334 Mr. Hatchett’s additional Experiments 
which being in a state approaching to coal, is probably the 
cause of the blackness of the common liquorice. 
As the formation of the tanning substance has been my prin- 
cipal object, I have not thought it necessary to enter at present 
into too minute a detail of other particulars, and have therefore 
only thus cursorily noticed some of the principal effects pro- 
duced by nitric acid on the resins, balsams, &c. Those however 
who are conversant with chemistry, will undoubtedly perceive 
that these effects deserve to be accurately investigated, and that 
the resins, balsams, gum resins, and gums, should be regularly 
examined by every possible method, not merely on account of 
the individual substances which may become the subjects of 
experiment, but because there is reason to expect that from 
such an investigation, medicine, with the arts, and manufactures, 
may derive many advantages, whilst the mysterious processes 
and effects of vegetation may very probably receive consi- 
derable elucidation. 
Concerning the third variety of the tanning substance, which 
is produced by the action of sulphuric acid on the resins, gum 
resins, &c. I shall here add but little to that which I have 
already stated in the latter part of the second section of my 
first paper, and in the account which I have lately given of 
an experiment on camphor. 
This variety appears to be uniformly produced during a 
certain period of the process, but by a long continuance of the 
digestion, I have reason to believe that it is destroyed. 
Substances, such as the gums, which afford much oxalic 
add by treatment with other acids, do not apparently yield any 
of this tanning substance. 
The energy of its action on gelatine and skin is certainly 
