SOME ob: the latest achievements of science. 147 



rays venerated in high vacua have more penetrative power than when 

 the vacuum is less high. These facts are familiar to all who have 

 exhausted focus tubes on their own pumps. Rontgen suggests a con- 

 venient phraseology; he calls a low-vaciuim tube, which does not emit 

 the highly penetrating rays, a "soft'' tube, and a tube in which the 

 exhaustion has been pushed to an extreme degree, in which highly 

 penetrating rays predominate, a '""hard'' tube. Using a "hard"' tube, 

 he took a photograph of a double-barreled rifle, and showed not only 

 the leaden bullets within the steel barrels, but even the wads and the 

 charges. 



Benoit has reexamined the alleged relation between density and 

 opacity to the rays, and finds certain discrepancies. Thus, the opacity 

 of equal thicknesses of palladium and platinum are nearly equal, while 

 their densities and atomic weights are very different, those of pal- 

 ladium being about half those of platinum. 



At the last meeting of the British Association visitors saw — at the 

 McGill University — Professors Cox and Callendar's apparatus for 

 measuring the velocity of Rontgen rays. They found it to be cer- 

 tainly greater than 2()(> kilometers per second. Majorana has made 

 an independent determination, and finds the velocity to be 600 kilo- 

 meters per second with an inferior limit certainly of not less than 150 

 kilometers per second. It may be remembered that J. J. Thomson 

 has found for cathode rays a velocity of more than 10,000 kilometers 

 per second, and it is extremely unlikely that the velocity of Rontgen 

 rays will prove to be less. 



Trowbridge has verified the fact, previously announced b}' Prof. S. P. 

 Thompson, that fluor-spar, which by prolonged heating has lost its 

 power of luminescing when reheated, regains the power of thermo- 

 luminescence when exposed to Rontgen raj's. He finds that this res- 

 toration is also effected b}^ exposure to the electric-glow discharge, but 

 not by exposure to the ultra-violet light. The difference is suggestive. 



A« for the action of Rontgen ra3's on bactei'ia, often asserted and 

 often denied, the latest statement by Dr. H. Rieder, of Munich, is to 

 the effect that bacteria are killed l)v the discharge from "hard" tubes. 

 Whether the observation will lead to results of pathologic importance 

 remains to be seen. The circumstance that the normal retina of the eye 

 is slightly sensitive to the rays is confirmed by Dorn and by Rontgen 

 himself. 



The essential wave nature of the Rontgen rays appears to ])e con- 

 firmed by the fact, ascertained by several of our great mathematical 

 physicists, that light of excessively short wave-length would be but 

 slightly absorbed by ordinary material media and would not in the 

 ordinary sense be refracted at all. In fact, a theoretic basis for a com- 

 prehension of the Rontgen ra3's had been propounded before the rays 

 had been discovered. At the Liverpool meeting of the British Asso- 



