1878.] the Vapours of the Chlorides of Thallium and Lead. 427 



to ascertain the specific gravity of vapours at a high temperature that, 

 in spite of the interest which attaches to this subject, but few addi- 

 tions have been made in our knowledge in this direction since the 

 researches of Deville and Troost. 



The present experiments, of which this notice contains the first 

 results, have been made with the object of so simplifying the process 

 as to render it easy to determine the specific gravity of the vapours of 

 bodies possessing high boiling-points with a degree of accuracy suffi- 

 cient for the purpose of controlling their molecular weights. 



The method consists in vaporizing the substance under examina- 

 tion in long-necked glazed porcelain globes of known capacity placed 

 in a muffle raised to bright redness. The temperature of the globe is 

 ascertained by a calorimetric determination made with heavy platinum 

 weights placed in the muffle, this determination being checked by the 

 simultaneous insertion in the muffle of a second globe containing 

 mercury. 



The porcelain globes having a capacity of about 300 cub. cen- 

 tims., and containing from 3 to 9 grams of substance, are closed by 

 loosely fitting stoppers of baked clay, and then gradually introduced 

 in the muffle. After remaining there until no further escape of vapour 

 is observed, and until the temperature has become constant, the globes 

 are quickly withdrawn from the muffle and their contents removed 

 and analysed, the temperature being in each case ascertained by the 

 calorimetric method at the time of withdrawal of the globe. The fol- 

 lowing determinations of the specific gravity of mercury vapour serve 

 to show the reliability of the method : 



Temperature 

 determined 

 calorimetrically. 



Experiment 1 1019° . , 



II 894° . 



III 815° ., 



IV 972° 



Y 1047° . 



Specific gravity of 

 mercury yapour. 



•92 

 •75 

 •91 

 •77 



•05 



the calculated specific gravity (Hg=198'8) being 6"728. 



Before determining the specific gravity of the vapour of thallium 

 chloride it was ascertained that this compound does not give off free 

 chlorine when volatilized at a red-heat, and that the sublimate contains 

 thallium and chlorine in the atomic ratio of equality. 



In each experiment the total amount of thallium and of chlorine 

 remaining in the globe was determined by analysis, and the specific 

 gravity calculated from their sum. 



2 f2 



