100 DIFFUSION OF GASES 265 



Since this time observations have been made to decide 

 this question, first by Waltz 1 and von Obermayer 2 

 simultaneously, and somewhat later by Winkelmann. 3 

 All three observers found that the assumption of a constant 

 value of the coefficient of diffusion of two gases was not 

 sufficient if the experiments were made under varying cir- 

 cumstances. 



Waitz, who did not, like his predecessors, chemically 

 analyse the gaseous mixture that resulted at the end of an 

 experiment, but determined its composition by measuring its 

 refractivity during the course of the experiment, was able to 

 determine the value of the coefficient at different places of 

 the diffusion-apparatus at which the mixtures were not alike. 

 He thus obtained, as the means of a number of experiments 

 for three different heights in his apparatus, the values, 

 referred to the centimetre and second, 



0-151, 0-154, 0-158, 



for the coefficient of diffusion of carbonic acid into air at 

 19 C. and under the pressure 7 51 '6 mm. These figures 

 show an increase of the coefficient which is certainly not 

 great, but is still marked, and this increase of the coefficient 

 corresponds to an increase of the amount of carbonic acid 

 in the mixture, which agrees with the theoretical formula. 

 Similar results were obtained by the other observers named, 

 von Obermayer and Winkelmann ; their observations, 

 too, confirmed indubitably the fact that the constant of 

 diffusion varies with the ratio of the mixture of the gases. 



But when Waitz tried to go into details and to show 

 whether the observed variations of the coefficient conformed 

 numerically to the theoretical formula, a considerable devia- 

 tion seemed to appear. But it does not at all follow from 

 this that the formula is incorrect ; for an objection that is 

 mathematically justified may be raised against the mode of 

 calculation employed by Waitz. With regard to this, 



1 Wied. Ann, 1882, xvii. p. 201 ; Inaug. Diss. Berl. 1882. 



2 Wien. Sitzungsber. Abth. 2, 1882, Ixxxv. pp. 147, 748 ; Ixxxvii. p. 188. 



3 Wied. Ann. 1884, xxii. pp. 1, 152 ; xxiii. p. 203 ; 1885, xxvi. p. 105 ; 1886, 

 xxvii. p. 479. 



