40 G. Jounstone Stoney On Polarization Stress in Gases. 
to resume the subject and present the theory freed from the error that has been 
pointed out. In doing this I have taken the opportunity of introducing the 
conception of the reflecting tube, which greatly facilitates the inquiry into the 
mechanical effect of the interpenetration; and I have also availed myself of the 
admirable method of treating the problem described by Mr. George F- Fitzgerald 
in “Nature,” vol. 17, p.200, to obtain a complete expression for the stress, and 
to show that my theory is not at variance with results established by Professor 
Clausius, as had been asserted by Professor Osborne Reynolds in “ Nature,” vol. 17, 
p. 122. 
Part ].—TREATMENT oF THE PropiemM BY GENERAL MECHANICAL ConsIDERATIONS. 
1. Ifa drop of water or other volatile liquid is allowed to fall into a smooth and 
sufficiently hot metal dish, it continues a liquid drop instead of spreading out or 
flashing off into vapour, and it exhibits an appearence of great mobility. The drop 
is then in what has been called the “Spheroidal state.” Now when a drop of liquid 
is so situated, a chink may be observed between it and the hot surface beneath ; so 
that the drop does not rest directly upon the metal, but is in reality floating upon 
a layer of vapour. We further learn from these observations that after the brief 
interval of adjustment is over the layer of vapour presses upwards and downwards 
more than it presses sideways ; for the pressure sideways must equal the pressure of 
the atmosphere so soon as the adjustment is over, otherwise air would still be enter- 
ing or leaving the chink, whereas the pressure upwards must exceed the pressure of 
the atmosphere by an amount able to support the drop. _ It is my object to explain 
how this difference of pressures, this Crookes’s pressure as it has been called, comes 
into existence. 
2. The thermal conditions of the problem are easily traced, but need not detain 
us here. It is enough to state that they show the metal dish and drop to be at 
different temperatures, so that they are a heater and cooler on either side of the 
layer of vapour. Experiment further shows that the heater and cooler may be 
either one a liquid and one a solid, as in the case already considered, or both liquids, 
or both solid, and that the intervening layer may be either vapour or permanent 
gas. This last important fact has been established by Mr. Richard Moss in an 
admirable series of experiments lately made by him to test the theory of the present 
communication (see Scientific Proceedings of the Royal Dublin Society, vol. 1. 
p. 89). It is also found to be immaterial whether the heater or cooler is upper- 
most, or whether they face one another sideways. i 
Other facts of importance have been elicited by the experiments at low tensions: 
of which the most significant are that when the heater and cooler are maintained 
at given temperatures the Crookes’s stress between them may be increased either 
by bringing the heater and cooler closer together, or by attenuating the gas until 
a certain point is reached which varies from one gas to another; and that when 
that point is passed the force decreases and apparently without limit. 
