20 
He appears to have adopted, as tlie basis of his essay, 
Dr. Hamilton's theory of spontaneous evaporation. This 
affords him a supply of air, but investigates not the quality of 
that air : however, as it seems necessary that this air should 
contain a large portion of oxygen gas, to supply the daily 
consumption of that gas by various processes, he lays 
down an hypothesis, that the atmosphere in its different 
" circumstances, acquires a different affinity for the oxygen 
" or hydrogen part of the water, and converts it into air as 
circumstances require." This hypothesis he thinks con- 
firmed or rendered highly probable, from some eudiometrical 
experiments of Dr. Priestley: by the notion of Dr. Girtanner, 
that atmospheric air was composed of oxygen and hydrogen 
gases in very near the same proportion as water ; and by 
additional experiments with the eudiometer, by M. Humboldt. 
By the aid of those high authorities, he satisfactorily 
concludes that the atmosphere is a chemical composition or 
solution of gases ; that it may be, and frequently is, composed 
of the gases oxygen and hydrogen ; the proportional parts of 
which are subject to considerable variation in different seasons 
and temperatures ; and hence, together with the spontaneous 
evaporation of water, he deduces the principal cause of its 
greater or less fitness for the purposes of combustion, par- 
ticularly as applied to the production of iron from the blast 
furnace. 
Having understood, five years after the publication of 
this essay, that he still maintained the same opinions, we 
took the subject into a more particular consideration ; and 
after a minute examination of his hypothesis, and a copious 
comparison of its supports with more recent and correct 
authorities (at a length indeed so great, as quite to forbid its 
introduction on this occasion, but which will, with much 
pleasure, be submitted to the inspection of any gentleman here,) 
we felt bound to differ most essentially from these opinions 
of Mr. Dawson ; particulars we must here wave, and only 
