On the Constitution of mixed Gases, Sc. 549 
as an axiom by modern philosophers, has been 
the cause of more error and perplexity perhaps 
than any other ungrounded opinion. 
Lavoisier thought that; in the higher regions 
of the atmosphere a stratum of inflammable fluid 
exists in which the aurora borealis and other 
fiery appearances are produced; this opinion is 
plausible enough; but that fluid cannot be hy- 
drogenous gas, because its particles are not repul- 
sive of those of the other atmospheric gases, as 
appears by its intimate and almost instantaneous 
diffusion amongst them. 
There may be gases in the higher regions of 
which we have not the principles below, the 
whole stock of matter being spent in their for- 
mation; and being constituted of particles repul- 
sive of those of the atmosphere, according .to 
the first or second hypothesis, and of less specific 
gravity than the other gases, they. must float 
upon the surface of the common .atmosphere, 
and consequently for ever elude the investigation 
of philosophy. He.observes that even metallic 
substances may be found within the regions of 
the atmosphere: I have myself shewn,.. in 
my Meteorological Essays, page 180, that a 
fluid possessing magnetic properties constantly 
holds a place in the higher regions of the atmos- 
phere, and which therefore we cannot help con- 
