serve as a Sulstitute for Oak Bark. i45 



stitutes one half, or even more, and sometimes contains a 

 portion of fixed alkaline salt. In dry fruits, juices, and 

 bulbous roots, this proportion suffers Some exceptions. It 

 is easy to conceive that the knowledge of these component 

 parts, of their respective quantities, and of their properties, 

 which are well known to chemists, may lead to that of 

 their effects, and of the manner in which they produce- 

 them. We shall be able then to distinguish a false tanning 

 plant from a real one, or to lay aside such as are too weak 

 for that purpose. There are some, for example, that are 

 much fitter for giving a fine dye to leather than for tanning 

 it. 



Nor is it difficult, after what we have said concerning 

 the principles contained in the plants, to form an idea oi 

 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, undergo 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 va- 

 rious degrees of heat- They remove imperceptibly frohi 

 each other, and extend in every direction in a very gentle 

 manner, which renders them fit for softly penetrating the 

 Substance of the skins, and producing gradually an alter- 

 ation 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-gunmiy, 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 

 tanning, we shall confine ourselves to our object, which is 

 the indication of tanning plants, and shall mention another 

 property of them, whereby they are plainly distinguishable 

 from all others. This property occurs in their dust, or iu 

 a decoction of them, when mixed with copperas (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 produced by this 

 mixture is sometimes reddish or of a dark red, and some- 

 times blue or black. The cause of this phaenomenon is 

 known to chemists, who know also how to make these 



Vol. XVII. No. 66. K decoctions 



