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Prof. 0. Reynolds on certain Dimensional [Feb. 6, 



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 as regards both 

 thermal transpiration and the law of corresponding densities for 

 transpiration and impulsion. 



Having made these deductions I 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 render- 

 ing 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 in this paper is to describe the reasoning by which 1 

 was led to undertake the experiments, as well as the experiments 

 themselves ; but as the theory will be better understood after an 

 acquaintance with the facts, I have inverted the natural order and 

 given the experiments first. I include here, however, a somewhat full 

 account of the results to be expected as deduced from the theory. 



Then follows a statement of the laws of transpiration and impulsion 

 as deduced from theory : — 



Section II is devoted to the description of the experiments on 

 thermal transpiration ; Section III to the experiments on transpiration 

 under pressure, and Section IV to the experiments on impulsion. 



In this abstract it will not be possible to give more than a sketch 

 of the matter contained in these sections. The numerous precautions 

 and tests will have to be left unnoticed, and only a few of the experi- 

 mental results can be given. The investigation occupied from 

 February, 1878, till the beginning of August, every result being 

 verified by repeated experiments. 



The Apparatus for Thermal Transpiration. 



This consisted principally of an instrument called a thermo-diffiisio- 

 meter, of which the essential feature is two chambers, separated by a 

 plate of porous material, means being provided for keeping the- 

 chambers at constant, but different, temperatures for many hours at a 



