THE COMBINING VOLUMES OF HYDROGEN AND OXYGEN. 
405 
This value was checked by running out mercury in a single operation from the 
3-way tap to the dead-space point. 
Weight of mercury from 3-way tap 
to dead-space point. 
Temperature. 
Volume. 
g r - 
4173-36 | 
0 C. 
0 in bulb 
21 in dead-space 
c.c. 
J 306-996 
Mean total volume . 
306-995 
For our purpose it was unnecessary to determine the volume of the pipette with 
very great exactness. Let the volume be represented by x, and let 2 x + y be the 
volume of hydrogen that unites with x of oxygen. Then the ratio of the combining 
volumes is + -~ , where x is more than three hundred times as great as y. A change 
of 0'5 c.c. in the value of x alters the ratio by only 1 part in 400,000. 
No sensible error is introduced, therefore, by calling the volume of the pipette 307 c.c. 
in round numbers. Again, there was no need to reduce the mercury weighings to 
the vacuum standard, since the small volume increments (represented by y) were also 
estimated by running out mercury which was counterpoised by the same set of weights. 
These weights were carefully compared with each other, but the deviations from their 
face values were far too small to involve a significant correction. 
Introduction and Measurement of the Gas. 
The exit tubes from the hydrogen and oxygen trains united beyond the taps, X and 
Y, at the T-piece, Q. The common channel then divided again, one branch leading to 
the mercury pump through the tap, It, and the other to the measuring bulb. After 
the large bath had been filled with powdered ice, gas was allowed to enter the pipette, 
displacing mercury into the reservoir, O, until the mercury surfaces in the dead-space 
and manometer chamber were approximately set to the two points. The tap, X or Y, 
as the case might be, was then closed. The ice in the large bath was pressed down 
and replenished, and the upper bath was also charged with ice. Finally, both baths 
were filled with ice-cold water, and the dead-space was cooled with brine. The fine 
adjustment was now made by means of the pressure-adjuster, S. By holding the 
small reservoir, T, at a suitable height and rapidly turning the tap, V, minute 
quantities of gas could be introduced into or withdrawn from the pipette. By the 
alternate manipulation of the pressure-adjuster and of the screws controlling a wooden 
clip which compressed the rubber tubing connecting the reservoir to the pipette, the 
