SOME OF THE LATEST ACHIEVEMENTS OF SCIENCE. 14'J 



travt'ling- with the speed of light, l)iit of a length compura))le 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 wa}^ in which the molecules of any solid not at 

 absolute zero may possibly emit such rays. 



If the target onto which the electrically-charged atoms impinge is 

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

 rhythmical vibration, the energy thus absorbed reappears in the form 

 of light, and the bod}^ 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 phos- 

 phor! belong to certain well-detined classes, such as the sulphides of 

 the alkine-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 pro- 

 duced by traces of calcium or lead. The persistence of the samarium 

 spectrum in presence of overwhelming quantities of other metals is 

 almost unexampled in spectroscopy; thus 1 part of samaria can easily 

 be seen when mixed with 3,000,000 parts of lime. 



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

 phorescing 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 

 onlj^ a small fraction of a second, as with the constituents of yttria, 

 where the duration of the different lines varies l)etween 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 

 phosphorescent bodies can be excited by light waves, but most of them 

 require the stimulus of electrical excitement. 



It now appears that some bodies, even without special stinmlation, 

 are capable of giving out rays closely allied, if not in some cases iden- 

 tical, with those of Professor Rontgen. Uranium and thorium com- 

 pounds are of this character, and it would almost seem, from the 

 important researches of Dr. Russell, that this ray-emitting power may 

 be a general property of matter, for he has shown that nearly every 

 substance is capal)le of affecting the photographic plate if exposed in 

 darkness for sufficient time. 



No other source for Rcintgen rays but the Crookes tube has yet been 

 discovered, but rays of kindred sorts are recognized. The Becquerel 

 rays, emitted ])y uranium and its compounds, have now found their 

 companions in rays — discovered almost simultaneously by Curie and 



