244 Prof. Bunsen on Ccssium. 



21*0513 per cent, of chlorine was contained in the salt. This 

 number very closely agrees with the mean from Johnson and 

 Allen's experiments, viz. 21*0452. The more exact atomic 

 weight of caesium is therefore — 



According to Johnson and Allen's determinations . 133*03 

 According to my own determinations 132*99 



Hence, until a more exact determination be made, we may take 

 the atomic weight of caesium, as the mean of these two determi- 

 nations, to be 133. 



In the paper already referred to*, I remarked that chloride of 

 caesium readily attracts moisture from the air and deliquesces. 

 Johnson and Allen, on the contrary, state that the salt prepared 

 by them, and used for their determination of the atomic weight, 

 did not possess this property. They say in their memoir, " We 

 thus obtained an amorphous mass, of a pure white colour, which, 

 unlike Bunsen's chloride, was not perceptibly deliquescent even in 

 a very moist atmosphere.-" And further, " As to the properties 

 of the chloride of caesium, we observed that not only is it not 

 deliquescent, but it is hardly hygroscopic. The unfused and 

 porous salt may be weighed in moist air with as much accuracy 

 as chloride of sodium/' In spite of this statement, I must 

 adhere to my previous observation. Even the chloride of cae- 

 sium which may be considered to be spectrum-analytically 

 pure, having the atomic weight of 133, exhibits a high de- 

 gree of deliquescence in moist air, as is seen from the fol- 

 lowing experiments: — 0*02715 grm. of powdered chloride of 

 caesium, which had been evaporated with a few drops of hydro- 

 chloric acid and dried for two hours at 130°, when spread 

 out upon a platinum capsule deliquesced in a few hours to a 

 liquid in which no solid substance could be seen. In the first 

 two hours the weight of the salt had increased by 0*0068 grm., 

 in the second two hours 0'0065 grm., and in the third two hours 

 0*0030 grm., thus taking up within six hours a quantity of 

 water from the air equal to half its own weight. 22 milli- 

 grammes of the fused salt deliquesced quickly in the air, and 

 after fourteen hours had taken up 19 milligrammes, or nearly its 

 own weight of water. In dry air only, this liquid dries up to a 

 solid mass. The chlorides of potassium and sodium, portions of 

 which were exposed simultaneously to the same air as the chlo- 

 ride of caesium, did not exhibit any deliquescence even after an 

 exposure of twenty-four hours. Hence Messrs. Allen and John- 

 son seem to have made their experiments in comparatively dry 

 air, in which, as is well known, many deliquescent substances 



* Phil. Mag. vol. xxii. p. 502. 



