RICHARDS AND ROGERS. — ATOMIC WEIGHT OF ZINC. 173 



The attempt was made also to distil the bromide in a vacuum, but 

 the reduction of the pressure lowered the boiling point too nearly to 

 the proximity of the melting point for convenient manipulation. 



In this connection it may be well to state the atomic weight of zinc 

 obtained from these specimens, in order to show their identity. The 

 details of these figures are given later. 



From the substance used in the preliminary determinations 



(Expt. 17) 65.410 



From new substance not crystallized from water but distilled 



in carbon dioxide (Expt. 14) 65.403 



From extreme mother liquors from crystallization (C) sub- 

 limed in carbon dioxide (Expt. 18) 65.404 



From purest crystals (A), twice crystallized from water and 



sublimed (Expt. 15, 19) 65.404 



From purest crystals (A) twice crystallized from water and 



twice distilled in carbon dioxide (Expt. 16) ... 65.398 

 Average Zn = 65.404 



Siloer. — The silver used in the final determinations was repeatedly 

 purified by the methods already described. Finallj^ the beautiful 

 electrolytic crystals were fused in a small crucible of pure lime in a 

 vacuum. In this way the metal may be obtaine.l in the purest possi- 

 ble state, for if it is distilled according to Stas there is always danger 

 of impurity from the oxygen and illuminating gas or hydrogen used in 

 the oxygen blowpipe. 



Other Materials. — The acids were purified in the usual fashion and 

 the greater part of the water used was only distilled twice, rejecting 

 the first portions. For experiment 16, all the water used was dis- 

 tilled three times, once over potassic permanganate. Of course the 

 platinum condenser which has been already described served for all of 

 these distillations.* 



Phosphoric pentoxide was sublimed in a stream of pure oxygen. 

 Since the presence of oxides of nitrogen in carbon dioxide might assist 

 the partial decomposition of zincic bromide, nitric acid was rejected as 

 a means of decomposing marble, and very dilute hydrochloric acid 

 was used instead. The gas was purified by passing through a solu- 

 tion of sodic hydric carbonate, long tubes containing argentic nitrate, 

 and much pure water. Since the last tube containing water gave 

 absolutely no test for chlorine after over a hundred litres of the gas 



* These Proceedings, XXX. 380. 



