138 Dr. Henry on the a'eriform compounds 
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 phi- 
losophers, 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 proportions indeed as subject to no limita- 
tion. 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 only 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 
• Manchester Society’s Memoirs, new Series, vol. III. 
