424 
MR. W. CROOKES ON THE VISCOSITY 
690. I have plotted the above observations in diagrams A, B, and C. The portion 
between 760 millims. and 0'32 millim. not varying in logarithmic decrement is 
represented by a vertical line, shown in diagrams B and C below the principal curve. 
The remarkable character of hydrogen is its stiffness. It obeys Maxwell’s law 
almost absolutely up to an exhaustion of about 700 M. To this point the line of 
viscosity is almost perfectly vertical. It then commences to curve over, and when 
the mean free path begins to assume proportions comparable with the dimensions of 
the bull), and approaches infinity, the viscosity curve in like manner draws near the 
zero line. 
691. The repulsive force of radiation is higher in hydrogen than in any other gas. 
It commences at as low an exhaustion as 14 mill im s., but does not increase to any 
great extent till an exhaustion of 200 M is attained; it then rises rapidly to a maxi¬ 
mum at between 40 and 60 M, after which it falls away to zero. The maximum 
repulsion exerted by radiation in hydrogen is to that in air as 70 to 42"6. This fact 
is now utilised in the construction of radiometers and similar instruments when great 
sensitiveness is required. 
692. Taking the viscosity of air at 760 millims. as 0T124, and hydrogen as 0-0499, 
the proportion between them is 0'4439. Graham* gives the transpiration time for 
hydrogen as 0"4375 taking oxygen as unity, or 0'4855 taking air as unity (706). 
693. Professor Clerk Maxwell! found dry hydrogen to be much less viscous than 
air, the ratio of its viscosity to that of air being 0'5156. He says, referring to this 
result : “It appears, from the experiments of Mr. Graham, that the ratio of the 
transpiration time of hydrogen to that of air is O'4855, and that of carbonic acid to 
air 0‘807. These numbers are both smaller than those of this paper. I think that 
the discrepancy arises from the gases being less pure in my experiments than in those 
of Graham.” 
694. MM. Kundt and Warburg, in the paper already mentioned, assert that 
according to theory the viscosity should not begin appreciably to diminish as long as 
the layer of gas is thicker than fourteen times the mean length of path. They also 
declare that even when the thickness is as much as 300 times the mean length of path 
a perceptible diminution of the retarding forces commences, becoming greater as the 
pressure diminishes. They find the ratio between air and hydrogen to be 0*488, and 
the viscosity to vary according to the following table :— 
•e in millims. 
Log. decrement. 
380-0 
0-0652 
20-0 
0-0638 
8-8 
0-0629 
2-4 
0-0601 
1-53 
0-0557 
Above this exhaustion they found the viscosity rapidly diminished, but no measure¬ 
ments of pressure were taken. 
* ‘Chemical and Physical Researches,’ by Thomas Graham, p. 179. 
f “On the Viscosity or Internal Friction of Air and other Gases,” Phil. Trans., 1866, Part I., p. 257. 
