550 
DR. A. SCOTT ON THE COMPOSITION OP WATER BY VOLUME. 
when the gas now generated might be regarded as absolutely free from admixed air. 
Idle same was repeated with the oxygen apparatus. It was easy to arrange before 
raising the reservoir so that there was no “ untraversed space'’ by filling the 
tubes with mercury from the bulbs Nh and Nq. 
The mode of using the apparatus will be liest understood liy the description of an 
actual experiment. 
Oxygen was evolved from the oxide of silver in O b}^ heating till it was almost 
equal in pressure to that of the external atmospjhere, as indicated by the gauge and 
safety valve Vo, when taps 3 and 2 were opened, being about the level indicated, 
and the heating continued till B and A were almost filled to the mark on I), when 
the flame was extinguished and tap 2 shut, and then mercury was run into the fine 
tubes from No, so that it passed the tap 4 a short distance, that tap was then closed, 
and 2 opened carefully, so that the gas in the tube was driven into the calibrated 
space, and the mercury rvas stopped at the mark by closing taps 2 and 4. Tap 1 on 
the manometer was ojoened generally before filling the volume with gas, as one could 
judge better of the exact pressure by so doing. All being noAv ready for the final 
adjustments the mercury reservoir was raised or lowered by a fine screw on 
its holder till the mercury came exactly to the mark on D, and, after thorough 
Stirling of the water in L, left for a quarter of an hour, and the adjustments looked 
to, and then repeatedly stirred, and readings taken in about half an hour after filling 
if the volume was quite steady. The temperature and liarometric pressure were then 
read. Closing tap 1 on the manometer, and opening taps 2 and 7, the oxygen was next 
expelled into the jar H (which had been completely filled with mercury) by raising 
Mj sufficiently. When all the gas has been thus driven over into H, tap 7 is closed, 
is returned to its former position, tap 1 opened again, and the same operation 
repeated twice for the hydrogen. By opening tap 8 quantities of the mixed gases 
are drawn over into the explosion-tube J, and there caused to combine under 
diminished pressure by passing a spark, and this continued till nearly all has 
combined, when what remains in the explosion-tube is passed back into the jar H, 
and mixed and drawn back again, and exploded as Ijefore. A small residue of one or 
other gas in excess now remains. This was now passed into H, but the water formed 
was expelled by closing tap 8 and opening tap 9 when the water flowed out on to the 
surface of the mercury ; it was easy to arrange so that nothing but the gaseous residue 
was driven into H, from which it was now drawn into B, and its volume accurately 
determined. After measurement it was again expelled into H, and, if h^’drogen, S0D)e 
oxygen in excess was drawn into B, and measured as l)efore, passed into H, and the 
mixture exploded, and the new residue, measured as before, again passed back into H, 
and then into J, and expelled into an absorption-tube, measured and treated first 
with pure potassium hydrate solution to estimate any carbon dioxide, and then with 
solution of pyrogallol to absorb the oxygen and determine the amount of impurity, if 
any, present. The final residue gave thus a maximum value for any impurity which 
