ii6 PHYSICAL SCIENCE 



and arsenic are good examples. If a solution of 

 arsenious acid be allowed to flow into water kept 

 saturated with sulphuretted hydrogen by means 

 of a current of that gas, a colloidal hydrosulphide 

 is formed. Many hydrates, too, are colloids, 

 ferric hydrate, for instance, which can readily be 

 prepared from the corresponding salts of iron. 

 By treating dilute solutions of gold chloride with 

 reducing agents, such as a few drops of a solution 

 of phosphorus in ether, the gold is set free in 

 the colloidal condition, forming a ruby-coloured 

 solution. Silver, bismuth, and mercury can also 

 be obtained in colloidal solution. 



Crystalloids diffuse much more rapidly through 

 water and other solvents than do colloids. If 

 a mixture of crystalloids and colloids be placed 

 in a drum covered with a colloidal membrane, 

 such as bladder or parchment, complete separa- 

 tion can be effected, for the dissolved colloids 

 seem quite incapable of passing through such 

 membranes. This process probably plays a great 

 part in animal and vegetable physiology. 



Solutions of colloids in crystalloid solvents, 

 such as water or alcohol, seem to be divisible into 

 two classes. Both classes appear to mix with 

 warm water in all proportions, and the mass will 

 solidify under certain conditions to form a solid 

 which may be called a gel. One class, represented 

 by gelatine and agar jelly, will, when solidified, re- 

 dissolve on warming or dilution, while the other 

 class, containing such substances as hydrated 

 silica, albumen, and metallic hydro-sulphides, will, 

 under the influence of heat or on the addition of 

 electrolytes, form gels which cannot be redissolved. 

 The solidification of members of the first class into 



