1905.] on the Pressure due to Radiation. 103 



and the pressure exerted by a concentrated beam from an arc electric 

 lamp on one vane of a delicately suspended torsion balance, was 

 shown to the audience ; and likewise the character of the disturbing 

 action due to the gases present in the balance chamber. 



Lebedew and Nichols and Hull succeeded in detecting radiation 

 pressure, only because they were the first to systematically eliminate 

 the disturbing forces due to the residual gases. Thus the cause of 

 failure of the well directed efforts of earlier observers to isolate 

 radiation pressure from the relatively powerful and uncertain gas 

 forces exemphfied in the Crookes radiometer was made clear. 



A vacuum tube built by Nichols and Hull to illustrate the 

 repulsion of comet tails by the sun was also shown. The form of the 

 tube was that of an hour-glass. A very fine dust, prepared by cal- 

 cining puff-ball spores, was mixed with the sand, and the pressure of 

 a very powerful beam of light directed horizontally against the stream, 

 just below the neck, drove the finer dust particles backward, while 

 the heavier sand grains fell vertically. 



The verification of the radiation pressure theory affords a means 

 of extending our knowledge of many celestial phenomena and of 

 l)roadening our theories concerning them. In the first place, the 

 hitherto mysterious behaviour of comet tails is satisfactorily explained. 

 The Newtonian gravitation theory is seen no longer to express the 

 whole mutual action between bodies, for if either or both of the 

 bodies be at a temperature above that of their surroundings, a correc- 

 tion must be added to include the radiation pressure between them. 

 If the bodies are massive, and not too hot, the correction is insignifi- 

 cant ; but Professor Poynting has shown that for two spherical block 

 bodies of unit density 8 inches in diameter, if at a temperature of 30° 

 Centigrade, the radiation pressure would just balance gravitational 

 attraction, and the bodies would be entirely indifferent to each other, 

 however near or far apart they might be. 



A law of action between two bodies, alone in space, which takes 

 account of both gravitation and radiation pressure is — 



P=-!f^;^(,'^*_.(T,' + T/)) 



in which F is the force between two spheres of radii, a and J, of 

 densities Pi and P2, r units distance apart, and at temperatures Tj 

 and T2 absolute ; y is the gravitation constant, and o- is four times 

 the radiation pressure between two spheres of unit radius and at two 

 units distance apart when Ti = 0° and T2 = 1°, or the reverse. 



Hereafter, therefore, in dealing with flocks of small stones or 

 meteorites in space, and in computing the forces of condensation in 

 a heated nebula, radiation pressure must be taken into the account. 



Furthermore, radiation pressure is reciprocal. As a body which 

 stops radiation feels a pressure, so also a body which is sending out 



