ON ENERGY OF RONTGEN AND BECQUEREL RAYS, ETC. 
33 
with X rays than with light for equal intensities of radiation. The deflection of the 
galvanometer is thus less for X rays than for light-waves of equal energy. 
From these considerations it is obvious that the solid type of thermopile is most 
unsuitable for such work ; but a modified thermopile of thin plane sheets of metal, c.p., 
iron and Constantin, would probably give better results and be simpler to manipulate 
than the bolometer. With thin sheets the heat would be equally distributed over 
the cross-section due to diffusion, and no appreciable error would arise. The method, 
however, has the objection that the amount of heat must be standardised by a known 
lamp or source of radiant energy. 
Total Energy of the Rays emitted 'per Second. 
When X rays fall on a metal plate, the plate is heated, and tlie question at once 
arises whether we are justified in assuming that the energy of the rays stopped by 
the metal plate is transformed into heat in the j^late. The experiments of Perrin, 
Sagnac, and J. J. Thomson have clearly shown that when X rays strike a solid 
body, secondary rays are set up which ionize the gas and act on a photographic plate. 
These secondary rays are of a far less penetrating character than the rays that excited 
them ; but on account of the ease with which they are absorbed in the gas, the amount 
of ionization per cub. centim. in the gas near the surface of the body may be greater 
than that due to the direct rays. The total number of ions produced by the scattered 
rays depends to a great extent on the density of the metal as well as on the intensity 
of the incident rays. The total number of ions produced by complete absorption of 
the scattered rays is generally only a small proportion of the number produced liy 
complete absorption of the direct rays. Assuming that an ion in both cases requires 
the same expenditure of energy to produce it, the energy of the scattered rays is thus 
only a small projiortion of the total energy of the incident rays. 
The secondary rays are set uji both at the points of incidence and emergence of 
the rays falling on the grid. The heating effect on the grid is thus less than the 
heat equivalent of the energy of the rays stopped by the grid by the portion of tlie 
energy used uji in exciting secondary rays. The correction is probably small, and 
has been neglected in these experiments, but it is hoped in a future investigation to 
determine its value. 
There is no evidence that the chemical energy of j^latinuni is iii any way altered 
by the passage of the rays through it, and, as far as our present knowledge goes, the 
energy of the rays stopjied minus the energy of the scattered radiation, is transformed 
into heat within the platinum. 
In a case where there is a chemical change, e.g., when the rays fall on a photo¬ 
graphic film, the heating effect would not be the equivalent of the energy absorbed. 
It has been shown by Fontgen and other observers that the intensity of the rays 
given out from the front surface of a platinum plate of a focus tube is approximately 
VOL. CXCVI.-A. F 
