:ME. T. GEAHA3I OX LIQIHD DIFFUSION APPLIED TO ANALYSIS. 
221 
for instance, that salt is first rendered basic by the addition of peroxide of iron. The 
comparatively stable perchloride of iron is transformed, by such treatment, into a feebly- 
constituted colloidal hydrochlorate. The latter compound breaks up under the purely 
physical agency of difiusion, and divides on the dialyser into colloidal peroxide of iron 
and free hydrochloric acid. The super-induction of the colloidal condition may possibly 
form a stage in many analogous organic decompositions. 
A tendency to spontaneous change, which is observed occasionally in crystalloids, 
appears to be general in the other class. The fiuid colloid becomes pectous and inso- 
luble by contact ■uith certain other substances, without combining with these substances, 
and often under the infiuence of time alone. The pectizing substance appears to hasten 
merely an impending change. Even while fluid a colloid may alter sensibly, from colour- 
less becoming opalescent ; and while pectous the degree of hydration may become re- 
duced from internal change. The gradual progress of alteration in the colloid effected 
by the agency of time, is an investigation yet to be entered upon. 
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 but judging from the small proportions of Hme 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 smaller 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 whether the basis of colloidality may not really be this composite 
character of the molecule. 
With silicic acid, which can exist in combination both as a crystalloid and colloid, we 
have two series of compounds, silicates and cosilicates, the acid of the latter appearing 
to have an equivalent much gi’eater (thirty-six times greater in one salt) than the acid 
of the former. The apparently small proportion of acid in a variety of metallic salts, 
such as certain red salts of iron, is accounted for by the high colloidal equivalent of 
their bases. The effect of such an insoluble colloid as prussian blue in carrying down 
small proportions of the precipitating salts, may admit of a similar explanation. 
Gelatine appears to hold an important place as a colloidal base. This base unites 
with colloidal acids, giring a class of stable compounds, of which tanno-gelatine only 
appears to be hitherto known. Gelatine is precipitated entirely by a solution of meta- 
phosphoric acid added drop by drop, 100 parts of gelatine uniting Avith 3’6 parts of 
the acid. The compound formed is a semitransparent, soft, elastic, and stringy solid 
mass, presenting a startling resemblance to animal fibrin. It will be an interesting 
inquiry whether metaphosphoric acid is a colloid, and enters into the compound described 
in that character, or is a crystalloid, as the small proportion and low equivalent of 
the acid would suggest. Gelatine is also precipitated by carbolic acid. 
The hardness of the crystalloid, Avith its crystalline planes and angles, is replaced in 
