ADDRESS. 25 



produced in favour of the doctrine that the essential fact in the pheno- 

 menon is electrified Radiant Matter. 



The speed of these molecular streams has been approximately measured, 

 chiefly by aid of my own discovery nearly twenty years ago, that their 

 path is curved in a magnetic field, and that they produce phosphorescence 

 where they impinge on an obstacle. The two unknown quantities, the 

 charge and the speed of each atom, are measurable from the amount of 

 curvature and by means of one other independent experiment. 



It cannot be said that a complete and conclusive theory of these rays 

 has yet been formulated. It is generally accepted that collisions among 

 particles, especially the violent collisions due to their impact on a massive 

 target placed in their path, give rise to the interesting kind of extremely 

 high frequency radiation discovered by Rontgen. It has, indeed, for some 

 time been known that whereas a charged body in motion constitutes an 

 electric current, the sudden stoppage, or any violent acceleration of such 

 a body, must cause an alternating electric disturbance, which, though so 

 rapidly decaying in intensity as to be practically ' dead beat,' yet must 

 give rise to an ethereal wave or pulse travelling with the speed of light, 

 but of a length comparable to the size of the body whose sudden change 

 of motion caused the disturbance. The emission of a high-pitched musical 

 sound from the jolting of a dustman's cart (with a spring bell hung on it) 

 has been suggested as an illustration of the way in which the molecules of 

 any solid not at absolute zero may possibly emit such rays. 



If the target on to which the electrically-charged atoms impinge is so 

 constituted that some of its minute parts can thereby be set into rhyth- 

 mical vibration, the energy thus absorbed reappears in the form of light, 

 and the body is said to phosphoresce. The eflicient action of the phos- 

 phorescent target appears to depend as much on its physical and mole- 

 cular as on its chemical constitution. The best known phosphor! belong 

 to certain well-defined classes, such as the sulphides of the alkaline-earthy 

 metals, and some of the so-called rare earths ; but the phosphorescent 

 properties of each of these groups are profoundly modified by an admix- 

 ture of foreign bodies — witness the effect on the lines in the phosphor- 

 escent spectrum of yttrium and samarium produced by traces of calcium 

 or lead. The persistence of the samarium spectrum in presence of over- 

 whelming quantities of other metals, is almost unexampled in spectro- 

 scopy : thus one part of samaria can easily be seen when mixed with three 

 million parts of lime. 



Without stating it as a general rule, it seems as if with a non- 

 phosphorescing target the energy of molecular impact reappears as pulses 

 so abrupt and irregular that, when resolved, they furnish a copious supply 

 of waves of excessively short wave-length, in fact, the now well-known 

 Rontgen rays. The phosphorescence so excited may last only a small 

 fraction of a second, as with the constituents of yttria, where the duration 

 of the different lines varies between the 0-003 and the 0'0009 second; 



