446 
PROFESSOR G. G. STOKES ON MR. CROOKES’S EXPERIMENTS. 
for which /=0'0495. The ratios of corresponding pressures resulting from the numbers 
for the relative viscosities given on page 443, and the numbers to which they lead, are 
as follows :— 
Gas. 
Air. 
O. 
N. 
COo. 
CO. 
H. 
Ratios of corresponding pressures 
0-1387 
0-1396 
0-1386 
0-0748 
0-1386 
1 
Pressures corresponding to 330 M in H . 
45-8 
46T 
45 - 7 
24-7 
45‘7 
330-0 
1 calculated from l in H. 
•0990 
•1106 
•0961 
•0815 
•0961 
1 observed . 
•0758 
•0829 
•0722 
•0525 
•0734 
0-0495 
It would seem as if when a gas may be treated as a continuous mass with con¬ 
tinuously varying conditions of pressure, velocity, &c., as is done in the application of 
the hydrodynamical equations, a gas is completely defined'" as to its mechanical action 
by two constants, suppose the density at a standard pressure and the coefficient of 
viscosity; but when the conditions are such as oblige us to take account of the 
finiteness of path of the molecules, specific differences are manifested which oblige us 
to introduce at least one constant more in order that the gas may be even mechanically 
defined; for of course I am not contemplating the chemical properties. It is worthy 
of note in this connexion that the two gases, oxygen and kerosoline vapour, which 
showed the phenomenon of a rate of decrease of arc of vibration increasing with a 
decreasing density, are just those which lie at the two extremes as regards viscosity, 
while as regards density at a given pressure they are separated by carbonic anhydride, 
which nevertheless does not show the phenomenon in question. It may well be that 
the mode of encounter of such complex structures as the molecules of a gas varies from 
one gas to another; and that while some of the laws of gases admit of explanation 
when the molecules are regarded as elastic spheres, or as particles repelling one another 
according to some definite law of force, other properties fail to receive an explanation 
when such a simplification of conception is adopted. 
* I here leave out of account such, differences as the small deviations from Boyle’s law which have been 
observed with different gases. 
