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142 ABSORPTION OF SALINE SUBSTANCES BY CHARCOAL. 
I have still to mention the experiments made by Messrs. 
Garrod and Weppen ; but the results obtained by these two 
chemists are so little in accordance, and differ so much from 
those published by Prof. Graham, that I have considered it in- 
dispensable to repeat their experiments and in different 
ways, in order to ascertain whether the difference of the 
results at which they arrived should not be principally attribu- 
ted to the varied conditions under which they operated. All 
my experiments were made with three distinct kinds of 
charcoal : — 
1. With animal black, freed from the carbonate of lime 
which it contained, but still retaining the phosphate. 
2. With animal charcoal, digested with hydrochloric acid, 
and washed with distilled water until what passed through no 
longer furnished a precipitate with oxalate of ammonia or with 
nitrate of silver : a considerable quantity of this charcoal, on 
calcination in a platinum crucible, left but an insignificant 
r esidue of silica. 
3. W T ith blood-charcoal, calcined twice with a mixture of 
carbonate of soda and potash, and then washed with distilled 
water until it no longer furnished a trace of alkaline cyanide. 
I tried the action of these charcoals in three different man- 
ners : 1st, by mixing a known weight of charcoal with a cer- 
tain quantity of saline solution, and setting the mixture aside; 
2nd, by boiling a known weight of c arcoal with the saline so- 
lution ; 3rd, by filtering the metallic solution through a layer 
of charcoal placed in the neck of a retort, and acting thus by 
way of displacement. 
Of these different methods, the third is that which appeared 
to be the most advantageous, only it has the inconvenience of 
being rather tedious and also somewhat capricious. To obtain 
a satisfactory result, it is requisite that the filtration should 
proceed slowly, otherwise some of the liquid passes without 
being decomposed. The second process is undoubtedly quicker, 
but it always appeared to require a greater proportion of char- 
coal for the same quantity of salt. 
