Mr. W. S. Jevons on the Cirrous form of Cloud. 27 
apart from those of another stratum for which they have less 
cohesion, and though we may conceive the two moving strata 
thus segregated into distinct threads, I do not think that this 
is at all the cause of the phenomenon. It is rather the much 
greater facility with which any two fluid bodies can move among 
each other when distinct channels are preserved, than when each 
particle of one is opposed to a particle of the other moving in 
the opposite direction,—just as in a street or a large crowd of 
people, the passage of all is much impeded unless those moving 
in opposite directions proceed along different sides or along di- 
stinct channels. It is probably a simple mechanical effect of the 
motions of small bodies of each fluid, produced immediately that 
the perfect equilibrium of the horizontal strata is in any way 
disturbed. 
And again, I do not know that it is proved that cohesion is 
entirely non-existent in gases; by definition, a gas is matter in 
which ihe repulsive forces entirely overcome the cohesive or attractive 
forces between the particles; but supposing the gas so restrained 
by superincumbent pressure that no further expansion can take 
place, it is not impossible, as far as I see, that some difference of 
attractive or repulsive forces between particles of the same and 
particles of different gases, or the same gas in different conditions, 
may come into play, which, if not to be called cohesion, will at 
least produce the same apparent effects as that force. The un- 
doubted attraction which many solids exercise on gases, as seen, 
for instance, in the cohesion of air to the glass tube of the baro- 
meter, and the absorption and condensation of gases by charcoal, 
platinum black, &c., show that gases do not possess repulsive 
forces only. 
Another distinct property or force existing equally in liquids 
and gases is the diffusive, which would certainly tend to cause 
mixture of two different strata; but this is evidently not the 
least concerned in this phenomenon, since diffusion acts quite 
independently of, or contrary to gravity, and would therefore 
produce nearly as much effect in the second experiment, where 
no cirrous appearance (or rather only flat cirrus or cirrostratus) 
at all was seen, as in the first experiment. 
I think it is pretty evident, then, that when two horizontal 
and tranquil strata of gases are in contact, the upper one being 
very slightly the denser, they will tend to change places, or to 
mix by filtering into each other in distinct portions, which in 
moving will assume the form of small channels or threads. If 
this do not take place, the strata could only remain the denser 
supported upon the lighter, until the half-chemical process of 
diffusion would cause their complete mixture; but no one can 
suppose the strata so equally poised that the difference of specific 
gravity would not cause mechanical movement. 
