468 REPORT— -1888. 



molecules witli FeCIs molecules at temperatures above tlie volatilisation- 

 point of ferric chloride is, therefore, an hypothesis for which there appears 

 at present to be no sufficient experimental proof; but the cases of acetic 

 acid and nitric peroxide are somewhat similar, and in the latter case 

 Ramsay has by Raoult's method found,' besides the formula NO2 at high 

 temperatures, that there is a formula N^204 ^or low temperatures, although 

 the vapour-densities at low temperatures above the boiling-point are 

 always intermediate between those required by the two formulae. 



Vapour-densities — Friedel and Crafts. 



In Crafts' study relative to the vapour-density of iodine - he showed 

 that, contrary to the opinion of V. Meyer at that time, the results got by 

 V. Meyer's process must differ from those by Dumas' process, where in 

 the former process the vapour-density can vary with the tension i.e., 

 vapour-pressure (of the iodine), as well as when there is dissociation. 

 Thus, in V. Meyer's process it is generally conceived that the substance 

 which is volatilised in the vapour-pressure cylinder acts by forming a 

 more or less dense layer of vapour at the bottom of the cylinder, which, 

 expanding rapidly, acts like a piston in expelling a quantity of (generally) 

 nitrogen, the volume of which at the high temperature employed is equal 

 to that of the expanded vapour, if, however, the temperature is very 

 much above the boiling-point or the volatilising-point of the substance 

 vapourised, the conditions are different, the vapour diffusing to a con- 

 siderable extent in the space occupied by the nitrogen ; in this case there 

 is no longer reason to expect that the volume of the nitrogen expelled 

 measures the amount of the substance volatilised in the manner contem- 

 plated by V. Meyer's method. 



Troost ^ had found a variety of different densities of iodine-vapour at 

 400°, and at pressures varying from 768 mm. to 34'52, these densities 

 varying from 870 to 7'35, the density corresponding to I, compared 

 with air being 88 ; and attributed these results to a deviation from 

 Gay-Lussac's law of dilatation. 



Now Crafts admits a difference of the vapour-density due to reduction 

 of vapour-pressure by diffusion, the diffusion having the effect of increas- 

 ing its volume, diminishing its pressure, and thus bringing it nearer to 

 the state of a gas considerably above its boiling-point at the reduced 

 pressure, and more nearly to the state of a perfect gas ; the results of 

 Crafts and Meier in one set of experiments for iodine were obtained by a 

 ^modification of Dumas' method, the globe being filled partly with air, 

 partly with vapour of iodine, the vapour- pressure of the iodine in some 

 of the experiments being a tenth of an atmosphere. In correctly calcu- 

 lating from the experimental results the densities of iodine-vapour, 

 account has to be taken of the deviation of iodine from Boyle's and Gay- 

 Lussac's laws ; but Crafts and Meier ^ found that at the reduced pressure 

 the rate of dilatation from 355° upwards was the same as that of air. 

 The vapour-densities (referred to air at the same pressure and tempera- 

 ture) at still higher temperatures were attributed by him to a gradual 

 dissociation, the rapidity of which was increased by diminished pressure, 

 as happens in the case of Friedei's hydrochlorate of methyl oxide, and as 

 shown by Lemoine for HI. 



' a S. J. 1888, 621. - C. R. 02, 1881, p. .39. 



s a li. 91, 1880, p. 51. * C. It. 92, 1881, p. 39. 



