394 
DR. F. P. BURT AND DR. E. C. EDGAR ON 
the two gases. Morley( 5 ), by a less direct method, arrived at the value 2'00269. 
(Clarke, on Morley’s data, gives 2 '00274.) A few experiments by Leduc( 6 ) 
yielded a considerably higher figure, 2'0037, and later, 2'0034, whilst Rayleigh( 7 ), 
from compressibility measurements, obtained the slightly lower value 2 '0026. 
The agreement between the values of Scott and Morley is exceedingly close, but 
in view of the uncertainty of some necessary corrections in Morley’s work, there is 
some excuse for a further series of determinations in which the erases, before combina- 
tion, are measured at 0° C. and 760 mm. pressure, that is to say, under the conditions 
of the density determinations. By adopting this procedure any errors due to uncer¬ 
tainty in the temperature and pressure coefficients are eliminated. This is the object 
of the present research. 
The experience of previous workers shows that the chief difficulty presented by 
the problem is the preparation of the two gases in a state of purity. To ensure the 
absence of traces of foreign gases, in particular chemically inert gases such as nitrogen 
or paraffin hydro-carbons which cannot readily be removed by chemical re-agents, the 
most stringent precautions must be taken. In selecting a method for preparing a 
gas it is less important to secure a high initial state of purity than to avoid the 
introduction of substances that are afterwards difficult to separate ; the presence of 
a trace of nitrogen may be much more troublesome than a considerable quantity of 
carbon dioxide. Treatment of gases by strongly heated solids is often unsatisfactory, 
because, although a particular impurity may be eliminated, some other impurity may 
be acquired. 
In this investigation we have relied on physical rather than chemical methods for 
the ultimate purification of the gases. The case of hydrogen is peculiar in that it 
cannot conveniently be liquefied, but advantage may be taken of this very fact to 
separate impurities that are more readily condensible. 
We began this investigation in July, 1911, and completed the experimental work 
in July, 1915. The apparatus was set up in an underground cellar below the 
Schunck Laboratory, and this room was reserved exclusively for our use. 
Preparation of Hydrogen. 
The work of H. B. Baker has shown that very pure gas is obtained by the 
electrolysis of an aqueous solution of barium hydroxide, and this is the method we 
have adopted throughout for generating hydrogen. The electrolysis was carried out 
in a U-tube constructed of boro-silicate glass (fig. 1, A), fitted with electrodes of 
stout platinum foil, about 27 mm. square. The platinum wire seals at B and B were 
mercury cupped. The two limbs of the U-tube were sealed to the soda glass 
connecting tubes by means of a glass of intermediate composition. The extension of 
each limb was bent over to form a short manometer which dipped into mercury; these 
manometers could be closed by the taps, C, C. T-pieces at E and F conducted the 
