730 
PROFESSOR O. REYNOLDS ON CERTAIN DIMENSIONAL 
There exists a whole class of very marked phenomena which have escaped the 
notice of Graham and subsequent observers; while several of the most marked and 
important facts discovered by Graham have hitherto remained unconnected by any 
theory. 
2. Amongst the best known of the phenomena is the difference in the rates at 
which different gases transpire through minute channels, and the consequent difference 
of pressure which ensues when two different gases initially at the same pressure are 
separated by a porous plate. It does not appear, however, that hitherto any attempt 
has been made to ascertain the existence of what may be considered a closely analogous 
phenomenon—that a difference of temperature on the two sides of the plate might 
cause gas, without any initial difference of pressure or any difference in chemical con¬ 
stitution, to pass through the plate—nor am I aware that such a result from a difference 
of temperature has been in any way surmised (see Appendix, note 1). 
I have, however, now ascertained, by experiments which will be described at lengtlq 
that a difference of temperature may be a very potent cause of transpiration through 
porous plates. So much so that with hydrogen on both sides of a porous plate, the 
pressure on one side being that of the atmosphere, a difference of 160° F. (from 52° to 
212°) in the temperature on the two sides of the plate secured a permanent difference 
in the pressure on the two sides equal to an inch of mercury; the higher pressure 
being on the hotter side. With different gases and different plates various results 
were obtained, which are however, as will be seen, connected by definite laws. 
I propose to call the motion of the gas caused by a difference of temperature 
Thermal Transpiration (see Appendix, note 2). 
3. Again, although Graham found that he obtained not only very different results 
but also very different laws of motion with plates of different coarseness or with plates 
and capillary tubes,* neither he nor any subsequent observer appears to have 
followed up this lead. As regards Graham this appears to me to be somewhat sur¬ 
prising. For although he may have considered the mere difference in the results to 
have been analogous to the difference found by Poiseiulle for liquids, it would seem 
as though the difference in the laws of motion which he obtained should have excited 
his curiosity; and then, as he was avowedly of opinion that gas is molecular, he could 
hardly have failed to observe that so long as the distance separating the molecules in 
the gas bore a fixed relation to the breadth of the openings in his plates he should 
have had the same laws of motion. This view, however, appears to have escaped him 
as well as all subsequent observers. Otherwise it would have been seen that with a 
simple gas such as hydrogen, similar results must be obtained so long as the density of 
the gas is inversely proportional to the lateral dimensions of the passages through the 
plates. 
By experiments, to be described, I have now fully established this law. I find that 
with different plates similar results are obtained when the densities of the gas with 
* Phil. Trans., 1863. 
