16 G. Jonystone Stoney on the Penetration of Heat across Layers of Gas. 
unchanged. Through each element 6S of an isothermal surface, the molecules 
will still travel in equal numbers inwards and outwards, because when the 
adjustment is once over, the density of the gas will not anywhere undergo further 
change ; but the molecules making their way outwards, 7.¢., from A towards B, will 
on the whole be swifter than those tending inwards, because there should be a 
complete Crookes’s layer to enable the swifter class of molecules rebounding from 
A to keep back the whole of the slower kind which constantly tend to crowd in 
(see Phil. Mag., April, 1876, p. 308, §§. 15, 16, and 17). Accordingly, if the 
molecules at any one moment within an element of volume be considered, the 
portion of them which form a procession travelling inwards will now be found more 
numerous than those advancing outwards, and at the same time so much slower 
that the momentum in the two directions is the same; in other words, there is no 
molar motion of the gas, nothing in the nature of a wind. But that there is 
a continual transfer of kinetic energy from A to B across the intervening gas is 
evident, because members of the procession of colder molecules crowding up to A 
will cause the temperature 0,—A6, of the inner surface of the Crookes’s layer to be 
lower than 6,, the temperature of A; while, at the same time, the members of the 
swift procession which reach B will cause 0,+A6,, the temperature of the outside 
surface of the Crookes’s layer, to be warmer than 6,, the temperature of B. The 
Crookes’s layer, accordingly, must acquire heat by its contact with A, and impart 
heat where in contact with B; and as adjustments within the layer are made with 
a speed comparable with the velocity of sound in the gas, it is possible to arrange 
experiments in which the differences of temperature 40, and A0, shall have any 
amounts from 0 (when the interval between A and B equals or exceeds the width of 
an unrestricted Crookes’s layer) up to values bordering upon 4(6,—0,), (which, in the 
cases where the temperatures 6, and @, are not far asunder, is close to the limiting 
value produced by diminishing the interval between A and B, or by attenuating 
the gas). Tae 
Accordingly, if the variations of tempera- 
ture were plotted down on a diagram, the or- 
dinates representing temperatures, and the 
abscissas distances measured perpendicularly 
to the isothermal surfaces within the gas, we 
should obtain a figure something like that in 
the margin. It is, moreover, manifest that 
the curve mn, representing the variations of 
temperature across a compressed Crookes’s 
layer, will approximate more and more to a 
horizontal line, the greater the tenuity of the |e aay 
gas. ee eee ee 
6, Some idea will be formed of the quantity of heat which will pass from A to 
