370 REPORTS ON THE STATE OF SCIENCE.—1912. 
only resulted in stained crystals in four instances.1°° The nitrates 
of the barium group may be stained with methylene blue, these being 
perhaps the most successful instances of the kind.'*!' Potassium iodide 
crystals absorb iodine vapour, and the salt separates in coloured crystals 
from solutions containing iodine, but microscopical examination shows 
that the distribution within the crystal is very far from uniform.15? 
Somewhat more exhaustive experiments have peen made with 
meconic acid. Crystals of this acid may be stained if allowed to 
erystallise from solutions containing ‘ Fettfarben,’ but if the colourless 
crystals are placed in a solution of the dye the colour is not absorbed.15* 
It was objected 1** that the experiment was only conclusive if it were 
also shown that the same solvent (benzene or light petroleum) did not 
extract the dye from previously stained crystals. ‘This proved to be 
the case,** and the evidence therefore shows that in this instance . 
diffusion of the colouring matter does not occur, even when the con- 
ditions are apparently favourable. 
Diffusion in Colloidal Gels. 
The diffusion of salts in gels, such as gelatin, agar-agar, and colloidal 
silica, differs little from the same process in water, the diffusion follow- 
ing the same laws, and the velocity being diminished to a comparatively 
small extent. This was first studied by T. Graham,'*® and confirmed 
in detail by F. Voigtlinder.1°’? The velocity diminishes with increasing 
concentration of the colloid,'®* and is also influenced by the presence 
vf foreign substances. 
Liesegang’s Phenomenon. \ 
An interesting phenomenon, which is of importance for the theory 
of diffusion in many cases, was first observed by R. EH. Liesegang.'** 
In the most convenient manner of conducting the experiment, a thin 
layer of gelatin, spread on a glass plate, is impregnated with potassium 
dichromate and allowed to dry. A drop of silver nitrate solution is 
then placed on the gelatin surface. As the solution diffuses outwards 
silver chromate is precipitated, but instead of forming a ring round 
the original spot, diminishing steadily in intensity towards the circum- 
ference, the silver chromate is deposited intermittently, forming a 
series of concentric rings, which under favourable conditions are of 
very remarkable regularity. Many other salts behave in the same way 
as silver chromate. It is not even necessary that a precipitate be 
formed, as similar rings may be obtained by the diffusion of a single 
10 J. W. Retgers, Zeitsch. Physikal. Chem., 1894, 12, 583. 
£'! P. Gaubert, Bull. Soc. Franc. Min., 1901, 28, 211. 
, 2 BH, Sommerfeld, N. Jahrb. Min., 1902, ii. 43. 
168 QO. Lehmann, Ann. Physik, 1894 [iii.], 51, 47. 
| 4 W. Ostwald, Zeitsch. Physikal. Chem., 1894, 18, 758. 
165 §. Ruzi ka, ibid., 1910, 72, 381. 
66 Phil. Trans., 1866, 156, 399. 
11 Zeitsch. Physikal. Chem., 1889, 8, 316. 
168 K. Meyer, Beitr. Chem. Physiol. Path., 1905, 7, 393 ; H. Bechhold and J. Ziegler, 
Zeitsch. Physikal. Chem., 1906, 56, 105. 
160 * Chemische Reaktionen in Gallerten ’ (Diisseldorf, 1898) ;" ‘ Ueher die Sehicht - 
ungen bei Diffusionen ’ (Leipzig, 1907). 
