140 PHYSIOLOGY 



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. An 

 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 molecular weight 

 of hemoglobin 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 O 3 and A1 2 O 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 was not precipitated, but remained in suspension or pseudo- 

 solution, giving a deep red * or a blue liquid, according to the con- 

 ditions under which the reaction was effected. This solution was homo- 

 geneous in that it could be filtered without change, and could be kept for 

 months without deposition of the gold. The latter was, however, thrown 

 down on addition of mere traces of impurity, though greater stability could 

 be conferred on the solution by adding to it a little ' jelly,' i.e. a weak solution 

 of gelatin. In 1899 Bredig showed how similar hydrosols might be prepared 

 from a number of different metals, viz. by the passage of a small arc or 

 electric sparks between metallic terminals submerged in distilled water. 

 If, for example, the terminals be of platinum, the passage of the current 

 is seen to be accompanied by the giving off of brown clouds, which spread 

 into the surrounding fluid. These clouds consist of particles of platinum 

 of all sizes. The larger settle at the bottom of the vessel, the smaller 

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



