488 Proportions of Carton in Aftneral Coal, i^c. 



Coals and bitumens arc, however, fubftanccs that refift the ufual modes of analyfis ; they 

 elude the ailion of aqueous, acid, alkaline, or fpirituous menftruums ; and diftiilation, the 

 only mode hitherto ufed, confounds and varies their natural contents. 



Refle£ling on thefe obftaclcs to an exa£l difcrimination of bitumens and coals, and of the 

 various kinds of thefe laft, it occurred to me, that partly by combullion and partly by their 

 elEcacy in dccompofing nitre, the fecrct of their internal compo(ition might poflibly be 

 unveiled. 



I. Combullion. I have obferved that all the fpecies of folid bitumen properly fo called, 

 when laid on a red hot iron, burn with a large bright flame, fmokc and foot, leaving none 

 or fcarcc any coaly refiduum, and only a little alhes : 



That the fofter bitumens, as maltha, burn in the fame manner, leaving no coal, but only 

 a little alhes, and requiring no increafe of heat for their entire confumption : 



That afphalt burns witli flame and foot, but melts and fwclls, and requires for its entire 

 confumption an increafe of heat, leaving fcarce any coal, and but little aflies. 



It is moreover well known that liquid bitumens contain inflammable air and carbon : 

 that they abforb atmofpheric air when long expofed to it and light : that, in confequencc 

 of this abforption, they arc thickened, blackened, and condenfed, firft into mineral tar, then 

 into mineral pitch or maltha, and laftly into afphalt : that almoft all fpecies of mineral 

 coal yield more or lefs of both fpecies of bitumen in diftiilation, leaving a fliining coaly 

 refiduum ; but that the proportion is variable in every fpecies according to the degree of 

 heat applied; that the refiduum always obftinately retains a proportion of bitumen, and that 

 confequently diftiilation, in addition to its other imperfedions, is an infuflicient medium 

 whereby to dlfcern the proportion of carbon and bitumen, and confequently to dilcriminatc 

 the various forts of mineral coal from each other. 



a. Decompofition of nitre. It has long ago been remarked by the juftly celebrated 

 Macquer*, that nitre detonates with no oily inflammable matter, until fuch matter is re- 

 duced to a coal, and then only in proportion to the carbonaceous matter it contains. An 

 obfervation the truth of which will fully appear in the fubfequent experiments. 



Hence it occurred to me that, fince in the aft of detonation nitre is always totally or par- 

 tially decompofed, and fince, where carbonaceous compounds are employed, this decompo- 

 fition arifcs folely from the mere carbonaceous parts, and, every thing elfe being equal, is 

 proportioned to the quantity of mere carbon they contain ; and fince moft fpecies of coals 

 are compounds of mere carbon and bitumen, as appears by the products of their diftii- 

 lation ; it fliould follow, that, by the decompofition of nitre, the quantity of mere carbon in 

 a given quantity of every fpecies of coal may be difcovered ; and this being known, that of 

 bitumen may be inferred ; and, tlie other uncflential ingredients being detCiTled by incine- 

 ration, the whole contents of coaly fubftanccs might be afccrtained. 



The compofition of bitumens alfo, as far as relates to their proportion of carbon and oil, 

 may be evidenced in the fame manner; and here it is to be obfcrvcd, that the bitumens I 

 here confider are thofe that are found in a dry or folid ftate, and that thefe contain a larger 

 proportion of carbon than the liquid bitumens ; for, though thefe laft alfo contain carbon, it 

 being an efii;ntial component part of all oils, yet this portion does net extricate or educe 



* DiJIion. Clivm. fcconJ edition, 4S1. 



