138 REPOET— 1886. 



pears to obey the above laws up to a certain temperature, to deviate 

 from them for a range of higher temperatures, and tu obey them for a 

 still higher range, all of which facts receive an obvious and natural ex- 

 planation on the supposition that Sn2Cl4 splits up gradually into two 

 molecules SnCl2 as the temperature rises. 



In face of all the above facts, which are of somewhat recent develop- 

 ment, and which result from the long-continued labours of V. Meyer, 

 Crafts and Meier, and others, it is difficult to hold the view expressed by 

 Berthelot,' that Boyle's (or Mariotte's) law, and Gay-Lussac's law have 

 only been proved for hydrogen, oxygen, and nitrogen, with the implied in- 

 ference that iodine is probably merely one of very numerous exceptions, and 

 that therefore Avogadro's law does not hold good for the halogens, and in 

 the other cases which are apparent exceptions to the other two laws. 



Deville and Troost — Vapour-densities determined by them in 1860 — Bearing 

 of their Results on the Behaviour of Iodine. 



Dumas, in his paper already mentioned, recognised the importance of 

 determinations of vapour densities as aiding the solution of chemical pro- 

 blems, and particularly in helping to give the cori'ect formula to a com- 

 pound. Deville and Troost- used substantially the same method as 

 Damas, except that by using porcelain globes instead of glass globes 

 they were able to determine vapour-densities of bodies which have very 

 high boiling-points. The matter of chief importance is to have a fixed 

 temperature above the boiling-point of the substance in the flask at 

 which the flask and its contents can be kept before closing it when ifc 

 is full of the vapour at the constant temperature. They used for constant 

 temperatures the boiling-points of mei'cury, sulphur, cadmium, and zinc ; 

 taken as 350°, 440°, 860°, and 1040° respectively. In this way Deville 

 and Troost determine the vapour- densities of water and aluminium chlo- 

 ride at the temperature of boiling mercury ; again, at the temperature 

 of boiling sulphur, the densities of air, iodine, mercurous chloride (4 vols.), 

 aluminium chloride, aluminium bromide, aluminium iodide, zirconium 

 chloride, ferric chloride ; in the vapour of boiling cadmium, 860°, the 

 densities of the following : iodine, air, sulphur, selenium ; in the vapour 

 of boiling zinc, the densities of iodine, air, ammonium chloride (4 vols.), 

 phosphorus, cadmium, selenium, and sulphur. 



The boiling-points of the substances above mentioned, whose vapour- 

 densities have been determined by Deville and Troost, were taken as : 

 water 100° ; aluminium chloride 180° ; aluminium bromide 260° ; zirco- 

 nium chloride (?) ; ferric chloride 306° ; iodine 250° ; sulphur 440° ; 

 selenium 665° ; phosphorus 287°; cadmium 860°. 



It will be seen that even if these boiling-points are not so accurate as 

 could be wished, in each case the temperature at which the vapour-density 

 was determined was far above the boiling-point of the substance. 



The substances chosen for giving invariable temperatures of boiling- 

 point were all elements : these were probably selected, among other 

 reasons, because elements were not likely to show any alteration at high 

 temperatures, and, therefore, any serious deviation from Gay-Lussac's 

 law. 



The vapour-density of iodine was used thi'ee times, viz., at tempera- 



' Annales de Chinue et de Physique, 1881 (5), xxii. p. 456. 

 = Ibid. (3), Iviii. p. 257. 1860. 



