606 



SCIENGE. 



[N. S. Vol. VIII. No. 201. 



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 

 Eontgen. 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 accele- 

 ration of such a body, must cause an alter- 

 nating 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 com- 

 parable to the size of the body whose sud- 

 den change of motion caused the disturb- 

 ance. The emission of a high-pitched 

 musical sound from the jolting of a dust- 

 man's cart (with a spring bell hnng 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 rythmical vibration, the enei'gy 

 thus absorbed reappears in the form of 

 light, and the body is said to phosphoresce. 

 The efficient action of the phosphorescent 

 target appears to depend as much on its 

 physical and molecular 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 admixture of foreign bodies 

 — witness the effect on the lines in the 

 phosphorescent spectrum of yttrium and 

 samarium produced by traces of calcium or 

 lead. The persistence of the samarium 

 spectrum in presence of overwhelming 

 quantities of other metals is almost unex- 

 ampled in spectroscopy ; 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 tar- 

 get the energy of molecular impact reap- 

 pears as pulses so abrupt and irregular 

 that, when resolved, they furnish a co- 

 pious supply of waves of excessively 

 short wave-length — in fact, the now well- 

 known Eontgen rays. The phosphores- 

 cence so excited may last only a small frac- 

 tion of a second, as with the constituents 

 of yttria, where the duratiou of the differ- 

 ent lines varies between the 0.003 and the 

 0.0009 second ; or it may linger for hours, 

 as in the case of some of the yttria earths, 

 and especially with the earthy sulphides, 

 where the glow lasts bright enough to be 

 commercially useful. Excessively phos- 

 phorescent bodies can be excited by light 

 waves, but most of them require the stimu- 

 lus of electrical excitement. 



It now appears that some bodies, even 

 without special stimulation, are capable of 

 giving out rays closely allied, if not in some 

 cases identical, with those of Professor 

 Edntgen. Uranium and thorium com- 

 pounds are of this character, and it would 

 almost seem, from the important researches 

 of Dr. Eussell, that this ray-emitting power 

 may be a general property of matter, for he 

 has shown that nearly every substance is 

 capable of aifecting the photographic plate 

 if exposed in darkness for sufficient time. 



No other source for Eontgen rays but the 

 Crookes tube has yet been discovered, but 

 rays of kindred sorts are recognized. The 

 Becquerel rays, emitted by uranium and its 

 compounds, have now found their compan- 

 ions in rays — discovered almost simul- 

 taneously by Curie and Schmidt — emitted 

 by thorium and its compounds. 'The tho- 

 rium rays affect photographic plates through 

 screens of paper or aluminium, and are ab- 

 sorbed by metals and other dense bodies. 

 They ionize the air, making it ah electrical 



