578 Prof. J. Larmor on the Intensity of the 
showed that in the case of platinum the Thomson effect 
vanishes at —70° C.*] 
In conclusion I desire to express my best thanks to Prof. 
J. J. Thomson for his advice and suggestions throughout 
the course of the work, and to my brother, Mr. H. E. Laws, 
to whom I am indebted for the analyses of the specimens 
used. 
Cavendish Laboratory, 
March 25th, 1904. 




LXIIIL. On the Intensity of the Natural Radiation from Moving 
Bodies and tts Mechanical Reaction. By Prof. J. LARMor, 
Sec. RS.+ 
fhe subject of the pressure of radiation, which was first 
reduced into a definite formula by Maxwell, was placed 
in new and most fruitful light when Boltzmann showed, by 
following out an idea of Bartoli, that it stood in intimate 
relation to the law connecting the radiation of a body with 
its temperature. In a recent memoir { Poynting has based 
very remarkable results, as regards cosmical dynamics, on the 
operation of a retarding force due to the back pressure of its 
own radiation when the radiating body is in motion. The 
main object of the present note is to treat this aspect of 
radiation-pressure by more direct methods, and thereby con- 
firm the expression for the mechanical reaction against a 
moving radiating surface, that has been deduced by Poynting 
from general considerations, naturally somewhat uncertain, 
relating to flux of energy. 
The pressure exerted by radiation is essentially connected 
with opacity to it. From formule developed on other 
occasions § it appears that, in the case of a medium which 
may vary in its properties in any manner along the direction 
of propagation x, when it is the seat of electric disturbances 
of simple harmonic period 27/n, polarized so that the electric 
force is (0, Q, 0) and the magnetic (0, 0, y), the dynamical 
equations being thus in Maxwell’s notation 
* Haga, loc. cit. 111. p. 48. 
+ Reprinted from the Loltzmann-Festschrift. Communicated by the 
Author. 
{ Roy. Soc. Proc. 1903; Phil. Trans. zbzd. 
§ Phil. Trans. 1897 A ; or more fully in ‘ Auther and Matter,’ Camb. 
Uniy. Press, 1900, pp. 180-133. 
