$$8 On several indigenous Plants which may 
bo exaggeration to say that there are sixty species of 
them. 
It is very easy to discover the chemical principles in 
virtue of which these plants are fit for tanning, if one has 
a knowledge of those of sumach, galls, and different sorts 
of bark. With regard to this point, the plants may be 
divided into two principal classes. The principles that 
are chiefly to be considered are found generally in all of 
them; they are of zl fixed, but still active , terreo-gummy t 
or terreo-resinoso-gummy nature. Besides these com- 
mon principles, some other very active ones exist in some 
of these plants, in a greater or less quantity ; and this is 
what constitutes the difference that we establish between 
the plants that can be used in tanning. 
Those of the first sort have no smell, or at most a very 
weak one, but they have a very sharp and astringent 
taste. They contain only the active and fixed principles 
which we have mentioned, or at most an inconsiderable 
mixture of oleo-inflammable parts, which give a weak 
balsamic smell to the water distilled from them, without * 
any sharp or styptic taste. The proportion of these parts 
varies in the terreo-resinoso-gummy substance; but that 
which commonly exists in the greatest part of the coria- 
ceous plants is such, that, for instance, in a pound of them 
the terreous parts constitute one-third, or even one-half ; 
and the gummy principle about one -fourth or one-third, 
and in some as much as one-half, while the proportion of 
rosin is the smallest of all, being only from twenty to 
fifty grains, or at most a drachm and about twenty 
grains. 
In the second sort of these plants we find, indeed, the 
above-mentioned fixed active principles, but not in the 
same proportion, because they are mixed with other prin- 
ciples both volatile and fixed, so as to constitute the 
smaller part of the whole compound® Besides the fixvd 
