VAPOR-DENSITIES. 399 



formula. In fact, the densities obtained by Cahours at atmospheric 

 pressure and those obtained by Troost and Hautefeuille at pressures 

 a little less than one-third of an atmosphere seem to form a continuous 

 series, notwithstanding the abrupt change of pressure. Yet it is 

 difficult to admit that the density is independent of the pressure. So 

 radical a difference between the behavior of this substance and that of 

 the others which we have been considering requires unequivocal evi- 

 dence. Now it is worthy of notice that the experiment at 182, in 

 which the greatest discrepancy is seen, is not given in the first record 

 of the experiments, which was in the Cwnptes Rendus in 1845. It is 

 given in the Annales de Chimie et de Physique in 1847, where it is 

 called the first experiment. (The experiment at 336 is also omitted 

 in the Comptes Rendus and that at 208 in the Annales, otherwise 

 the lists are the same.) If it was the first experiment in point of time, 

 which is apparently the meaning, it was made before the publication 

 in the Comptes Rendus, and we can only account for its omission" by 

 supposing that it was a preliminary experiment, in which its distin- 

 guished author did not feel sufficient confidence to include it at first 

 with his other determinations, although he afterwards concluded to 

 insert it. If we reject this observation as doubtful, the disagreement 

 between the formula and observation appears to be within the limits 

 of possible error, but additional experiments will be necessary to 

 confirm the formula.* 



Experiments have also been made by M. Wurtz in which the vapor 

 of the perchloride of phosphorus was diluted with that of the proto- 

 chloride.t These experiments may be used to test equation (8), 

 which, when the values of its constants are determined by equation 

 (13), reduces to the form 



log A = JJ -18-751, (14) 



6 



where p 5 , p 2 , and p 3 denote the partial pressures due respectively to 

 the PC1 5 , the C1 2 , and the PC1 3 , existing as such in the gas-mixture. 

 Since these quantities cannot be the subjects of immediate observa- 

 tion, a farther transformation of the equation will be convenient. 

 Let M 8 , M 2 denote the quantities of the protochloride and of chlorine 

 of which the mixture may be formed, and P 3 , P 2 the pressure which 



* Additional experiments on the density of this vapor have been made by M. Cahours, 

 concerning which he says in 1866 : " Les determinations qui je viens d'effectuer a 170 et 

 172 degres (ce corps bout vers 160 a 165 degre"s) m'ont donn4 des nombres qui, bien que 

 notablement plus forts que ceux que j'ai obtenus ante'rieurement & 182 et 185 degr&, 

 sont encore bien eloignes de celui que correspond 4 volumes," Comptes Rendus, t. 63, 

 p. 16. So far as the present writer has been able to ascertain, these determinations 

 have not been published. The formula gives 6 '025 for 170 and 5*973 for 172, at 

 atmospheric pressure. The number corresponding to four volumes is 7*20. 



t Comptes Rendiu, vol. Ixxvi (1873), p. 601. 



