146 absorption of saline substances by charcoal. 
there any reduction? or is it merely a simple phenomenon 
of mechanical absorption, as in dyeing? I believe the lat- 
ter occurs most frequently; but would except the salts of 
silver, and all those metals the oxides of which are readily 
reduced. I would also except the salts of lead, which ap- 
pear to be converted, for the greater part at least, into the 
state of carbonate. This is partly indicated by the white 
deposit which covers the surface,and even penetrates pretty 
far into the interior of the layer of charcoal through which 
a solution of a lead salt has been filtered. 
There is likewise frequently a change in the composition 
of the salt; sometimes a subsalt is formed, and free acid is 
found in the liquid ; sometimes a subsalt is formed, which is 
retained by the charcoal, whilst a very acid salt passes 
through in the liquid : this is what undoubtedly happens 
with the sulphate of iron and the sulphate of zinc, which 
renders it almost impossible to precipitate the last portions 
of these salts; frequently, however, it is merely a simple 
absorption. Thus if 1 grm. of acetate of copper is di- 
solved in 100 grms. of distilled water, the liquor acidulated 
with one drop of acetic acid, and the solution poured over 
20 grms. of well-washed animal charcoal placed in the neck 
of a retort, not a trace of copper will be found in the liquid 
if the filtration proceeds regularly, and not too rapidly ; the 
charcoal may even be washed with 100 grms. of distilled 
water, the liquids united and concentrated, and still no cop- 
per be indicated by reagents. But if the charcoal be tritu- 
rated in a glass mortar with water, so as to isolate the mole- 
cules and thus to assist the solvent action of the water, it 
removes some of the salt which the charcoal had retained, 
and its presence is now readily detected. 
The following experiment will serve still better to estab 
lish my view. 1 grm. of corrosive sublimate was dissolved 
in 100 grms. of distilled water, and the solution filtered 
through 20 grms. of well washed animal charcoal. At the 
end of thirty-six hours half the liquid had passed through ; 
