the clmractenst'ic Properties ofTanmn. 105 



100 grains of the same mastich only gave 4^ grains of 

 coal when simply distilled. 



And 100 grains of amber, when treated with sulphuric 

 acid, yielded 5G grains of coal. 



But from 100 grains of the same amber, when distilled, 

 only 3-i- grains could be obtained. 



Many other examples might be adduced, but these appear 

 lo be sufficient : and I must here observe, that the case is 

 very different in respect ro the gums ; for the difference be- 

 tween the proportions of coal obtained from them by the 

 humid and dry ways is not very considerable, although it is 

 always the greatest in the former process when conducted 

 with precaution. Moreover it is to be remarked, tliat, in 

 either process, variations in the quantity of coal are pro- 

 duced by difference of temperature, by the figure and size of 

 the vessels, and many other circumstances. 



But it is not only in the proportion that there is so great 

 a difference between the coal obtained from the resinous sub- 

 stances by the humid way or by fire, for the quality is also 

 most commonly different ; and this not only applies to re- 

 sins, but also to ligneous matter. 



The coal obtained by the humid process from many of 

 the resins was shining, hard, and occasionally iridescent. 

 Few of the coals obtained from the sanje bodies by fire, had 

 any of these properties. The combustion of the former was 

 slow, in the manner of some of the mineral coals; whilst, 

 on the contrary, the latter were speedily consumed, like 

 charcoal. This difference I was at first inclined to attribute 

 to a small portion of the acid which might not have been 

 completely separated, and I therefore purposely made some 

 experiments, which convinced me that this was not the case. 



Having remarked this difference in the coals afforded bv 

 the resins, I was desirous to make some comparative expe- 

 riments on wood, and for this pnriiose I selected oak. 



1. On 180 grains of oak sawdust I poured two ounces of 

 sulphuric acid diluted with six ounces of water, and placed 

 the matrass on a sand-bath, wliero it remaiticd from liic he- 

 ginning of last June to the cud of September. During this 



time 



