TANNING MATTER., &C. |9 



speedily destroyed, and becomes mere coal. Tn the lat- Kxneriments, 



tor case, therefore, the same agent M'hich at lirst pro- &c. on an arti- 



diiccd it becomes at length the cause of its destruction, f^'^' substance 



^ ^ having the cha- 



and thus we find, tluit although a tanning substance may racter of t-m- 



be obtained from resinous and other bodies by means of"'"g matter. 



sulphuric 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 apparent change 



c^n be produced by any continuation of the process *. 



§111. 



I have already stated, that caoutchouc, and elastic bi- 

 tumen. Mere only superficially acted upon when digested 

 for a very long time in sulphuric acid; and it is remark- 

 able, that these substances, which in their external cha- 

 racters so much resemble each other, should be simdar 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 preceding experiments, and which were 

 almost immediately dissolved when the acid was poured 

 upon them, these on the contrary remained undissolved, 

 and only became partially carbonized on their surfaces, 

 lllven nitric acid does not so rapidly effect a change in the 

 clastic bitumen as it docs when applied to the other bitu- 

 minous substances. 



1. 



One hundred grains of pure soft elastic bitumen m ere 

 digested during three weeks in one ounce of nitric acid, 

 fiilutcd with an equal quantity of water ; a tough and 

 slightly elastic orange-coloured mass then remained. 

 Another ounce of the acid, not dduted, was poured upon 

 this mass, and the digestion was continued until the 

 whole was evaporated. The residuum was tenacious, 

 and of the colour above mentioned. Water partially dis- 

 solved it, and formed a deep yellow liquid, which copi- 

 ously precipitated gelatine, and possessed the other pro- 

 perties of the tanning substance which is produced from 

 the resins, &c. by nitric acid. 



* Ih the former Papers upon this subject I have observed, that 

 the tanning substance produced by sulphuric add, is very inferior 

 in energy to that, which is formed by nitric acid. 



D2 An 



