EQUILIBRIUM OF HETEROGENEOUS SUBSTANCES. 177 



The determinations of MM. Deville and Troost are satisfactorily 

 represented by the equation 



(3178 -D)*p at , 3118-6 



which gives D = 3178 + 9- V9(3178 + 9), 



where Iog 10 9 = 9'47056 - 



In the first part of the following table are given in successive 

 columns the temperature and pressure of the gas in the several 

 experiments of MM. Deville and Troost, the relative densities calcu- 

 lated from these numbers by equation (336), the relative densities 

 as observed, and the difference of the observed and calculated relative 

 densities. It will be observed that these differences are quite small, 

 in no case reaching '03, and on the average scarcely exceeding -01. 

 The significance of such correspondence in favour of the hypothesis 

 by means of which equation (336) has been established is of course 

 diminished, by the fact that two constants in the equation have been 

 determined from these experiments. If the same equation can be 

 shown to give correctly the relative densities at other pressures than 

 that for which the constants have been determined, such correspon- 

 dence will be much more decisive. 



Messrs. Play fair and Wanklyn have published* four determinations 

 of the relative density of peroxide of nitrogen at various temperatures 



* Transactions of the Royal Society of Edinburgh, vol. xxii. p. 441. 

 G. I. M 



