EQUILIBRIUM OF HETEROGENEOUS SUBSTANCES. 179 



at variance with the facts observed in the case of other gases, and 

 in the case of this gas at lower temperatures, as appears from the 

 three other determinations of Playfair and Wanklyn, it cannot possibly 

 be admitted on the strength of a single observation. The first experi- 

 ment of this series cannot therefore properly be used as a test of our 

 equations. Similar considerations apply with somewhat less force to 

 the last experiment. By comparing the temperatures and pressures 

 of the three last experiments with the observed relative densities, the 

 reader may easily convince himself that if we admit the substantial 

 accuracy of the determinations in the two first of these experiments 

 (the second and third of the series, which have the greatest weight) 

 the last determination of relative density 2 '588 must be too small. In 

 fact, it should evidently be greater than the number in the preceding 

 experiment 2'645. 



If we confine our attention to the second and third experiments of 

 the series, the agreement is as good as could be desired. Nor will 

 the admission of errors of '152 and '120 (certainly not large in deter- 

 minations of this kind) in the first and fourth experiments involve 

 any serious doubt of the substantial accuracy of the second and third, 

 when the difference of weight of the determinations is considered. 

 Yet it is much to be desired that the relation expressed by (336), or 

 with more generality by (334), should be tested by more numerous 

 experiments. 



It should be stated that the numbers in the column of pressures 

 are not quite accurate. In the experiments of Deville and Troost 

 the gas was subject to the actual atmospheric pressure at the time of 

 the experiment. This varied from 747 to 764 millimeters of mercury. 

 The precise pressure for each experiment is not given. In the 

 experiments of Playfair and Wanklyn the mixture of nitrogen and 

 peroxide of nitrogen was subject to the actual atmospheric pressure 

 at the time of the experiment. The numbers in the column of pres- 

 sures express the fraction of the whole pressure which remains after 

 subtracting the part due to the free nitrogen. But no indication is 

 given in the published account of the experiments in regard to the 

 height of the barometer. Now it may easily be shown that a varia- 

 tion of n^ in the value of p can in no case cause a variation of more 

 than "005 in the value of D as calculated by equation (336). In any 

 of the experiments of Playfair and Wanklyn a variation of more than 

 3Qmm i n the height of the barometer would be necessary to produce 

 a variation of '01 in the value of D. The errors due to this source 

 cannot therefore be very serious. They might have been avoided 

 altogether in the discussion of the experiments of Deville and Troost 

 by using instead of (336) a formula expressing the relation between 

 the relative density, the temperature, and the actual density, as the 



