646 Proceedings of the Boyal Society 
power of animal charcoal, when separated from the phosphate of 
lime by means of hydrochloric acid, to what it should be theoreti- 
cally, may be partly due to the presence of this acid ; for if animal 
charcoal, which has been submitted to three days’ digesting in 
ordinary hydrochloric acid, be washed in water till nothing further 
is removed, and be then placed in contact with ammonia for a short 
time, a slight hut decided crystalline precipitate, soluble in acetic 
acid, is obtained by adding chloride of ammonium, and sulphate of 
magnesia in excess of ammonia; while, if water or weak hydro- 
chloric acid was substituted for the ammonia, and suffered to remain 
in contact with the charcoal for many hours, not the least indication 
of phosphoric acid was obtained by the application of the above 
test. It may be mentioned, the water employed contained a minute 
quantity of carbonic acid, which would probably substitute itself 
for a portion of the phosphoric acid absorbed to the charcoal, and 
thus render its detectiorr more difficult. 
The absortion of hydriodic acid by coal from acidulated solu- 
tions renders it very probable that, wherever any coal-bed is acid 
from the presence of the stronger acid, and has a flow of water 
through it, such bed will be charged with hydriodic acid in those 
parts which first receive the underground flow ; and there, also, 
we may reasonably expect to find an unusual proportion of other 
acids, such as phosphoric, arsenic, hydrosulphuric, and hydroarsenic 
acids. 
The property of brown coals, &c., to absorb sulphuretted hydro- 
gen, affords a probable solution, as before shown, of the difficult 
problem. In what form does the sulphur exist in those highly sul- 
phurized coals, which are comparatively free from either iron 
pyrites, sulphuric acid, or sulphates ? That it does exist, in com- 
bination with hydrogen, to form sulphuretted hydrogen, received 
confirmation from further experiments, which went to prove that 
such coals evolved considerable quantities of this gas when sub- 
jected to temperatures ranging from 212° to 300° Fahr. 
In regard to the action of decomposing organic matters upon 
sulphates furnishing the gas, we have only to take into considera- 
tion the general absorptive power of coal to explain the frequent 
association of sulphuretted hydrogen with it. 
It may be stated that many of the older coals, as also -samples 
