ORGANIC MATTER. 17 



is no longer soluble in water; and it may bo suddenly thus 

 coagulated by a minute portion of an alkaline carbonate, as 

 well as by gelatine, alumina, and peroxide of iron. This last- 

 named substance, too — peroxide of iron — which is an ingre- 

 dient in the blood of mammals and composes the shells of 

 certain Protozoa, has a colloid condition. " Water containing 

 about one per cent, of hydrated peroxide of iron in solution, 

 has the dark red colour of venous blood." * * * « rjij^^ 

 red solution is coagulated in the cold by traces of sulphuric 

 acid, alkalies, alkaline carbonates, sulphates, and neutral salts 

 in general." * * * " rpj^^ coagulum is a deep red-coloured 

 jelly, resembling the clot of blood, but more transparent. 

 Indeed, the coagulum of this colloid is highly suggestive 

 of that of blood, from the feeble agencies which suffice to 

 effect the change in question, as well as from the appearance 

 of the product." The jelly thus formed soon becomes, like 

 the last, insoluble in water. Lime also, which is so important 

 a mineral element in living bodies, animal and vegetal, 

 enters into a compound belonging to this class. " The 

 well-known solution of lime in sugar forms a solid coagulum 

 when heated. It is probably, at a high temperature, entirely 

 colloidal." 



Generalizing some of the facts which he gives. Professor 

 Graham says : — " The equivalent of a colloid appears to be 

 always high, although the ratio between the elements of the 

 substance may be simple. Gummic acid, for instance, may 

 be represented by C^^ H^^ 0^^ ; but, judging from the small 

 proportions of lime and potash which suffice to neutralize this 

 acid, the true numbers of its formula must be several times 

 greater. It is difficult to avoid associating the inertness of 

 colloids with their high equivalents, particularly where the 

 high number appears to be attained by the repetition of a 

 small number. The inquiry suggests itself whether the 

 colloid molecule may not be constituted by the grouping 

 together of a number of smaller crystalloid molecules, and 



