178 EQUILIBRIUM OF HETEROGENEOUS SUBSTANCES. 



when diluted with nitrogen. Since the relations expressed by equa- 

 tions (319) and (320) are not affected by the presence of a third gas 

 which is different from the gases G l and 6r 2 (to which m x and m 2 

 relate) and neutral to them (see the remark at the foot of page 171), 

 provided that we take p to denote the pressure which we attribute 

 to the gases G l and G 2 , i.e., the total pressure diminished by the 

 pressure which the third gas would exert if occupying alone the 

 same space at the same temperature, it follows that the relations 

 expressed for peroxide of nitrogen by (333), (334), and (336) will 

 not be affected by the presence of free nitrogen, if the pressure 

 expressed by p or p at and contained implicitly in the symbol D (see 

 equation (326) by which D is defined) is understood to denote the 

 total pressure diminished by the pressure due to the free nitrogen. 

 The determinations of Playfair and Wanklyn are given in the latter 

 part of the above table. The pressures given are those obtained by 

 subtracting the pressure due to the free nitrogen from the total 

 pressure. We may suppose such reduced pressures to have been 

 used in the reduction of the observations by which the numbers 

 in the column of observed relative densities were obtained. Besides 

 the relative densities calculated by equation (336) for the temperatures 

 and (reduced) pressures of the observations, the table contains the 

 relative densities calculated for the same temperatures and the pressure 

 of one atmosphere. 



The reader will observe that in the second and third experiments 

 of Playfair and Wanklyn there is a very close accordance between 

 the calculated and observed values of D, while in the second and 

 fourth experiments there is a considerable difference. Now the weight 

 to be attributed to the several determinations is very different. The 

 quantities of peroxide of nitrogen which were used in the several 

 experiments were respectively '2410, *5893, '3166, and '2016 grammes. 

 For a rough approximation, we may assume that the probable errors 

 of the relative densities are inversely proportional to these numbers. 

 This would make the probable error of the first and fourth observations 

 two or three times as great as that of the second and considerably 

 greater than that of the third. We must also observe that in the 

 first of these experiments, the observed relative density 1*783 is 

 greater than 1*687, the relative density calculated by equation (336) 

 for the temperature of the experiment and the pressure of one 

 atmosphere. Now the number 1*687 we may regard as established 

 directly by the experiments of Deville and Troost. For in seven 

 successive experiments in this part of the series the calculated relative 

 densities differ from the observed by less than *01. If then we accept 

 the numbers given by experiment, the effect of diluting the gas with 

 nitrogen is to increase its relative density. As this result is entirely 



