645 
o/ Edmhurgh^ Session 1865-66. 
perties are not too well defined. The substances of this class can 
generally be abstracted from the absorbing body by the application 
of a suitable acid or base, as the case may be. 
The fact of the absorption of acids being often facilitated by the 
presence of stronger acids, and that of bases by the presence of 
stronger bases, the application of these being indeed often abso- 
lutely necessary to produce absorption, may perhaps be accounted 
for by the greater affinity these stronger chemicals have for water. 
Thus the solvent powers of this liquid for the body we wish to 
determine to the coal, &c., is reduced, or altogether removed, and 
that state most favourable for absorption obtains. If this is so, we 
can perceive why sulphuric acid and the caustic alkalies are not 
capable of being retained by coal or charcoal ; their affinity for water 
being so intense that it cannot be overcome by absorptive power 
alone, and we are not in the possession of means to remove or lower 
the affinity of these substances, as we have in the case of others. 
But it is particularly at this stage, in the investigation of assisted 
absorption, that, as I have before observed, we are enabled to trace 
differences in the intensity of the absorptive power of charcoal and 
coal ; the former body being able to absorb many acids from 
solution without that assistance from stronger acids required so 
frequently by coals, the absorbing power of the charcoal being 
superior to the affinity subsisting between the acid and the water, 
while that of coal is generally inferior. However, there is this to 
consider, that when we have determined the absorption of any 
acid to coal by the assistance of a stronger, we can remove the 
latter without effecting the solution of the former to any consider- 
able extent. 
General Remarks. 
It is worthy of remark, as indicating a practical application of 
these observations, that the absorption of arson ious acid by carbo- 
naceous substances would allow of its separation from solution for 
analytical purposes if desirable ; arsenic acid, too, would no doubt 
be also absorbed, being isomorphous with one which is so, that is, 
phosphoric acid. 
In reference to the property of phosphoric acid, of being absorbed 
by charcoal, &c., it is not improbable that the low decolorising 
