222 PHENOMENA DEPENDENT ON MOLECULAR PATHS 86 



exactness. For it may be concluded, from Boyle's law 

 holding, that the assumptions on which the gaseous theory 

 rests are right ; and if this is the case, the chief law of 

 gaseous friction must also hold, viz. that the coefficient of 

 viscosity is independent of the pressure. 



Further experiments on the friction of vapours have 

 been made by Puluj 1 and Schumann 2 by the oscillation 

 method ; numerous experiments also on the transpiration of 

 vapours have been carried out by Lothar Meyer, 3 partly 

 by himself and partly in co-operation with Schumann 

 and Steudel. The law of the non-dependence of the coeffi- 

 cients of viscosity and transpiration on the pressure of the 

 vapour was proved, by the measurements of these coeffi- 

 cients, for the cases when the conditions of the experiments 

 were so chosen that the vapour was sufficiently far from its 

 point of condensation. The temperature, therefore, must be 

 so high and the pressure so low that the vapour can follow 

 the laws of perfect gases, and, therefore, can exhibit the 

 normal density, at least approximately. 



The values of the coefficient of viscosity measured under 

 such circumstances stand in a simple relation to the chemical 

 nature of the substances. Substances of similar constitution 

 have equal, or at least approximately equal, coefficients of 

 viscosity. Thus the following values of the coefficient of 

 viscosity were found always with tolerable exactness for the 

 different kinds of compounds, whatever value n might have 

 in the formulae, provided only that it exceeds 1 : 



Alcohols . . CnH^+^O TJ = 0-000142 



Chlorides . . C M .H 2W+1 C1 150 



Esters . . . C n H 2n 2 155 



Bromides . . C^H^+^Br 182 



Iodides . . . CnH^+J. 210 



Against the correctness of these numbers we may remark, 

 as Lothar Meyer did himself, that they do not agree 

 with the observations of Graham for methyl chloride and 

 ethyl chloride which are cited in 79. I believe, however, 



1 Wiener Sitzungsber. 1878, Ixxviii. Abth. 2, p. 279 ; Carl's Eepert. 1878, 

 xiv. p. 573. 2 Wied. Ann. 1884, xxiii. p. 353. 



;< Ibid. 1879, vii. p. 497 ; 1881, xiii. p. 1 ; 1882, xvi. pp. 369, 394. 



