ABSORPTION OF SALINE SUBSTANCES BY CHARCOAL. 147 
the presence of a salt of mercury was sought for in 
it with potash, sulphuret of ammonium, and iodide of pot- 
assium, but no change was produced by these reagents. 
At the end of twice forty-eight hours the whole of the 
liquid had filtered, the charcoal was washed with distilled 
water, and the same tests applied again with the same re- 
sult. But perhaps the bichloride of mercury had undergone 
some alteration in its composition in contact with the char- 
coal ; perhaps it had been reduced to a state of protochlo- 
ride, and consequently become insoluble. Such is the opin- 
ion of Weppen ; which, however, does not agree with the 
observation made by Fourcroy, who in his Systeme des 
Connaisances Chirniques, states in a positive manner that 
the charcoal has no action upon corrosive sublimate, either 
with or without the assistance of heat. 
The following experiment will, I think, decide the question. 
The charcoal which had taken up the corrosive sublimate 
was washed with a mixture of alcohol and ether, which 
quickly dissolved the mercurial salt in the state of corrosive 
sublimate and in such quantity, that it became possible to 
trace, with a tube dipped in it, characters of a brilliant red 
upon a plate of porcelain which had been moistened with 
iodide of potassium. The ethereal solution left on evapora- 
tion a white po\vder,which, dissolved in distilled water, fur- 
nished with potash an orange precipitate; with hydrosulphate 
of ammonia, a white precipitate, which soon turned black ; 
and lastly, with iodide of potassium a red precipitate. 
It appears evident, therefore, that there was no reduction, 
but a simple phenomenon of absorption, analogous to what 
takes place in the decoloration of indigo, where the charcoal 
can be deprived of the coloring matter which it had com- 
bined with by an alkaline solution ; that is to say, by a 
substance having a greater affinity for it than the charcoal. 
The same occurs when the affinity of the charcoal for the 
corrosive sublimate is overcome by the stronger affinity of 
the ether or alcohol; the salt can then be re-dissolved ; or 
