Mr. Dalton on the constitution of the atmosphere . 1 75 
nical effects, we may safely adopt the above hypothesis within 
the fore-mentioned limits of condensation and rarefaction. 
The existence of elastic fluids totally different from atmo- 
spheric air in their chemical nature, but agreeing with it in 
mechanical properties, was unknown in Newton’s time. Such 
fluids are now known to exist ; they may be mixed together, 
and, in case no obvious chemical action ensues, they are 
found to occupy the same space after as before mixture, and 
in due time to be uniformly diffused through the space the 
mixture occupies, whatever may be the difference of their 
volumes and specific gravities : and such mixtures have all the 
mechanical properties o f simple elastic fluids, as the atmosphere 
itself evinces, which is a mixture of at least four such elastic 
fluids. 
Whether the uniform diffusion of elastic fluids through 
each other is occasioned by the repulsion of the elementary 
particles of the same kind, which appears to force them 
through most bodies, as well solid and liquid as aerial, except 
glass and the metals ; or whether it is caused by attraction 
or chemical affinity, may be doubted. Difficulties attend 
both views. I have long been inclined to adopt the former 
notion as most consistent with the phenomena. According 
to this view the particles of any elastic fluid (A) are endued 
with repulsion to each other by the Newtonian law above 
stated; also those of any other (B) repel each other in like 
manner ; but the particles of (A) do not repel those of (B), 
or rather are inelastic in regard to them. Such mixture must 
evidently possess the mechanical law of condensation which 
the atmosphere possesses, and which Newton’s imaginary 
homogeneous fluid possesses. 
