T. MANSELL — THE RoNTGEN OR "x" RAYS. 137 



The essential differences and similarities between the cathode 

 rays and Rciutgen's " X " rays are : — 



"X" rays are not deflected by magnets, cathodic rays are; 

 "X" rays are not absorbed or diffused so readily as cathodic 

 rays; "X" rays will traverse several inches of wood and even 

 several inches of aluminium or glass, while cathodic rays fail to 

 pass through any but thin films of glass and aluminium; "X" 

 rays excite fluorescence, and give shadowgrams at a distance of 

 eighty inches fi'om the tube, whilst cathodic rays are absorbed or 

 diffused at a distance of fifty inches. 



The main points of similarity between the two are their powerful 

 action on photographically sensitive films, and their rectilinear 

 propagation as shown by the sharply-defined shadows. 



In a darkened room, paper covered on one side with barium 

 platino-cyanide lights up with brilliant fluorescence when brought 

 into the neighbourhood of the Crookes tube ; and, with its aid, 

 paper is very transparent ; objects can be seen through a book of 

 over 1,000 pages, two packs of cards, and a thick block of wood. 

 A sheet of aluminium 15mm. thick still allows these "X" rays 

 to pass; water, and several other fluids, are very transparent to 

 them ; salts of metals, either solid or in solution, behave as a rule 

 as the metals themselves. 



Barium platino-cyanide is not the only fluorescent substance 

 acted on by the "X" rays. Calcium sulphide, uranium-glass, 

 rock-salt, and Iceland-spar exhibit fluorescence just as well, and 

 have the advantage of being much cheaper. 



The tubes must be constructed of glass which is of sufficient 

 thickness to withstand the heat ; otherwise, as in the case of 

 those made with lead glass, they easily crack. To avoid such 

 an accident we are obliged to give intervals of rest during the 

 exposure, which is a great drawback in the case of a surgical 

 shadowgraph. 



Some workers have suggested that the rays proceeded in direct 

 lines fi'om the cathode through the sides of the tube, and not 

 from the surface of the tube itself. I exposed a plate in front 

 of the tube (opposite the cathode) and another at the side of the 

 tube, its surface parallel with the main axis of the rays, if pro- 

 ceeding, as suggested, in a direct line from the cathode. The 

 results, developed together, gave every sign of equal exposure. 



Lenard, in his investigations on cathode rays, has shown that 

 they belong to the ether, and can pass through all bodies, and 

 "X" rays behave in the same manner. 



Professor Thompson has shown that a photographic plate enclosed 

 in a vacuum-tube is not acted upon by the cathode rays produced 

 in the tube. This is no doubt due to the absence of oxygen on 

 the plate. 



The discovery of Pratt and Bennett that the rays have killed 

 certain bacteria, viz., those of cholera, anthrax, pneumonia, diph- 

 theria, and others, is, if true, of great interest to the medical 

 profession. 



VOL. IX. — PART IV. 10 



