«7 0 Mr. Davy’s Experiments and Observations 
this substance diffused through water, it became of a red-brown 
colour, and the fluid obtained by filtration produced a distinct 
precipitate with solution of galls. The acids have less affinity for 
tannin than for gelatine; and, in cases where compounds of the 
acids and tannin are acted on by solution of gelatine, an equi- 
librium of affinity is established, in consequence of which, by 
far the greatest quantity of tannin is carried down in the inso- 
luble combination. The different neutral salts have, compara- 
tively, feeble powers of attraction for the tanning principle ; but, 
that the precipitation they occasion in astringent solutions, is 
not simply owing to the circumstance of their uniting to a por- 
tion of the water which held the vegetable substances in so- 
lution, is evident from many facts, besides those which have 
been already stated. The solutions of alum, and of some 
other salts which are less soluble in water than tannin, pro- 
duce, in many astringent infusions, precipitates as copious as 
the more soluble saline matters ; and sulphate of lime, and other 
earthy neutral compounds, which are, comparatively speaking, 
insoluble in water, speedily deprive them of their tanning prin- 
ciple. 
From the different facts that have been stated, it is evident 
that tannin may exist in a state of combination in different sub- 
stances, in which its presence cannot be made evident by means 
of solution of gelatine ; and, in this case, to detect its existence, 
it is necessary to have recourse to the action of the diluted acids. 
In considering the relations of the different facts that have 
been detailed, to the processes of tanning and of leather-making, 
it will appear sufficiently evident, that when skin is tanned in 
astringent infusions that contain, as well as tannin, extractive 
matters, portions of these matters enter, with the tannin, into 
