398 



VAPOR-DENSITIES. 



irregularity which is characteristic of errors of observation. We 

 should expect large errors in the observed densities, on account of the 

 difficulty of obtaining the substance in a state of purity, and because 

 the large value of the density renders it very sensitive to the effect of 

 impurities which diminish the density, also because the specific heat 

 of the vapor is great, as shown by the numerator of the fraction in 

 the second member of (13),* and because the density varies very 

 rapidly with the temperature as seen by the numbers in the third 

 column of Table VIII. 



TABLE VIII. PERCHLORIDE OF PHOSPHORUS. 



EXPERIMENTS OF MITSCHEBLICH, CAHOURS, WURTZ, AND TROOST 

 AND HAUTEFEUILLE. 



But at the two lowest temperatures of Cahours' experiments, the 

 differences of the observed and calculated densities ('381 and -568) are 

 not only great, but exhibit, in connection with the adjacent numbers, 

 a regularity which suggests a very different law from that of the 



* Compare Equilib. Het. Subs., this volume p. 180, and supra pp. 380-382. 



