236 CHEMICAL DISCOVERY AND INVENTION 



diffusate a liquid is ultimately left within the bag which contains 

 the whole of the silicic acid with mere traces of the chlorides. 

 Such a liquid is clear as water, it may be concentrated by evapor- 

 ation, provided no solid crust is allowed to be formed at the sides 

 of the dish, till it flows like syrup, and ultimately it will dry up 

 into a glass-like mass. The dissolved substance exhibits ex- 

 tremely small osmotic pressure and neither reduces the freezing 

 point of water nor raises the boiling point. The liquid is, how- 

 ever, very sensitive to the presence of small quantities of salts, 

 and acids and speedily sets into a gelatinous mass when any 

 electrolyte is added. The pseudo-solution of silicic acid thus 

 obtained, called by Graham a " colloidal solution," is now 

 simply called a " sol," while the gelatinous mass resulting from 

 slow change or the addition of saline substances is called a " gel." 



By similar processes which involved the preparation of the 

 colloid form and its purification from the attendant crystalloids 

 by the method of dialysis through a membrane, Graham suc- 

 ceeded in preparing sols of the hydroxides of iron, chromium, 

 and aluminium, tungstic acid and other substances. Some of 

 these preparations may be made by other methods. Thus alu- 

 minium hydroxide (hydrated alumina) may be obtained in 

 the form of a sol by making first a solution of aluminium acetate 

 and then boiling the solution in an open dish till the acetic acid 

 produced has been completely driven off. Water must be added 

 from time to time to replace that which is lost by evaporation. 

 The residual hydrous alumina is the basis of the aluminous 

 mordant used in calico printing. 



A very interesting sol is produced by boiling white arsenic 

 (arsenious oxide As 4 6 ) in water and adding hydrogen sulphide 

 to the solution. A bright yellow colour is immediately developed 

 but no precipitate is formed, although arsenious sulphide, As 4 S 6 , is 

 insoluble in water. If now a few drops of hydrochloric acid are 

 added to the clear liquid a yellow precipitate of the sulphide is 

 immediately formed, and after it has had time to subside the 

 liquid is seen to be colourless. 



The most remarkable sols, however, are those which are pro- 

 ducible from certain metals, especially those which have been so 

 long called the " noble metals," namely gold, silver, and platinum. 



Faraday discovered in the earlier half of the nineteenth cen- 

 tury that a solution of gold chloride on which is floated a solution 

 of phosphorus in ether will yield a liquid of various colours, blue, 



