VAPOR-DENSITIES. 397 



pressure as determined by Regnault's experiments, have been cal- 

 culated by the present writer by multiplying the numbers of the 



third column by 



As the height of the barometer in Horstmann's experimente is not 

 given, it has been necessary to assume P = 760. The inaccuracy due 

 to this circumstance is evidently trifling. The last two columns of 

 the table, which relate to different series of experiments by Horstmann 

 (a distinction not observed in other parts of the table), give the excess 

 of the densities thus obtained from Horstmann's and Regnault's 

 experiments above the values calculated from equation (12) with the 

 use of Regnault's determinations of pressure. 



The densities obtained by experiment are without exception less 

 than those obtained from equation (12). At the highest temperatures, 

 where the liability to error is the least, both in respect to the measure- 

 ment of the pressure of saturated vapor and in respect to the analysis 

 of the product of distillation, the results of experiment are most 

 uniform, and most nearly approach the numbers required by the 

 formula. At the lowest temperatures, the greatest observed density 

 is about one-eleventh less than that required by the formula, the 

 difference being about the same as between the highest and lowest 

 observed values for the same temperature. 



Since each successive purification of the substance employed by 

 Regnault diminished the pressure of its vapor, it is not improbable 

 that the pressures might have been still farther diminished by farther 

 purification of the substance. The pressures which we have used are 

 therefore liable to the suspicion of being too high, and it is quite 

 possible that more accurate values of the pressure would still farther 

 reduce the deficiency of observed density. 



Perchloride of phosphorus. For this substance, we have at 

 atmospheric pressure a single determination of vapor-density by 

 Mitscherlich,* and a series of determinations by Cahours;t at lower 

 pressures we have determinations by WurtzJ and by Troost and 

 Hautefeuille. In the experiments of Wurtz the pressure was reduced 

 by mixing the vapor with air. In Table VIII all these determinations 

 are compared with the formula 



- 3*6) 5441 



The differences between the calculated and observed values are often 

 large, in six cases exceeding '30; but they exhibit in general that 



* Pogg. Ann., vol. xxix (1833), p. 221. 



t Com/ptes Rendus, vol. xxi (1845), p. 625 ; and Annales de Chimie et de Physique, 

 ser. 3, vol. xx (1847), p. 369. 

 Gomptes Eendus, vol. Ixxvi (1873), p. 601. Ibid., vol. Ixxxiii (1876), p. 977. 



