168 Dr. Henry on the Aeriform Compounds [Sept. 



In the second Memoir (Philosophical Transactions, 1808), I 

 described a series of experiments on the gases obtained from 

 several different varieties of pit-coal, and from the same kind of 

 coal under different circumstances. Various species of that 

 mineral were found to yield aeriform products, differing greatly 

 in specific gravity, combustibility, and illuminating power ; the 

 cannel coal of Wigan, in Lancashire, being best adapted to the 

 purpose, and the stone-coal of South Wales the least so. In 

 decomposing any one species of coal, the gaseous fluids were 

 ascertained not to be of uniform quality throughout the process, 

 but to vary greatly at different stages ; the heavier and more 

 combustible gases coming over first, and the lighter and less 

 combustible afterwards. By subsequent experiments on the 

 gases obtained from coal on the large scale of manufacture, it 

 was found that a similar decline in the value of the products 

 takes place, but not to the same extent, owing, probably, to the 

 greater uniformity of temperature, which is attainable in large 

 operations.* 



On the practical conclusions, which it was the object of the 

 last mentioned Essay to establish, I forbear to dwell, because 

 they are unconnected with my present purpose, which is limited 

 to the chemical constitution of these compound gases, and to 

 the methods of separating them accurately from each other. 

 The view of their nature and composition, which was taken in 

 the first Essay, was opposed by those able philosophers, M. 

 Berthollet, and Dr. Murray, of Edinburgh, who both contended 

 for greater latitude as to the proportions in which hydrogen and 

 charcoal are capable of uniting, and considered these propor- 

 tions indeed as subject to no limitation. The facts, however, 

 which have since been multiplied in this, as well as in other 

 departments of chemistry, tending to prove, that bodies capable 

 of energetic combination unite in a few definite proportions 

 only, leave little doubt that the same law holds good with 

 respect to the compounds of hydrogen and charcoal. Not that 

 it is meant that the known compounds of those elements are the 

 onlv possible ones ; for others will probably be discovered which 

 will still be found conformable to the general law, that when one 

 body combines with another in different proportions, the greater 

 proportions are multiples of the less by an entire number. 



A different view of the subject has lately been taken by the 

 ingenious author of the Bakerian Lecture, published in the Phi- 

 losophical Transactions for 1820. In that paper, Mr. Brande 

 has endeavoured to prove that the gas called light carburetted 

 hydrogen, or simply carburetted hydrogen, or hydrocarburet, is 

 not entitled to be considered as a distinct species ; that the only 

 aeriform compound of charcoal and hydrogen, which is with 

 certainty known to exist, is the gas called defiant, or bicarbu- 



* Manchester Society's Memoirs, New Series, vol. iii. 



