434 
MR. W. CROOKES OK THE VISCOSITY OF GASES. 
Molecules ”"" I drew attention to the fact that a molecular raj producing green 
phosphorescence can be projected 102 millims. .from the negative pole when the 
pressure is as high as (P324 millim. or 427 M. In this case the mean free path of the 
molecules is 0'23 millim. ; and it is not surprising that with more powerful induction 
discharges, and with special appliances for exalting the faint action to be detected, the 
above-named phenomena can be produced at still higher pressures. 
719. It must be remembered that we know nothing of the absolute length of the free 
path or the absolute velocity of a molecule ; these may vary almost from zero to infinity. 
We must l im it ourselves to the mean free path and the mean velocity, and all that 
these experiments show is that a few molecules can travel more than a hundred times 
the mean free path, and with perhaps a corresponding increase over the mean velocity, 
before they are stopped by collisions. With weak electrical power, the special phos- 
phorogenic action of these few molecules is too faint to be noticed ; but by intensifying 
the discharge the action of the molecules can be so increased as to render their presence 
visible. It is also probable that the absolute velocity of the molecules is increased so 
as to make the mean velocity with which they leave the negative pole greater than 
that of ordinary gaseous molecules. This being the case, they will not easily be 
stopped or deflected by collisions, but will drive through obstacles, and so travel to a 
greater distance. 
If this view is correct, it does not follow that gas and ultra-gas can co-exist in the 
same vessel. All that can be legitimately inferred is, that the two states insensibly 
merge one into the other, so that at an intermediate point we can by appropriate 
means exalt either the phenomena due to gas or to ultra-gas. The same thing occurs 
between the states of solid and liquid, and liquid and gas. Tresca’s experiments on 
the flow of solids prove that lead and even iron, at the common temperature, possess 
properties which strictly appertain to liquids, whilst Andrews has shown that liquid 
and gas may be made to merge gradually one into the other, so that at an intermediate 
point the substance partakes of the properties of both states. 
* Phil. Trans., Parti., 1879: The Bakerian Lecture. 
