344 CAKNEGIE INSTITUTION OF WASHINGTON. 



CHEMISTRY. 



Baxter, Gregory P., Harvard University, Cambridge, Massachusetts. 

 Determination of atomic iveiyhts. (For previous reports see Year 

 Books Nos. 3-14.) 



During the past year investigations upon the atomic weights of 

 lead and zinc were continued and a new investigation upon that of 

 tin was both begun and completed. 



Mr. L. W. Parsons, after finding that lead amalgam, obtained by 

 electrolysis, could be satisfactorily washed with ether and dried in a 

 vacuum, analyzed fused lead chloride by electrolytic deposition of 

 tlie metal in a mercury cathode. Eleven experiments with quantities 

 of chloride varying from 5 to 19 grams yielded concordant results for 

 the atomic weight of lead, but these were somewhat higher than the 

 value found by Baxter, Grover, and Thorvaldson (see recent Year 

 Books) by determination of the halogen in lead chloride and bromide. 

 In a second series of experiments, in which a weighed quantity of 

 metal was electrolytically transported to a mercury cathode, a small 

 consistent gain in the product over the factors was found, amounting 

 on an average to 0.00013 gram per gram of lead. While the cause of 

 this gain has not yet been discovered, a suitable correction obviously 

 should be applied to the analyses of the chloride. When this is done, 

 the average result for the atomic weight of lead, 207.21, agrees almost 

 exactly with that obtained by comparison of the lead halides with 

 sUver, 207.20. 



Mr. J. H. Hodges appUed the electrolytic method of analysis to 

 zinc chloride. The great difficulty of preparing this salt in an anhy- 

 drous neutral condition was overcome by first dehydrating zinc 

 bromide by fusion in a dry atmosphere containing hydrobromic-acid 

 gas and then displacing the bromine in the fused salt by prolonged 

 fusion in a current of dry chlorine and hydrochloric-acid gases. Five 

 preliminary experiments, in which the metal was electrolytically depos- 

 ited in a weighed mercury cathode, yielded an average value for the 

 atomic weight of zinc of 65.38. This figure is essentially the same as 

 that found by the determination of both bromine (Richards and Rogers) 

 and metal (Baxter and Grose, see Year Book No. 14) in zinc bromide. 



Mr. H. W. Starkweather prepared tin tetrachloride by the action 

 of pure dry chlorine upon electrolytically purified tin. The product 

 was purified by fractional distillation in a vacuum at low temperature 

 and was collected in a series of sealed glass bulbs. Each bulb after 

 being weighed was broken under dilute hydrochloric-acid solution and 

 the metal was deposited in a weighed mercury cathode. The glass was 

 collected separately and weighed. Sixteen concordant analyses with 

 two series of preparations yielded the average result 118.70 for the 

 atomic weight of tin. This confirms very closely Briscoe's value recently 

 found by comparing the same compound of tin with silver. 



