480 On several indigenous Plants which may 
much fitter for giving a fine dye to leather than for tan- 
ning it. * 
Nor is it difficult, after what we have said concerning 
the principles contained in the plants, to form an idea of 
their action upon skins, properly cleansed and macerated. 
The skins being left steeping in a decoction of these 
plants, or merely along with the coarse dust of them, un- 
dergo a change in the tissue of their parts, whereby they 
become leather. In this operation the soluble and active 
parts of the vegetables are separated from the coarse 
mass, with the help of air, evaporating moisture, water, 
work, and various degrees of heat. They remove im- 
perceptibly from each other, and extend in every direc- 
tion in a very gentle manner, which renders them fit for 
softly penetrating the substance of the skins, and pro- 
ducing gradually an alteration in them. It is easy to 
comprehend the effects which, in such a case, a gentle 
acid is capable of producing, when dissolved, mixed, 
and put in action with other particles highly volatile, 
oleoso-ethereous, and of great mobility The skins are 
penetrated with these particles, and with those which we 
have called terreo-resino-gummy, as if with a sort of 
balsam, and are thereby condensed into leather. But as 
it is not our intention to enlarge upon the theory of tan- 
ning, we shall confine ourselves to our object, which is 
the indication of tanning plants, and shall mention ano- 
ther property of them, whereby they are plainly distin- 
guishable from all others. This property occurs in their 
dust, or in a decoction of them, when mixed with cop- 
peras (vitriol de mars). 
Take then these plants, and reduce them into dust, 
which you will throw into a solution of copperas ; or put 
some copperas into an infusion or decoction of the plants 
which has been previously filtrated. The colour pro- 
duced by this mixture is sometimes reddish or of a duck 
