ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 25 
chromate and a ‘metastable supersaturated solution’ of the same 
substance may (apart from the fact that we know hardly anything 
definite about the latter type of systems) seem rather subtle, further 
consideration shows that the former assumption immediately 
explains the specific effect of the gel. If electrolyte coagulation is 
the deciding factor, the protective effect of the gel must play an 
important part in promoting or inhibiting the formation of strata, 
and some such effect is easily traced in the comparative material 
collected principally by E. Hatschek. The consistent differences in 
the results obtained in gelatin, agar, and silicic acid gels point in this 
direction, as the protective effects of the three substances are widely 
different: taking Zsigmondy’s ‘gold figures’ as measuring this 
property, the protective effect of gelatin and agar is respectively 100 
and 2, while that of silicic acid is negligible. These facts, of course, 
form merely the starting-point for a theory of the Liesegang pheno- 
menon, and a very large amount of work would still be required to 
show its general validity. 
The lack of a general theory of the formation of the Liesegang 
stratifications does not, of course, preclude suggesting a similar origin 
of stratified structures in nature or attempts to reproduce such 
experimentally. Liesegang led the way by explaining the bands in 
agate in this fashion, 7.e, by diffusion of iron salts into the 
gelatinous silica from which the stone probably originates, in 
preference to the earlier assumption of alternate deposition. Many 
similar suggestions in regard to both geological and_ histological 
examples are thrown out in his book entitled ‘ Geological Diffusions’ 
and in his numerous papers. The importance of the phenomenon 
for botanical anatomy and histology has been insisted on chiefly by 
E. Kuester. Its possible bearing on the transformation of cartilage 
into bone has been pointed out by H. Bechhold, and experimental 
work on this subject should be very fruitful. E. Hatschek and 
A. Simon have shown that a whole series of apparently disconnected 
peculiarities of gold deposits in quartz can be explained by assuming 
the latter to have originally been gelatinous silicic acid, in which the 
reduction of the solutions of gold salts was brought about by one 
of many possible agents. 
In all these directions little more than a beginning has been 
made, and the expertmental reproduction and elucidation of natural 
periodic structures should for a long time to come be one of the 
most fruitful fields for applied colloidal chemistry. It seems 
probable that investigation will have to be extended to gels made 
anisotropic by stress, since it is evident that most of the gel-like 
constituents of organisms are in such a condition, at least during 
long periods. 
BIBLIOGRAPHY. 
WILHELM OstWALD, Allg. Chemie, 2nd ed., II. 778, 780. 
H. BECCHOLD, The formation of structures in jellies, Zeitschr. phys. Chem., 52, 
185 (1905). 
R. BE. LIESEGANG, The imitation of vital processes, Roux’s Arch. 7. Lntwickelungs- 
mechani, 33, 328 (1911). 
R. BE. LieseGanc, Reactions in gelatinous media, Zeitschr. f. anal. Chem., 50, 
82 (1910). 
