NOYES AND GOODWIN. — VISCOSITY OF MERCURY VAPOR. 235 



Second Series. 

 ^ = 0.08. '!h^ = 4.04. V^ = 1.94. 



The relative values for mercury and carbon dioxide agree within about 

 four per cent. That of the second series is, however, more reliable, for 

 the reason above stated, and will be used in the subsequent calculations. 

 It may be stated that the value of the ratio T/cog ■ Vnz ^* ordinary temper- 

 ature is 1.72,* somewhat smaller than that found by us at 357°, a result 

 which is in accordance with the greater temperature coefficient of carbon 

 dioxide established by several investigators. 



The corresponding values of the relative mean cross-sections as cal- 

 culated by formula (1) are 



?li = 1.02; ^-2?- = 2.48. 



That is to say, the average cross-section of the mercury molecule or 

 atom is very nearly the same as that of the carbon dioxide molecule, and 

 is about 2^ times as large as that of the hydrogen molecule. This last 

 result does not differ very greatly from that (2.68) corresponding to the 

 relative molecular volumes of mercury and hydrogen as calculated for 

 300° by Koch. 



These results indicate that atoms and molecules are of the same order 

 of magnitude, and that the spaces between the atoms within the mole- 

 cule, if any exist, are not large ip comparison with those occupied by the 

 atoms themselves, and consequently the viscosity of gases, or any other 

 property which like it is dependent only on the size or form of the 

 molecules, is not adapted for distinguishing between monatomic and 

 polyatomic molecules. 



In considering the significance of the above values of the cross-section, 

 the different masses of the various molecules, to be sure, ought not to be 

 entirely disregarded. The mercury and carbon dioxide molecules have, 

 as we have seen, the same cross-section, and therefore, assuming both to 

 be of the same general form, they occupy the same volume. The mass 

 of the former is, however, 4.55 times as great as that of the latter. The 

 density of the mercury molecule is consequently greater in this same 

 proportion. But this difference is not marked enough to make it neces- 



* O. E. Meyer, Kinetische Theorie der Gase, p. 142. 



