ATOMIC WEIGHTS 15 



It would be easy to point out the sources of error in the foregoing sets 

 of determinations, bnt it is hardly worth while to do so in detail. A few 

 leading suggestions are enough for present purposes. First, there is an 

 insignificant error due to the occlusion of hydrogen by metallic copper, 

 rendering the apparent weight of the latter a trifle too high. Secondly, 

 as shown by Dittmar and Henderson, hydrogen dried by passage through 

 sulphuric acid becomes perceptibly contaminated with sulphur dioxide. 

 In the third place, Morley ' has found that hydrogen prepared from zinc 

 always contains carbon compounds not removable by absorption and 

 washing. Erdmann and Marchand themselves note that their zinc con- 

 tained traces of carbon. Finally, copper oxide, especially when prepared 

 by the ignition of the nitrate, is very apt to contain gaseous impurities, 

 and particularly occluded nitrogen.^ Any or all of these sources of 

 error may have vitiated the three investigations so far considered, but it 

 would be useless to speculate as to the extent of their influence. They 

 amply account, however, for the differences between the older and the 

 later determinations of the constant under discussion. 



Leaving out of account all measurements of the relative densities of 

 hydrogen and oxygen, to be considered separately later, the next de- 

 termination to be noted is that published by J. Thomsen in 1870.' 

 Unfortunately this chemist has not published the details of his work, 

 but only the end results. Partly by the oxidation of hydrogen over 

 heated copper oxide, and partly by its direct union with oxygen, Thom- 

 sen finds that at the latitude of Copenhagen, and at sea level, one litre of 

 dry hydrogen at 0° and 760 mm. pressure will form .8041 gramme of 

 water. According to Eegnault, at this latitude, level, temperature, and 

 pressure, a litre of hydrogen weighs .08954 gramme. From these data, 

 = 15.9605. It will be seen at once that Thomsen's work depends in 

 great part upon that of Eegnault, and is therefore subject to the correc- 

 tions recently applied by Crafts and others to the latter. These cor- 

 rections, which Avill be discussed further on, reduce the value of from 

 15.9605 to 15.91. In order to combine this value with others, it is neces- 

 sary to assign it weight arbitrarily, and as Thomsen made eight experi- 

 ments, which are said to be concordant, it may be fair to rank his 

 determination with that of Erdmann and Marchand, and to assume for 

 it the same probable error. The value 15.91, ±.0113 will therefore be 

 taken as the outcome of Thomsen's research. 



In 1887 Cooke and Richards published the results of their elaborate 

 investigation.* These chemists weighed hydrogen, burned it over copper 



1 Amer. Chem. Journ., 12, 469. 1890. 



- See Richards' work cited in the chapter on copper. 



^Ber. Deutsch. chem. Ges., 3, 928. 1870. 



*Proc. Amer. Acad., 23, 149. Amer. Chem. Journ., 10, SI. 



