R?ntgen Photography At The Royal Society. A paper by Lord Kelvin on the generation of longi- tudinal waves in ether was read at Thursday's meeting of the Royal Society. Be described an arrangement for obtaining pressural disturbanee through a consider- able space of air, accompanied by a very bmall propor- tion of ordinary transverse waves. His apparatus would afford the means of exposing sensitive plates to these longitudinal vibrations and thus might assist in eluci- dating the nature of the P.ontgen rays. A paper by Professor J. J. Thomson was also read relating to ex- perimnents from which he concludeas that all substances when transmitting the RBntgen rays are conductors of electricity. A discussion followed the reading of these papers, in which details wvere given of many experimeats on these X ra y. Its generaL effect was, however, to show that, whie nmany iateresting points hare been noted, tile obscurity hanging over the subject has not been appreciably lightened. Considerable aifferences of opinion were manitest even upon the conditions af the Ebntgen experiments. While some advocated the use of very powerful currents, others had been successful with relat,vely weak ones, and while some were in favour of regarding tbe phosphorscsrence of tbe glass as the efficient sour e of the rays, others ascribed ttem to the glow of the electrodes. A new turn was given to the dis-ussion by Captain Abney,who ventured, amid some expressions of dissent, to doubt whether the action of the Runtgen r-ays on a sensitive plate could properly be described as photorraphic. lie cited several facta which, in his opinion, excluded the theory of dircet phlotographic action in any ordinary sense and indicated some nre- ference for the view that the Ronitgen rays acted by i;rst setting up phosphorescence or a;:tion of some uunown krind in the glass at the bacl; of the sensitive mlm. This view was corroborated bov an experiment described by Professor Dewr upon pletino-cyanide of ammoniumn at 1-jw temperatures. T'his salt, ordinarily Iluorcscent, only became phosphorescent at the tempera- ture of liquid air. On being exiposed to Rontgen rays instead of to ordinary light, while isnmerged in liquid air, it showved when the liquid air was poured off brilliant phosphorescence. This proved that, whatever might be the nainrez of the Btontgen rays, they were con- vertible into light rays affecting the humcan eye. A Jarge nvumber of experirnents were also described by Professor Dewar showing that reslistance to the passage of lRontgen rays increased with increase of atomic we-ight. Orgamo snbstances wvare nll relatively trans- parent, following the carbon, oxygen, hydrogen, and nitrogen of whicb they are composed. mere complexity of struceture made no difference, but'substitutionprodncts showed increasinzg opacity in the order of the atomic weighth of the combined chlorine, bromine, and iodine. R?ONTGBN PHOTOGRAPBY AT THE ROYAL SOCIETY.