1906.] on Studies on Charcoal and Liquid Air. 443 



and heating the cooled spot by contact with the hand, a permanent 

 brown part remains, whereas, if it had been all mercury it would have 

 completely volatilised. On standing for a little time the liquid air 

 cooling causes no metallic mirror, as all the mercury vapour has been 

 removed by combination with the sulphur, and the sulphide film re- 

 forms on the mercury, when the experiment may be repeated. 



Phosphoric Action at High Vacua. 



In the Friday Evening address of 1905 an experiment was shown 

 illustrative of the well-known fact that the phosphorous glow caused 

 by oxidation does not take place in dry oxygen at atmospheric pres- 

 sure, t.nd that it was only when the pressure was very considerably 

 reduced that such action took place. This may be shown more clearly 

 in the following way. A globe (Fig. 18), A, has a small capillary- 

 side tube, F, attached, containing a little phosphorus, that has been 

 melted at its end, and is at the same time connected by another 

 tube to a reservoir, D, containing charcoal covered with a layer of 

 phosphoric anhydride to absorb all traces of moisture. To 

 the side of this tube a small mercury gauge, G, is attached. 

 After the whole vessel is thoroughly exhausted it is filled up to 

 atmospheric pressure with pure oxygen gas and the whole sealed off. 

 On immersing the charcoal l3ulb in liquid air the pressure falls 

 rapidly to a fraction of a millimetre pressure, and soon the globe A 

 becomes filled with a phosphorescent glow, showing that chemical 

 action is taking place. As the absorption of the oxygen by the 

 charcoal continues the phosphorescence becomes less, and at last 

 disappears, nothing finally filling the bulb but the vapour of 

 phosphorus as it distils from P into the liquid air condenser. 

 Meanwhile the gauge Gr shows that the pressure at which the phos- 

 phorescence begins must be only a fraction of a millimetre, and that 

 it continues at a pressure below what could be measured on the 

 gauge. When the charcoal is taken out of the liquid air, and the 

 temperature allowed to rise, the course of the experiment is inverted, 

 the oxygen occluded in the charcoal being given ofl^, and as it meets 

 the vapour of phosphorus in the bulb A, suddenly combines with it 

 in a few oscillating flashes, and soon all is dark again. The ex- 

 periment may be repeated after weeks of keeping with the same 

 uniform results. The same thing is also very beautifully shown by 

 the arrangement in Fig. 19. Here the gauge is replaced by a 

 sparking tube B, having free access to the globe A by the tubes E 

 and F, the former having communication by means of the small hole 

 H ; D is the phosphorus. The high vacuum at which the glow 

 takes place in the bulb is shown by the character of the discharge 

 taking place in B. A curious phenomenon is observed on stopping 

 the electric discharge ; now the glow travels from the bulb along E 

 and F and meets in the middle of B. It looks as if the electric 



