156 Experiments and Remarks on a Substance 



common resin, indigo, dragon's blood, and various other 

 substances : and, 



3dly, That which is yielded to alcohol by common resin, 

 elemi, asa foetida, camphor, &c., after these bodies have 

 been for some time previously digested with sulphuric acid. 



Upon these three products I shall now make a few re- 

 marks, which 1 have hitherto postponed, in order that the 

 account of the experiments might not be interrupted. 



The first variety is that which is the most easily formed ; 

 and from some experiments which were purposely made, I 

 find that JOO grains of dry vegetable charcoal aflord 120 of 

 the tanning substance ; but, as it is extremely difficult com- 

 pletely to expel moisture, or even the whole of the nitric 

 dcid which has been employed*, an allowance of about three 

 or four grains ought to be made ; so that after this deduction 

 we may conclude, that 100 grains of vegetable charcoal yield 

 J 16 or 1 17 of the drj' tanning substance. 



The proportions of the constituent parts of this substance 

 1 have not as yet ascertained ; but, from the manner by 

 which it is produced, carbon is evidently the base of it, and 

 is the predominating essential ingredient. 



From § III. experiment F. it also appears, that the other 

 component parts are oxygen, hydrogen, and nitrogen ; for, 

 when the artificial tanning substance was distilled, ammonia 

 and carbonic acid were obtained, exclusive of a very small 

 portion of a yellow liquor, which stained the upper part of 

 the retort, and which, from its tenacity and insolubility in 

 water and alcohol, appeared to be of an oily nature. 



As I had taken every precaution respecting the charcoal 

 which had been emploved, I. was at first induced to consider 

 the above facts as almost positively demonstrative of the pre- 

 sence of hydrogen in charcoal ; but upon further reflection, 

 ^nd upon weighing some of the circumstances which attend 

 the formation of the artificial tanning substance, I still feel 

 -on this point very considerable doubt j for I have constantly 



* The most effectual method of expelling the nitric acid is, to reduce the 

 tanning substance to powder, and repeatedly evaporate different portions of 

 ^istilied water from it in a glass or porcelain basin. 



rcaci. 6bserved, 



