ORGANIC MATTER. 17 



and sundry of the oxy-hydro-carbons found along with them ; 

 but, significantly enough, it includes several of those sub- 

 stances classed as inorganic, which enter into organized 

 structures. Thus silica, which is a component of many 

 plants, and constitutes the spicules of sponges as well as the 

 shells of many foraminifera and infusoria, has -a colloid, as 

 well as a crystalloid, condition. A solution of hydrated silicic 

 acid, passes in the course of a few days into a solid jelly that 

 is no longer soluble in water ; and it may be 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." * * * " The 

 red solution is coagulated in the cold by traces of sulphuric 

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

 in general." * * * " The 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" O' ; but, judging from the small 

 proportions of lime and potash which suffice to neutralize this 



