THE VISCOSITY OF MERCURY VAPOR. 

 By a. a. Noyes and H. M. Goodwin. 



Presented October 14, 1896, by Charles R. Cross. 



The uncertainty which attaches to the specific heat ratio of gases as a 

 means of distinguishing between monatomic and polyatomic molecules 

 has been recently made evident by the extended discussions of the sig- 

 nificance of that property in connection with the atomic weights of argon 

 and helium. It is therefore of great interest to investigate other proper- 

 ties which may be expected to be related to the atomicity of the molecule. 

 Of such properties those dependent on the volume or cross-section of the 

 molecules seem most promising. We have therefore undertaken the 

 investigation of one of these, the viscosity or internal friction, in order 

 to determine whether a marked difference in its value exists in the case 

 of gases with monatomic and those with polyatomic molecules. To this 

 end we have made comparative measurements of the viscosity of hydro- 

 gen, carbon dioxide, and mercury vapor at the boiling temperature of the 

 last named substance. 



According to the Kinetic Theory of Gases the viscosity coefficient has 

 the theoretical significance expressed by the following equation,* 



77 = - NmLc, 



in which N is the number of molecules in the unit of volume, m the 

 mass of a single molecule, L the mean free path, and c the mean ve- 

 locity. Moreover, the free path L is dependent solely on the number of 

 molecules A^ and the mean cross-section 5' of a single molecule, or its 

 sphere of action. f 



* O. E. Meyer, Kinetische Theorie der Gase, 1st ed., pp. 130, 139. 



t Ibid., pp. 206, 218. The symbol Q used by the author represents the total 

 cross-section of all molecules in the unit of volume, and is therefore evidently 

 equal to Nq. 



