524 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



If the low vapour-densities were actually due to dissocia- 

 tion, they should be raised by adding one of the products 

 since this would tend to accelerate their recombination. 

 Wurtz found (foe. at. p. 608) that when the pentachloride 

 was vaporised at 160 to iy6 C. into an atmosphere of 

 phosphorus trichloride, it had the vapour density 7*226, 

 agreeing closely with the value of 7*217 calculated for the 

 formula PC1 5 . 



The dissociation of sulphur vapour. The decrease of 

 vapour density with rising temperature is shown in a still 

 more marked degree by sulphur, the vapour-density of which 

 falls from 6*57 at 524C. (Dumas, Ann. Chim. Phys., 1832, 

 50, 175) to 2*23 at 86oC. and io4oC. (Deville and Troost, 

 Ann. Chim. P/iys., 1860, 58, 287 and 298). The latter value 

 corresponds with the formula S 2 ; the former approximates 

 to that required by the formula S 6 ; later measurements at 

 lower temperatures and reduced pressures have given values 

 approximating to that required for the formula Sg. 1 It is 

 probable that the vapour contains molecules of two types, 

 as shown by the equation : 



but intermediate forms such as S 6 and S 4 are by no means 

 excluded. 



The dissociation of nitrogen peroxide discovered by 

 Playfair and Wanklyn (1862). Playfair and Wanklyn 

 discovered in 1862 (loc, cit. p. 156) that the vapour-density 

 of nitrogen peroxide (diluted with nitrogen) decreases with 

 rising temperature as follows : 



Temp ............. 4*2 11-3 24-5 97'5C. 



Density ........ 2-588 2*645 2 '5 2 I 7 8 3 



1 Bleier and Kohn (Ber., 1900, 33, 51) found S 7 . 8s at 193 C. and 

 2*1 mm. pressure. Biltz (Ber., 1901, 34, 2493) obtained an average 

 value 87-96 in ten readings at the normal boiling-point, under pressures 

 ranging from 200 to 540 mm. 



