﻿Attraction of Unlike Molecules. 



9 



The experimental determinations for any one pair of gases 

 are not numerous enough, or at widely enough separated 

 temperatures, to test this theoretical relation between D and 

 T, but as the corresponding relation in viscosity has been 

 verified experimentally over a temperature range of 1400 

 degrees, it is not necessary to have such verification here 

 before proceeding. From a single determination of the ratio 

 of D 2 to D x at any two temperatures T 2 and T x we can calculate 

 from (8) a value of xC 2 which is a measure of the potential 

 energy of molecules 1 and 2 in contact. Loschmidt was the 

 first to measure diffusion-coefficients at one, or at different 

 temperatures, but as von Obermayer's results are later and 

 more elaborately determined it will suffice to use them. He 

 has measured the diffusion-coefficients of six pairs of gases at 

 ordinary temperatures, and at 61°*5 C. (Sitz. Akad. Wien, 

 lxxxi.), namely, those of carbon dioxide with air, hydrogen, 

 and nitrogen monoxide N 2 0, and those of oxygen with 

 hydrogen, nitrogen, and carbonic oxide. In the following 

 table are given for each pair of gases the two temperatures 

 of v. Obermayer's experiments, the values of the diffusion- 

 coefficients at these temperatures, and the values of ^2 for 

 each pair calculated therefrom. The diffusion-coefficients are 

 given in terms of the centimetre and second as units, and are 

 the volumes of the gases in cub. centim. measured at the 

 temperature of the experiment and at a pressure of 1 atmo- 

 sphere which pass in one second in a diffusion-stream where 

 the fall of the partial pressure of each gas is 1 atmosphere 

 in 1 centimetre : — 



Pair of Gases. 



T. 



D. 



A- 



C0 o and Air 



281 

 334-5 



284 

 334-5 



284 

 334-5 



286 

 334-5 



286 

 334-5 



273 

 334-5 



•143 

 •201 



•580 

 •773 



•0996 

 •1394 



•736 

 •967 



•193 

 •256 



•187 

 •269 



250 



106 

 380 

 100 

 136 

 124 



C0 2 andH 2 



C0 2 andN 2 



0, and H 2 



2 andN 2 



2 andCO 





If the law of force is that of the inverse fourth power, or 



