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PROFESSOR 0. REYNOLDS ON CERTAIN DIMENSIONAL 
silk, and at once accounts for the observed fact that the positive motion with the 
spider line was not .obtained until the pressures were somewhat lower than those 
necessary for the fibre of silk. 
52. Both with the fibre of silk and the spider line the phenomena of impulsion (the 
excess of pressure against warm surfaces) were apparent and consistent at densities 
many hundred times greater than the highest densities at which like results are 
obtained with vanes several hundred times broader than the fibre of silk ; this verifies 
the theoretical conclusion on which this part of the investigation was based. The 
results in this case are not so definite as is the agreement of the logarithmic homologues 
in the instances of transpiration ; but the one fact supports the other, and we may 
consider the law of impulsion—Law^ VIII., Art. 9—to have been sufficiently proved. 
This concludes the experimental investigation. 
PART II.—(THEORETICAL). 
Section V.— Introduction to the Theory. 
53. In suggesting in a former paper that the results discovered by Mr. Crookes 
were due to the communication of heat from the surface of the solid bodies to the gas 
surrounding them, I pointed out as the fundamental fact on which I based my explana¬ 
tion, that when heat is communicated from a solid surface to a gas, the mean velocity 
of the molecules which rebound from the surface must be greater as they rebound than 
as they approach, and hence the momentum which these particular molecules com¬ 
municate to the surface must be greater than it would be if the surface were at the 
same temperature as the gas. 
So far the reasoning is incontrovertible. But in order to explain the experimental 
results, it was necessary to assume that the number of cold molecules which approached 
the hot surface would be the same as if the surface were at the same temperature as 
the gas, or at any rate if reduced the number would not be sufficiently reduced to 
counteract the effect of increased velocity of rebound. 
Although at that time I could not see any definite proof of this, nor any way of 
definitely examining the question, yet I had a strong impression that the assumption 
was legitimate ; and although I hoped at some future time to be able to complete the 
theoretical explanation, I was content for the time to rest the evidence of the truth of 
the assumptions involved on the adequacy of the reasoning to explain the experimental 
results obtained. 
As other suggestions respecting the cause of the phenomena, widely different in 
character from mine, had found supporters, and a good deal of scepticism was expressed 
as to the fitness of the cause which I had suggested, my attention was occupied in 
deducing the actions which must result from such a force, and comparing them with 
experimental results. Having, however, at length satisfied myself, and seeing that a 
