86 Dr. Thomson on the Composition of [Aug. 



constitution of coal, and he attempted to analyze it by determin- 

 ing the quantity of saltpetre which a given weight of each 

 species of coal is capable of decomposing ; for this quantity he 

 considered as proportional to the charcoal of the pit-coal ; the 

 bituminous portion being too volatile to be capable of being 

 raised to the temperature requisite to enable it to decompose the 

 nitre. 



It seems unnecessary to make any observations upon the 

 analyses of coal hitherto published. We have no evidence 

 whatever that pit-coal is a compound of charcoal and bitumen. 

 We cannot resolve it into these two constituents except by the 

 action of heat ; and these are not the only constituents obtained, 

 when we distil pit-coal ; for we always find water and ammonia 

 in the receiver as well as bitumen. Nor are we able to prevent 

 a considerable proportion of the coal from making its escape in 

 the state of gas. We have, therefore, exactly the same evidence 

 that pit-coal is composed of charcoal, bitumen, water, ammonia, 

 carburetted hydrogen gas, and olefiant gas, as we have that it is 

 a compound of bitumen and charcoal ; but when we attempt to 

 mix all these constituents together, we are quite unable to form 

 any substance exactly resembling the pit-coal from which we 

 procured them. Pit-coal seems to be nothing else than a pecu- 

 liar combustible substance formed by the union of certain 

 proportions of carbon, hydrogen, oxygen, and azote. In what 

 way these ultimate constituents are grouped together, we have 

 no accurate notion ; but it will appear from the following expe- 

 riments that every species of pit-coal is a definite compound of 

 a determinate number of atoms of each of its constituents. 



1 conducted the experiments to determine the constitution of 

 the different species of pit-coal in the following manner : 1. 1 as- 

 certained the quantity of earthy matter left when each species 

 was kept for many hours in a red heat in an open muffle. 

 2. I converted a given weight of each species into coke, by sub- 

 jecting it to a strong heat in a close vessel, and determined the 

 proportion of coke which each yielded. 3. I mixed a given 

 weight of each kind of coal with peroxide of copper, and sub- 

 jected the mixture to a red heat in a copper tube connected with 

 a mercurial trough, and determined the quantity of carbonic 

 acid and water disengaged, and the bulk of azotic gas evolved 

 — data which enabled me to determine the quantity of carbon, 

 hydrogen, azote, and oxygen, of which the coal was composed. 

 I shall state in succession the results obtained in each of these 

 three suits of experiment. ' 



1 . Earthy Matter contained in each Species. 



To determine this point, 20 gr. of each species of coal were 

 put into a platinum crucible or cup, which was introduced into 

 a muffle, and kept red-hot till every thing combustible was con- 

 sumed and dissipated. From four to six hours are requisite to 



