Vol. 58. | CONCRETIONS IN THE LOWER COAL-MEASURES. D3 
all of them, it will be more convenient to divide them into classes 
and discuss the salient points :— 
(1) On the precipitation of calecium-carbonate under varying conditions. 
(2) On the action of salts of lime and of iron upon wood, ete. 
(3) On the action of bacteria upon solutions containing calcium-sulphate in 
solution and ferric oxide in the deposit. 
(1) On the Precipitation of Calcium-Carbonate under 
Varying Conditions. 
It was found that calcium-carbonate precipitated from pure solu- 
tions in the cold by whatever means was almost always crystalline, 
the crystals being various forms of calcite; in a few cases only it 
was flocculent, showing no structure. At the boiling temperature 
crystals of aragonite were formed. 
When calcium-carbonate was precipitated from solution in presence 
of organic matter the results were quite different, and the substance 
considerably moditied ; in certain cases it was quite flocculent, but 
as a rule it separated in dumbbell-shaped crystals, or more or less 
perfect spheres, twinned spheres, or botryoidal groupings. Colloids 
such as glue, gum, dextrin, and albumen have the most marked 
effect, but urine and peaty matter also influence it considerably. 
The best results were obtained by the slow mixing of solutions 
of calcium-chloride and sodium-carbonate in strong gum-water. 
(See figs. 1 & 2, p. 52.) 
The first to point out this remarkable fact was Mr. George 
Rainey.’ He used for the purpose of the experiments two solutions— 
one a solution of calcium-chloride in gum-water, the other sodium- 
carbonate in the same fluid; and they were brought into a bottle in 
such a way that the one solution floated upon the other without 
mixture taking place: by slow diffusion through the liquid, spheres 
of calcium-carbonate were produced. 
This line of enquiry was also followed by Prof. Harting, of 
Utrecht, who used the hollow of a porcelain plate which was filled 
up with the colloid solution, and the solids were placed on opposite 
sides of the raised portion of the plate just in contact with the solu- 
tion. The plate was covered with a sheet of glass, and left for a 
few weeks, when the diffusion of the two substances caused the 
precipitation of calcium-carbonate in the colloid. Dr. W. M. Ord 
has extended the experiments in connection with the formation of 
urinary and other calculi, which are really concretions of calcium- 
carbonate and phosphate, etc. produced in the organic fluids of the 
body. 
Gone cvoink very often separates in perfect spheres from 
alkaline urines.2, Organic matter and especially colloidal substances 
* See Hogg ‘On the Microscope’ 4th ed. (1859) p. 606; also W. B. Car- 
penter ‘ The Microscope & its Revelations’ 7th ed. (1891) p. 1021. 
* A fine photograph of this is reproduced in R. W. Lucas’s ‘ Practical 
Pharmacy.’ 
