388 
MR. W. CROOKES OK THE VISCOSITY 
INTRODUCTION. 
6 35.* By the viscosity or internal friction of a gas, is meant the resistance it offers to 
the gliding of one portion over another. In a paper read before the British Association 
in 1859, Maxwell! gives the following explanation of the internal friction of gases :— 
“ Particles having the mean velocity of translation belonging to one layer of the gas, 
pass out of it into another layer having a different velocity of translation, and by 
striking against the particles of the second layer exert upon it a tangential force which 
constitutes the internal friction of the gas. The whole friction between two portions 
of gas separated by a plane surface, depends upon the total action between all the 
layers on the one side of that surface upon all the layers on the other side.” 
The research here recorded commenced with experiments instituted to discriminate 
between the friction of the pivot supporting the fly of a radiometer and the viscosity 
of the residual gas. In the paper just quoted, Maxwell presented the remarkable 
result that on theoretical grounds the coefficient of friction, or the viscosity, should be 
independent of the density of the gas, although at the same time he states that the 
only experiments he had met with on the subject did not seem to confirm his views. 
An elaborate series of experiments were undertaken by Maxwell to test so remark¬ 
able a consequence of a mathematical theory ; and in 1866, in the Bakerian lecture for 
that year,j. he published the results under the title of “ The Viscosity or Internal 
Friction of Air and other Gases.” He found the coefficient of friction in air to be 
practically constant for pressures between 30 inches and 0'5 inch ; in fact, numbers 
calculated on the hypothesis that the viscosity was independent of the density agreed 
very well with the observed values. 
The apparatus used by Maxwell was not of a character to admit of experiments 
with much lower pressures than 0‘5 inch. 
636. In the Philosophical Magazine for July, 1875, is a translation of a paper by 
MM. Kundt and Warburg, “ On Friction and Heat Conduction in Rarefied Gases,” 
in which the laws theoretically discovered by Maxwell were examined at higher 
exhaustions. 
Maxwell’s theory, that the viscosity of a gas is independent of the density, 
presupposes that the mean length of path of the molecules between their collisions is 
very small compared with the dimensions of the apparatus; but inasmuch as the mean 
length of path increases directly with the expansion, whilst the distance between the 
molecules only increases with the cube root of the expansion, it is not difficult with 
the Sprengel pump to produce an exhaustion in which the mean free path is 
* The research embodied in this paper is an outcome of work recorded in my various papers “ On 
Repulsion resulting from Radiation,” Parts 1 to 6, “ On the Illumination of Lines of Molecular Pressure, 
and the Trajectory of Molecules,” and “ Contributions to Molecular Physics in High Vacua.” The 
paragraphs are therefore numbered in continuation of the last-named paper. 
t Phil. Mag., 4th ser., vol. xix., p. 31. 
J Phil. Trans., 186(1, Part I., p. 249. 
