ADDRESS. 27 



sifting from tlio molecules of the surrounding air the quick from the slow 

 movers. This sifting of the swift moving molecules is effected in liquids 

 whenever they evaporate, and in the case of the constituents of the 

 atmosphere, wherever it contains constituents light enough to drift away 

 molecule by molecule. In my mind's eye I see such a target as a piece of 

 metal cooler than the surrounding air acquiring the energy that gradually 

 raises its temperature from the outstanding effect of all its encounters 

 with the molecules of the air about it ; I see another target of such a 

 structure that it throws off the slow moving molecules with little exchange 

 of energy, but is so influenced by the quick moving missiles that it 

 appropriates to itself some of their energy. Let uranium or polonium, 

 bodies of densest atoms, have a structure that enables them to throw off 

 the slow moving molecules of the atmosphere, while the quick moving 

 molecules, smashing on to the surface, have their energy reduced and that 

 of the target correspondingly increased. The energy thus gained seems 

 to be employed partly in dissociating some of the molecules of the gas (or 

 in inducing some other condition which has the effect of rendering the 

 neighbouring air in some degree a conductor of electricity) and partly in 

 originating an undulation through the ethei*, which, as it takes its rise in 

 phenomena so disconnected as the impacts of the molecules of the air, 

 must furnish a large contingent of light waves of short wave-length. The 

 shortness in the case of these Becquerel rays appears to approach without 

 attaining the extreme shortness of ordinary Rontgen rays. The reduction 

 of the speed of the quick moving molecules would cool the layer of air to 

 which they belong ; but this cooling would rapidly be compensated by 

 radiation and conduction from the surrounding atmosphere ; under ordi- 

 nary circumstances the difference of temperature would scarcely be per- 

 ceptible, and the uranium would thus appear to perpetually emit rays of 

 energy with no apparent means of restoration. 



The total energy of both the translational and internal motions of the 

 molecules locked up in quiescent air at ordinary pressure and temperature 

 is about 140,000 foot-pounds in each cubic yard of air. Accordingly the 

 quiet air within a room 12 feet high, 18 feet wide, and 22 feet long 

 contains energy enough to propel a one-horse engine for more than twelve 

 hours. The store drawn upon naturally by uranium and other heavy 

 atoms only awaits the touch of the magic wand of Science to enable 

 the Twentieth Centuiy to cast into the shade the marvels of the 

 iN'ineteenth. 



Whilst placing before you the labours and achievements of my com- 

 rades in Science I seize this chance of telling you of engrossing work of 

 my own on the fractionation of yttria to which for the last eighteen years 

 I have given ceaseless attention. In 1883, under the title of 'Radiant 

 Matter Spectroscopy,' I described a new series of spectra produced by 

 passing the phosphorescent glow of yttria, under molecular bombardment 

 in vacuo, through a train of prisms. The visible spectra in time gave up 



