RICHARDS. — ATOMIC WETGHT OF BARIUM. 61 



Hence the alkalimetric correction is found to be 0.00025 gram, and 

 after adding this weight to the observed weight of the dried salt, the 

 true per cent of water present in the original crystals is found to be 



14.7717. 



(17.) Again, 5.02356 grams (in vacuum) of the same sample were 

 dried in a platinum crucible at about 400° by means of the spirit lamp, 

 and then fused in a stream of pure dry hydrochloric acid gas in a double 

 crucible. The constant outflow of hydrochloric acid was sufficient to 

 prevent the absorption of sulphuric acid from the products of the com- 

 bustion of the illuminating gas, hence it was possible to use this source 

 of heat. Indeed, the highest temperature obtainable by means of a 

 powerful Bunsen burner is required to fuse the salt. After fusion, the 

 salt was found to weigh 4.28148 grams (in vacuum), having lost 14.7720 

 per cent. 



These results (14.7717 and 14.7720) agree within the limit of the 

 probable experimental error ; therefore very satisfactory confirmation 

 is siven to the conclusion arrived at before. Hence baric chloride is 

 a compound of sufficient definiteness to form a very satisfactory basis 

 for the determination of the atomic weight of barium. 



The other properties of the salt — its solubility in water and insol- 

 ubility in alcohol, the specific gravity of its solution, and so forth — 

 have been studied by Gay Lussac, Gerlach, Karsten, Kopp, Mulder, 

 and many others, and further remark upon them is unnecessary here. 



Preparation of Materials. 



Baric Chloride. — The ready crystallization of baric chloride upon 

 the addition of alcohol to a strong aqueous solution of the salt fur- 

 nishes an admirable method for the preparation of baric chloride 

 in a pure state. This method of crystallization was freely resorted 

 to in preparing the pure salt used in the preliminary experiments ; but 

 the details of the preparation need not be dwelt upon. The spectro- 

 scopic test of the purity of the salt has already been described at 

 length.* The purest specimens of baric chloride used in the final 

 experiments were prepared by three methods. According to the first 

 of these methods, after the salt had been crystallized two or three 

 times from water, it was precipitated five times in succession by the 

 addition of strong pure hydrochloric acid to its concentrated aqueous 

 solution. Each time the mother liquor was wholly discarded, and the 

 crystals were carefully drained. The last precipitate was then dis- 

 solved in water, and reprecipitated twice successively by means of the 



* These Proceedings, XXVIII. 7. 



