OF ARTS AND SCIENCES. 9 



and strontium was added to five grammes of crystallized baric 

 chloride. The mother liquor from the first precipation by alcohol 

 was evaporated to dryness and extracted with alcohol. Calcium 

 was very evident in the extract, but no trace of strontium. The 

 reason for the apparent absence of the latter metal is to be found in 

 the fact that the mother liquor was evaporated to dryness. To 

 prove this, the same amount of materials were fractionally precipi- 

 tated three times, and a very evident strontium spectrum was given 

 by the last mother liquor. In the fifth trial, only one two-hun- 

 dredth of a milligramme of strontium was used. Upon three frac- 

 tionations no strontium could be detected; but upon dissolving 

 and reprecipitating each of the precipitates once more a faint test 

 for the metal was found in the final mother liquor. This is evi- 

 dently about the limit so far as strontium is concerned. Calcium 

 may be detected when it is present in quantities much less than 

 the two-hundredth of a milligramme, because of the ready solu- 

 bility of its chloride in alcohol. The baric chloride used gave 

 no trace of the calcium or strontium lines after most careful 

 fractionation.* 



From these experiments it may be concluded that, when a baric 

 salt shows no trace of the allied metals upon the treatment just de- 

 scribed, it does not contain a weighable amount of them. Never- 

 theless, in the work which follows, the purification was usually 

 continued long after the visible traces of strontium and calcium 

 had been eliminated. 



The Choice of Material. 



It has been already stated that the most satisfactory determina- 

 tions of the atomic weight of barium have had baric chloride as a 

 starting point. In many respects this substance is well adapted 

 for the purpose; but one serious cause of error must be carefully 

 guarded against in the usual method adopted for its analj^sis. The 

 well known solubility of argentic chloride influences the accuracy 

 not only of the weight of chloride obtained, but also of the apparent 

 end point of the precipitation after the method of Gay Lussac. 

 Long ago Stas f pointed out this cause of error, and carefully de- 



* A trace of sodium was always found in even the purest specimens. It is 

 probable that this trace was derived from the air during the course of fractional 

 treatment necessary to eliminate the barium. 



t Aronstein's translation of Stas's Memoir, pp. 46, 56, 59, and especially 295 

 (Leipzig, 1807). 



