118 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



Siljestrom^ found departures from the law in the case of air and 

 oxygen. Amagat^ found the law obeyed, while Bohr^ found an 

 anomaly in the case of oxygen at 0.7 mm. pressure. In the curve 

 representing |> v plotted against p there is a sudden drop at this pres- 

 sure. Batelli's* experience corresponds well with that of Bohr. At 

 0.7 mm. with oxygen there is departure, while with air the departure 

 is at pressures between 2 mm. and 5 mm. Carbon dioxide departs from 

 the law, while hydrogen is found to obey it from one atmosphere to 

 0.002 mm. Baly and Ramsay ^ have pronounced the gauge worthless for 

 oxygen, while the results given for hydrogen are said to be reliable. 



Lord Rayleigh,^ working with values of p ranging from 1.5 mm. of 

 mercury to 0.01 mm., found no evidence of any anomaly with oxy- 

 gen, nitrogen, or hydrogen ; while at pressures ranging from 75 mm. to 

 150 mm. he found the law fully obeyed. 



There is, then, conflicting evidence, with much in favor of irregularity. 



Although with the gauge which Rayleigh employed in his investiga- 

 tion one can measure, with an accuracy of about five per cent, pressures 

 of 0.01 mm. of mercury, yet for pressures which in many cases must be 

 measured, this method is not sensitive enough, and then one is forced 

 back upon the use of the McLeod gauge, and that in a region where we 

 know nothing of its action. 



We are indebted to Sutherland 7 for the suggestion that the lowest 

 pressures may possibly be measured by measuring the friction of the 

 gas on a pendulum at pressures where the phenomenon of slip can be 

 detected experimentally. Having had some experience in determining 

 the absolute value of viscosity in gases, I have undertaken to investi- 

 gate the whole subject. 



As has been stated at the beginning of this paper, the problem 

 involves the solution of two others, which will now be discussed in 

 detail. 



The Friction Prohhm. 



When a gas flows over a solid surface at a uniform rate, or a solid 

 surface is made to move through a gas with uniform velocity, there is 

 brought into play a resistance to the motion due to friction. If, for 

 example, we have two parallel planes placed in a mass of gas at, say, 

 unit distance apart, and, while keeping one fixed, we cause the other to 

 move in a certain direction in its own plane, a certain force must be 



1 Pogg. Ann., 151, 1874. 



3 Wied. Ann., 27, 1880. 



6 Phil. Mag., 38, 18iH. 



f Phil. Mag., [5], 43, 1897. 



2 Ann. de Chim. et Phys., 28, 1883. 

 * Phys. Zeits., 3, 1901. 

 Phil. Trans., 196, 1901. 



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