Tkavers. — Presidential Address. 123 



"When Lenard made the kathode rays pass through 

 different gases, liquids, and sohds their behaviour proved 

 quite different from that of ordinary hght. Various sub- 

 stances are, we all know, not equally transparent to sunlight, 

 but their different degrees of transpai'ency depend upon their 

 inner structure or their chemical composition, not upon their 

 density. Glass has a greater density than paper, but it is 

 transparent to ordinary light, while paper is not." 



" With the kathode rays it was quite the reverse. Paper 

 was more transparent to them than glass, and aluminium, which 

 is slightly less dense than mica, was more transparent than 

 mica ; as to the denser metals, such as gold and silver, they 

 were quite opaque for the kathode rays even in very thin 

 leaves. The same was noticed with all gases : their trans- 

 parency, too, depended entirely upon their density. At the 

 ordinary atmospheric pressure the kathode rays ceased to act 

 upon the phosphorescent paper at a distance of a little over 

 2in., but in rarefied air they travelled a distance of 6ft. 

 without being absorbed ; and when Lenard experimented 

 upon gases of different densities, such as oxygen and hydrogen, 

 he found that it was sufficient to rarify oxygen to one- 

 sixteenth part of its usual density to render the two gases 

 equally transparent." 



" In short, the absorption of the kathode rays proved to be 

 in direct proportion to the density of the medium which they 

 passed through. ' Like inertia and gravity,' Lenard wrote 

 in December, 1895, ' the kathode rays depend in their ab- 

 sorption upon the mass of the matter they traverse. They 

 do not behave like light, but like a cannon-ball, which is 

 arrested in its course by the density of the heap of earth 

 which it has to pierce.' Moreover, while usual luminous 

 vibrations would take no heed of a magnet placed near their 

 path, the kathode rays explored by Lenard were deflected by 

 a magnet from their ordinary rectilinear directions. And yet — 

 such is, at least, Lenard' s opinion — the magnet acted not 

 upon the rays themselves, but upon the medium they passed 

 through ; and what seemed still more incomprehensible was 

 that the action of the magnet depended upon the way in which 

 the kathode rays were generated — the more the air was 

 rarefied in the vacuum tube where they took origin the 

 greater was the magnetic deflection." 



" At every step the physicist thus met with some new 

 problem which he could by no means explain under the now 

 cm'rent theory of luminous radiations. And finally, as if it 

 were to establish one more affinity between these extra- 

 ordinary rays and common light, Lenard discovered that 

 when a photographic plate was brought near to the aluminium 

 ' window ' the silver salts of the plate were decomposed by 



