OXYGEN. 19 



The mean of all the twenty-four determinations, taken as one series, 

 with the correction to the third series included, is = 15.8966, ± .0017. 

 In sum, there were consumed 18.-5983 grammes of hydrogen and 147.8145 

 of oxygen ; whence = 15.8955. 



Dittmar and Henderson,* who effected the synthesis of water over 

 copper oxide b}^ what was essentially the old method, begin their memoir 

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

 mann 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 chloride may still retain 

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

 they found to dr}^ 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 tlirough cotton wool, then 

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

 oxygen. In later experiments it first traA'ersed 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 sulphur. 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 the usual unit : 



Mean, 15.949, it .0103. 



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

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



* Proc. Roy. Soc. Glasgow, 22, 33. Communicated Dec. 17, 1890. 



