732 
PROFESSOR 0. REYNOLDS OX CERTAIN DIMENSIONAL 
or more strictly with some general property of matter. And the general conviction 
which may be said to prevail at the present time is owing to the simplicity of the 
assumptions on which the molecular hypothesis is based, and the completeness with 
which many of the properties of gases have been shown to follow from the molecular 
hypothesis. 
But it will be readily seen that however simple may be the assumptions of the 
kinetic theory, and however completely the properties of gases may be shown to follow 
from these assumptions, this is no disproof of the possibility that gas may be a con¬ 
tinuous substance, each elementary portion of which is endowed with all the properties 
of the whole, and unless this is disproved there may exist doubt as to the necessity for 
the kinetic theory. 
Any direct proof, therefore, that gas is not ultimately continuous altogether alters 
the position of the molecular hypothesis. 
The sufficiency of the demonstration that gas is not structureless, 
6. In order to prove that gas is not continuous it is not necessary that we should be 
able to perceive the actual structure; we have only to find some property of a certain 
quantity of gas which can be shown not to be possessed by all the parts—some property 
which is altered by a re-arrangement of the parts. 
Hitherto I believe that no such property has been recognised, or at all events the 
conclusions to be drawn from such a property have not been recognised. The pheno¬ 
mena of transpiration as well as those of the radiometer depend on such properties, 
but these properties have not been sufficiently understood to bring out the conclusion. 
This conclusion however follows directly from the law indicated in Art. 4, viz.: that 
the results of transpiration and impulsion depend on the relation between the size of 
the internal objects and the density of the gas. 
The force of this reasoning will be better seen after the results of the experi¬ 
ments have been described, but it is introduced here to show the importance which 
attaches to what otherwise might be considered secondary properties of gases. 
To these properties I must now return, not having yet indicated how I was led to 
make the experiments, and besides those already mentioned there remains an impor¬ 
tant class of phenomena to be noticed. 
The results deduced from theory. 
7. Although the existence of the phenomena of thermal transpiration and the exis¬ 
tence of the law of corresponding results at corresponding densities have been verified 
by experiment they were not so discovered. 
They followed from what appeared to me to be a successful attempt to complete the 
explanation I had previously given* of the forces which must result when heat is 
communicated from a surface to a gas and the phenomena of the radiometer. 
Proc. Roy. Soc., 1874, p. 402, 
