22 



SMITHSONIAN" MISCELLANEOUS COLLECTIONS 



VOL. 54 



with an exhaustive criticism of the work done by Dumas and by Erd- 

 maun and Marchand. They show, as I have already mentioned, that 

 hydrogen dried by sulphuric acid becomes contaminated with sulphur 

 dioxide, and also that a gas passed over calcium cliloride may still retain 

 as much as one milligramme of water per litre. Fused caustic potash 

 they found to dry a gas quite completely. 



In their first series of syntheses, Dittmar and Henderson generated 

 their hydrogen from zinc and acid, sometimes hydrochloric and some- 

 times sulphuric, and dried it by passage, first through cotton wool, then 

 through vitrioled pumice, then over red-hot metallic copper to remove 

 oxygen. In later experiments it first traversed a column of' fragments 

 of caustic soda to remove antimony derived from the zinc. The oxide 

 of copper used was prepared by heating chemically pure copper clip- 

 pings in a muffle, and was practically free from siilphur. In weighing 

 the several portions of apparatus it was tared with somewhat lighter 

 similar pieces of as nearly as possible the same displacement. The re- 

 sults of this series of experiments, which are vitiated by the presence, 

 unsuspected at first, of sulphur dioxide in the hydrogen, are stated in 

 values of H when = 16, but in the following table have been recalcu- 

 lated to conformity with the earlier determinations : 



Mean, 15.949, ± .0103 



Eeducing to a vacuum, this becomes 15.843, while a correction for the 

 sulphur dioxide estimated to be present in the hydrogen brings the value 

 up again to 15.865. Still another correction is suggested, namely, that 

 as the reduced copper in the combustion tube, before weighing, was ex- 

 posed to a long-continued current of dry air, it may have taken up traces 

 of oxygen chemically, thereby increasing its weight. As this correction, 

 however, is quantitatively uncertain, it may be neglected here, and the 

 result of this series will be taken as = 15. 865, ±.0103. Its weight, 

 relatively to some other series of experiments, is evidently small. 



