322 Chemical Society. 
January 10.—The following communication was read :— 
65. ‘On the Formation of Milk in the Animal Economy,” by 
Lyon Playfair, Ph.D. 
February 7.—The following communications were read :— 
66. “On anew Method of obtaining pure Silver in the Metallic 
State or in the Form of Oxide,” by William Gregory, M.D., F.R.S.E. 
All these communications will shortly appear in the Philosophical 
Magazine. 
67. “* Some Experimental Observations on the formation of Prus- 
sian Blue upon the surface of Gravel, through the medium of Ferro- 
cyanide of Calcium.” By Robert Warington. 
In a communication formerly made to the Society by Mr. Porrett 
on the above subject*, that gentleman considered the production 
of prussian blue to have arisen from some of the gas-lime employed 
to destroy the worms, &c., and placed under the fresh gravel, having 
been accidentally dropped on the surface, and that the peroxide 
of iron contained in the gravel had been deoxidized by some of the 
sulphur compounds of the gas-lime, giving rise to the formation of 
a combination of iron with cyanogen and calcium, and that this 
compound had been decomposed by the action of the carbonic acid 
of the atmosphere, or by the siliceous matter of the stone, thus 
causing the formation of prussian blue. An artificial ferrocyanide 
of calcium was formed by mixing hydrate of lime and prussian blue 
to the consistence of a cream ; and this was placed in an open part 
of a garden, and numerous white-coated siliceous pebbles, selected 
from the red gravel of the neighbourhood of London, then partly 
immersed in the mixture, so that the upper surfaces might be ex- 
posed to the action of the atmosphere and moisture ; in a few days 
the sides of the pebbles assumed the blue colour, which gradually 
spread itself to the summits, having the same bright tint as the 
pebbles presented to the Society by Mr. Porrett, proving therefore 
that the ferrocyanide had been drawn to the surface, either by that 
curious species of crystalline growth, if the expression may be 
allowed, which is exhibited by so many saline combinations during 
their crystallization, or by capillary attraction united with evapora- 
tion from the exposed parts of the pebbles, thus rendering it evident 
that the ferrocyanide might reach the summit of the gravel from 
below. 
Other substances were then submitted to the same action, to de- 
cide the question as to the siliceous matter of the stones being in 
any way instrumental in the production of colour. White limestone 
pebbles, from the south coast of Devon, and baked pipe-clay, un- 
derwent the same changes, with the exception that the blue tint 
was not so bright and clear as was the case on the siliceous surface ; 
but this is considered attributable more to the perfect whiteness of the 
siliceous coating, and the decidedly superficial film of prussian blue 
which was produced on it. Independent of this, the effect can only 
be attributable to the action of the carbonic acid gas present in the 
atmosphere slowly decomposing the ferrocyanide of calcium and 
generating the blue stain. 
* See Proceedings of Chemical Society, p. 35, vol. i. 
