22 REPORTS ON THE STATE OF SCIENCE.—1918. 
anticipate somewhat and to indicate at once the reasons for 
attaching importance to what at first sight may appear only a 
curious laboratory experiment. A large number of stratified 
structures occur both in organic and inorganic nature, the explana- 
tion of which has so far generally involved one or more of the 
following assumptions: a periodic supply of either or both 
components of a reaction ; a periodic supply of a catalyst, activating 
or inhibiting agent; or, finally, a periodic external agency, such 
as, é.g., variations of mean temperature. The Liesegang phenomenon, 
on the other hand, is proof that periodic structures may appear 
where all factors which can possibly exert any influence are 
constant. Where it is applicable as the basis of explanation it 
therefore leads to that economy of assumptions which is the 
desideratum of every hypothesis. 
The conditions of the experiment have been varied by Liesegang 
and subsequent observers. The gel containing one component may 
be allowed to set in a test tube and the solution of the other poured 
on top. Or the gel may fill the bend of a U-tube and two solutions 
may diffuse into it in opposite directions from the limbs. Finally, 
bodies of gel containing one component may be submerged in 
solutions of the other. The results are substantially the same in all 
cases, although a given reaction may not produce them in an equally 
marked manner with every arrangement. 
The phenomenon has also been studied in gels other than gelatin 
such as agar and silicic acid, and even in porous media of very much 
coarser structure than gels. The nature of the gel, and to a much 
slighter degree its concentration, are now fully proved to have a 
marked specific influence on the result with any given reaction. 
Thus, the reaction between silver nitrate and potassium bichromate 
leads to stratifications in gelatin, but not in agar, while on the other 
hand the reaction between lead nitrate and potassium chromate 
produces them in agar, but not in gelatin, while neither of the two 
leads to a stratified deposit if it takes place in silicic acid gel. A 
great number of reactions have also been investigated, some in all 
three gels and over wide ranges of concentrations, and well marked 
stratifications have been obtained particularly with the following 
precipitates ; lead iodide and lead chromate in agar, lead carbonate 
in agar and in silicic acid, phosphates of the heavy metals in various 
gels, sulphides in various gels and in sand, &c. 
Certain reactions, within wide limits of concentration, do not 
produce stratifications in certain gels. It has, however, been shown 
by Liesegang and by Hatschek—and the point is again of importance 
for the explanation of natural structures—that they can nevertheless 
be obtained as pseudo-morphoses after an intermediate product of 
reaction. Thus Liesegang places a drop of silver nitrate solution on 
a gelatin film containing sodium chloride; the resulting siiver 
chloride forms only a continuous band. If a small fragment of 
potassium bichromate is placed some distance from the edge of the 
drop of silver nitrate, the usual silver chromate strata are formed 
round it when the silver salt has diffused so far, but are promptly 
transformed into silver chloride. The final result is stratifications of 
silver chloride round the site of the bichromate, although such 
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