ADDKESS. 23 



not lead to success, nevertheless a first dim glimpse of the phenomenon was 

 obtained by M. Fievez, of the Royal Observatory at Brussels, in 1885. 



It would be imjiroper to pass without at least brief mention the 

 remarkable series of theoretic papers by Dr. J. Larmor, published by the 

 Royal Society, on the relationship between ether and matter. By the 

 time these researches become generally intelligible they may be found to 

 constitute a considerable step towards the further mathematical analysis 

 and interpretation of the physical universe on the lines initiated by 

 Xewton. 



In the mechanical construction of Rontgen ray tubes I can record a 

 few advances : the most successful being the adoption of Professor Silvanus 

 P. Thompson's suggestion of using for the anti-cathode a metal of high 

 atomic weight. Osmium and iridium have been used with advantage, 

 and osmium anti-cathode tubes are now a regular article of manufacture. 

 As long ago as June 1896, X-ray tubes with metallic ui'anium anti- 

 cathodes were made in my own laboratory, 3.nd were found to work better 

 than those with platinum. The difficulty of procuring metallic uranium 

 prevented these experiments from being continued. Thorium anti- 

 cathodes have also been tried. 



Rontgen has drawn fresh attention to a fact very early observed by 

 English experimenters — that of the non-homogeneity of the rays and the 

 dependence of their penetrating power on the degree of vacuum ; rays 

 generated in high vacua have more penetrative power than when the 

 vacuum is less high. These facts are familiar to all wlio liave exhausted 

 focus tubes on their own pumps. Rontgen suggests a convenient phrase- 

 ology ; he calls a low vacuum 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 pre- 

 dominate, a 'hard' tube. Using a 'hard' tube he took a photograph of a 

 double-barrelled rifle, and showed not only the leaden bullets within the 

 steel barrels but even the wads and the charges. 



Benoit has re-examined 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 whilst their 

 densities and atomic weights are very difterent, those of palladium 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 measur- 

 ing the velocity of Rontgen rays. They found it to be certainly greater 

 than 200 kilometres per second. Majorana has made an independent 

 determination, and finds the velocity to be 600 kilometres per second with 

 an inferior limit certainly of not less than 150 kilometres per second. It 

 may be remembered that J. J. Thomson has found for cathode rays a 

 velocity of more than 10,000 kilometres per second, and it is extremely 

 unlikely that the velocity of Rontgen rays will prove to be less. 



