Page:Philosophical Transactions of the Royal Society A - Volume 184.djvu/563

550 when the gas now generated might be regarded as absolutely free from admixed air. The same was repeated with the oxygen apparatus. It was easy to arrange before raising the reservoir M1, so that there was no “untraversed space” by filling the tubes with mercury from the bulbs NH and NO.

The mode of using the apparatus will be best understood by the description of an actual experiment.

Oxygen was evolved from the oxide of silver in O by heating till it was almost equal in pressure to that of the external atmosphere, as indicated by the gauge and safety valve VO, when taps 3 and 2 were opened, M1 being about the level indicated, and the heating continued till B and A were almost filled to the mark on D, 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 was stopped at the mark by closing taps 2 and 4. Tap 1 on the manometer was opened generally before filling the volume with gas, as one could judge better of the exact pressure by so doing. All being now ready for the final adjustments the mercury reservoir M1 was raised or lowered by a fine screw on its holder till the mercury came exactly to the mark on D, and, after thorough stirring 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 barometric 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 M1 sufficiently. When all the gas has been thus driven over into H, tap 7 is closed, M1 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 before. 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 hydrogen, some oxygen in excess was drawn into B, and measured as before, 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