1906.] on Studies on Charcoal and Liquid Air. 445 



Recent determinations by the charcoal method, made by Sir 

 W. Ramsay, give these proportions as one part in 80,790 of neon, 

 and one part in 245,:^>00 of helium or one part in 61,000 of both 

 these gases together. The absorption of 30 litres of air by 500 

 grammes of charcoal gives a residuary pressure amounting to 

 between 1 * 4 and 1 * 5 mm. of mercury pressure ; thus representing 

 a partial pressure of unabsorbed gases, helium, neon and hydrogen 

 amounting to 53^^00 of the volume of the original air used in the 

 experiment. 



Separation of the less volatile gases, Krypton and Xenon, ly Charcoal. 



The method of separating carbonic acid from air shown in the 

 Friday Evening Address for 1905 is equally applicable to the separa- 

 tion of krypton and xenon. A current of air passes through a series 

 of tubes immersed in liquid air for the purpose of purification, the 

 final tube containing cotton wool in order to retain any dust of the 

 solid condensed impurities. This pure air is passed through a tube 

 containing about 100 grammes of charcoal, the current being main- 

 tained for at least 24 hours. The charcoal tube is now removed and 

 placed in solid carbonic acid, any gas coming off being allowed to 

 escape. The gas remaining in the charcoal at - 78° C. is now got 

 out by heating and exhaustion, and all the carbon compounds and 

 oxygen removed from it. The remaining gas, nitrogen containing 

 krypton and xenon, is separated into its constituents by condensation 

 and fractionation. Instead of passing a cuiTent of air over char- 

 coal at - 183° C, a few hundred grammes of charcoal may be 

 covered with old liquid air, and the latter allowed to evaporate in a 

 silver vacuum vessel, the gases remaining in the charcoal being 

 separated as above. In this way krypton and xenon are readily 

 separated from air and spectrum tubes easily prepared. 



Charcoal Thermometer and Calorimeter. 



Charcoal saturated with gases at low temperatures may be used 

 as a thermoscope or calorimeter. Attention has been called to the 

 law^ of absorption, that when the volume is constant, the pressure falls 

 very rapidly with the temperature ; in fact, the curve connecting them, 

 when pressure is plotted as ordinate to temperature as abscissa, drops 

 almost vertically as the temperature diminishes to the absolute zero, 

 and a similar relation holds for the volume absorption at constant 

 pressure. Hence, a small change of temperature is accompanied by a 

 great change of pressure or volume absorption as the case may be. 

 This was exemplified in the following experiment. A charcoal 

 bulb A (Fig. 20) previously saturated with air at - 185° C. or hydrogen 

 at - 253° C. at any required pressure, the most convenient being 



