68 Experiment^ o?> a Sulstaiicc iiliich po5sTJ?s.> 



and at length total destruction. These effects are produced 

 at ditfcrent periods, according to the substance which may 

 be the subject oF the experiment, and therefore it is impos- 

 sible at present to state the utmost quantity ot" the tanning 

 fiihstance which, under equal circumstances, may be ob- 

 tained from each of the resins, balsams, Sec. 



The tanning substance appears to be always the same, 

 whether obtained from turpentine, or common resin, or 

 from the balsams, or from asa foetida, or camphor, or indeed 

 from any of the bodies which have been enumerated ; its 

 effects on the different reagents are similar j by the addition 

 of a small portion of nitric acid, and subsequent evaporation, 

 it is converted into that which I 'have called the first variety ; 

 or if digested with sulphuric acid, it is speedily destroyed, 

 and becomes mere coal. In the latter case, therefore, the 

 same agent which at first produced it becomes at length the 

 cause of its destruction ; and thus we find that although a 

 tanning substance may be obtained from resinous and other 

 bodies by means of sulphiaric and by nitric acid, yet in the 

 former case the product is variable, and is formed at or 

 about the mean period of the operation, whilst the latter is 

 an ultimate and invariable effect, beyond which no appa- 

 renf'change can be produced by any continuation of the 

 process*. 



§ III. 



I have already stated, that caovUchouc, and elastic bitu- 

 men, were only superficially acted upon when digested for a 

 very long time in sulphuric acid; arnl it is remarkable, that 

 these substances, which in their external characters so much 

 resemble each other, should be similar in their habits when 

 exposed to the effects of this acid ; for, unlike the resins and 

 most of the other bodies which were subjected to the pre- 

 ceding experiments, and which were almost immediately 

 dissolved when the acid was poured t^on them, these on 

 the contrary remained undissolved, and only became par- 

 tially carbonized on their surfaces. Even nitric acid docs 



* lii the former papers upon this subjrrt I have otiserveU, that the tan- 

 ning iulistance produced by sulphuric acid U very inferior in energy to that 

 ^vJiikh is formed bv nitrir acid. 



not 



