372 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Thus, Pelouze, Marigoac, and Dumas all obtained low results with 

 the method of Gay-Lussac ; in fact, the error sometimes exceeded the 

 tenth of one per cent. The cause of this error, which apjjeared also 

 in the work of these expeiimeuters upon barium, has already been 

 pointed out in another paper.* 



We are thus led to infer that the true molecular weight of strontic 

 chloride must exceed the usually accepted value 158.4 by about one 

 tenth of one per cent, and that the true atomic weight of strontium 

 must be nearly 87.7. This inference is confirmed by the result of the 

 investigation now to be described. 



The balance and weights, and the methods of weighing and of tabu- 

 lating results employed in the work recounted below have already 

 been described in sufficient detail. f The balance seems to have in- 

 creased slightly in sensitiveness during its four years' work, owing 

 perhaps to the smoothing of microscopic roughnesses in the bearings. 

 It is almost needless to say again that the weights were carefully stan- 

 dardized from time to time, and the small, surprisingly constant cor- 

 rections were always applied. The correction to the vacuum standard 

 was calculated by the usual formula : 



( "-""'^"^ 0.000156^ 2L 2P\ 



\sp. gr. substance / t 60 273° + t° 



= correction in grams for 1 gram of substance, j 



The values thus calculated for the appropriate substances at 20° 

 and 760 mm. were as follows : 



Correction to be applied to One Gram of Substance. 



Gram. 



Silver —0.000031 



Argentic bromide +0.000043 



Strontic bromide +0.000141 



The general plan of the following work was similar to that adopted 

 in the case of barium. For obvious reasons the bromide of strontium 

 was chosen as the starting point ; and the investigation began with a 

 study of the properties of the salt, in order to determine its fitness for 

 the purpose. 



The atomic weight of silver is assumed to be 107.93, and that of 

 bromine 79,955, unless a definite statement to the contrary is made. 



* Tliese Proceedings, XXIX. 80. 

 t These Proceedings, XXVI. 242 ; also XXVIII. 5. 



J H = atmosplieric pressure; <° = atmospheric temperature at the time of 

 weigliing ; 0.000156 = standard weiglit of air displaced by 1 gram of brass. 



