 

 is irreversible, in that it is not possible to bring the silicic acid into 

 solution again by removal of the electrolyte by means of dialysis. 

 If, however, it be allowed to stand with weak alkali for some time, it 

 gradually passes into solution. Analogous methods are employed for 

 the preparation of colloidal Fe 2 3 and A1 2 3 . 



Of special interest are the colloidal solutions of the metals. Faraday 

 long ago pointed out that, on treating a weak solution of gold chloride 

 with phosphorus, it underwent reduction with the formation of metallic 

 gold. The gold, however, was not precipitated, but remained in sus- 

 pension or pseudo- solution, giving a deep red * or a blue liquid, accord- 

 ing to the conditions under which the reaction was effected. This 



* Ruby glass is a colloidal ' solid ' solution of gold in a mixture of 

 silicates. 



