G. Jounstons Stoney On Polarization Stress in Gases. 47 
from their coexistence. Each stream is exposed within the slice to the mutual 
jostling of its own molecules, and itis also attacked by molecules of the other stream. 
The mutual jostling of its own molecules tends, as explained in section 10, both 
to maintain the onward velocity of the stream and to reduce the gas of which the 
stream consists still more towards the condition of unpolarized gas. These encounters, 
then, taken by themselves, tend to bring about the state of the gas described in section 
10. But the interference of the two streams with one another counteracts this. 
This interference modifies the effect of the encounters within the streams, but it is 
incompetent to annul it. For the two streams do not by their mere coexistence 
constitute stationary unpolarized gas, and hence they would need time before they 
could by their action upon one another reduce the gas to this condition. It is, how- 
ever, plain that whatever action they exert is a step towards bringing about this condi- 
tion ; for the gas would become depolarized if the cross sections which bound the 
slice could be rendered impervious both to energy and molecules, so as to leave the 
two streams time to act fully on one another. In reality, however, sufficient time 
is not allowed to them, because the streams pass one another, and the struggle is 
continually renewed within the slice by fresh portions of the streams which come 
up in the same state as those that had been obliged to pass on. These fresh 
portions keep in the same state because a sufficient supply of swift molecules is 
without intermission being thrown back along the tube from one end by the heater, 
and a corresponding supply of slow molecules from the other end by the cooler. 
13. The two streams, though not annulled, are, however, different from what 
they would have been if they had been without influence upon one another. They 
do not consist of the same molecules from one instant to another, for there is 
such a perpetual shifting of molecules between them, owing to the vast number 
of encounters that take place, that no one molecule is likely to remain long in 
one stream. Again, after an encounter between molecules of the two streams 
both of the colliding molecules will sometimes join the same stream, and it will 
most frequently havpen that the stream so joined is the hotter and swifter stream. 
Hence the stream from the heater to the cooler receives an accession to the 
number of its molecules as it travels forward, while the reverse effect is produced 
upon the stream making its way in the opposite direction. On both accounts 
there will be gradients of density and temperature along the tube between the 
heater and cooler. Again, every encounter between molecules of the two streams 
diminishes the momentum of one or both streams; but, as we have seen, the 
effect so produced does not go the length of reducing the streams to rest. 
14. Hence we must bear in mind the gradients of temperature and density 
along the two streams, and the continual fluctuation of the molecules that are 
to be referred to them, if we want to regard the condition of the gas throughout 
the whole length of the tube as arising out of the coexistence of two streams of 
gas less polarized than itself. But with these precautions the hypothesis may be 
made, and accordingly the condition of the gas at every cross section of the tube 
1s intermediate between a structure represented by the coexistence of the two streams 
I 
