Some time ago I communicated to the Society the results of an attempt to estimate the amounts of krypton and of xenon in air. The quantities were necessarily minimum estimates, for there is no doubt that both krypton and xenon must evaporate when air evaporates, even if that take place at a very low temperature. Dr. Travers and I guessed at the amounts of neon and helium, and supposed that the amount of helium was one or two parts per million, and that of neon one or two parts per 100,000. This guess is not very far from the truth, as the following account of recent experiments will show. The ingenious method discovered by Sir James Dewar of using cooled cocoanut charcoal as an absorbent for gases has made it easy to carry out the estimation. The process consists in cooling 100 grammes of such charcoal to — 100°, approximately, in a bulb from which all air has been removed by a pump. Such charcoal will absorb about three litres of air; at that temperature neither neon nor helium are absorbed in appreciable quantity, as special experiments showed. Hence on placing the cooled bulb containing the charcoal in communication with a Töpler pump, the uncondensed gases enter the barrel of the pump. On closing the connecting stop-cock, a further quantity of gas accumulates, and is again removed into the pump in the same manner. As the relative volumes of the pump and of the cooled bulb were approximately 4 to 1, after communication had been established four times, only 1/256 of the contents of the bulb were left in it. And as the gas in contact with charcoal exerts a kind of vapour-pressure, inasmuch as the pressure which it gives depends on the temperature as well as on the extent of the surface of charcoal to which it is exposed, it may be assumed that gas escapes from the charcoal on each reduction of pressure, and that the more volatile gases in the bulb should be expelled by the less volatile.
Influence of photoionization rates on positive streamer branching in atmospheric air
