PROPERTIES OF MATTER IN' THE GASEOUS STATE. 
733 
Having found, what I had not at first perceived, that according to the kinetic 
theory the force resulting from the communication of heat to a gas must depend on 
the surface from which it is communicated being of limited extent, and must follow a 
law depending on some relation between the mean path of a molecule and the size of 
the surface, it appeared that by using vanes of comparatively small size the force 
should be perceived at comparatively greater pressures of gas (see Appendix, note 3). 
On considering how this might be experimentally tested, it appeared that to obtain 
any result at measurable pressures the vanes would have to be very small indeed ; too 
small almost to admit of experiment. And it was while thinking of some means to 
obviate this difficulty that I came to perceive that if the vanes were fixed, then instead 
of the movement of the vanes we should have the gas moving past the vanes—a sort of 
inverse phenomenon—and then instead of having small vanes, small spaces might be 
allowed for the gas to pass. Whence it was at once obvious that in porous plugs I 
should have the means of verifying these conclusions. I followed up the idea, and by 
a method which I devised of taking into account the forces, tangential and normal, 
arising from a varying condition of molecular gas, I was able to deduce what appears 
to me to be a complete theory of transpiration. 
This theory appears to include all the results established by Graham, as well as 
the known phenomena of the radiometer, which for the sake of shortness I shall call 
the phenomena of impulsion. I was also able definitely to deduce the results to be 
expected, both as regards thermal transpiration and the law of corresponding densities 
both for transpiration and impulsion. 
Having made these deductions, I then commenced the experiments on transpiration, 
which so completely verified my theoretical deductions that I have been able to 
produce the theory in its original form, with some additions, but without any important 
modification. 
Moreover, having succeeded (not without some trouble) in rendering apparent the 
effect of differences of temperature in causing gas to move through fine apertures, 
I recurred to the original problem, and by suspending fibres of silk and spider lines to 
act as vanes, I have now succeeded in directly verifying the conclusion that the 
pressure of gas at which the force in the radiometer becomes apparent varies inversely 
as the size of the vanes. With the fibre of silk I obtained repulsion at pressures of 
half an atmosphere. 
The arrangement of the paper. 
8. My object is to describe the reasoning by which I was led to undertake the 
experiments as well as the experiments themselves; but as the theory will be better 
understood after acquaintance with the facts, I have inverted the natural order and 
given the experiments first. And in order that the reader may not be at a dis¬ 
advantage in reading the accounts of the experiments, I include here a somewhat fuller 
account of the results to be expected as deduced from the theory which is to fohow. 
