IIB THE PRESSURE DUE TO RADIATION. 



support of the wave theory by showing theoreticall}^ that a longitudi- 

 nal wave motion might produce a pressure in the direction of its 

 propagation upon a body which checked its progress. In 1825 Fres- 

 nel ^ made a series of experiments, but arrived at no more definite 

 conclusion than that the repulsive and attractive forces observed were 

 not of magnetic nor electric origin. 



Crookes^ believed in 1873 that he had found the true radiation pres- 

 sure in his newly invented radiometer, and cautiously suggested that 

 his experiments might have some bearing on the prevailing theory of 

 the nature of light. Crookes's later experiments and Zollner's*^ meas- 

 urements of radiometric repulsions showed that the radiometric forces 

 were in some cases 100,000 times greater than the light pressure forces 

 with which they had been temporaril}' confused. Zollner's experi- 

 ments are among the most ingenious ever tried in this field of work, 

 and he missed the discovery of the true radiation pressure by only the 

 narrowest margin. An excellent bibliography of the whole radio- 

 metric literature is given by Graetz,'' and an account of some of the 

 older experiments not mentioned above is given by Crookes. ^ 



In 1S73 Maxwell,-^ on the basis of the electromagnetic theorv, showed 

 that if light were an electromagnetic phenomenon, pi-essure should 

 result from the absorption or reflection of a beam of light. After a 

 discussion of the equations involved, he says: 



Hence in a medium in which waves are propagated there is a pressure in the 

 direction normal to the waves and numerically equal to the energy in unit volume. 



Maxwell computed the pressure exerted by the sun on the illuminated 

 surface of the earth, and added: 



It is probable that a nmch greater energy of radiation might be obtained by means 

 of the concentrated rays from an electric lamp. Such rays falling on a thin metallic 

 disk, delicately suspended in a vacuum, might perhaps produce an observable 

 mechanical effect. 



Apparentl}^ independent of Maxwell, Bartoli ^ announced in 1876 

 that the second law of thermodynamics required the existence of a 

 pressure due to radiation numerically equal in amount to that derived 

 by Maxwell. Bartoli's reasoning holds for all forms of energy streams 

 in space, and is of more general application than Maxwell's equations. 

 Bartoli contrived elaborate experiments to verify this theory, but was 

 balked in the search, as all before him had been, by the complicated 



« A. Fresnel, Ann. Chem. et Phys., 29, 57, 107, 1825. 



i'W. Crookes, Phil. Trans., p. 501, 1873. 



cR Zollner, Pogg. Ann., 160, 156, 296, 459, 1877. 



dh. Graetz, Winkelmann's Handbuch der Physik, 2b, p. 262. Breslau, 1896. 



«W. Crookes, loc. cit., p. 501. 



/J. C. Maxwell, A Treatise on Electricity and Magnetism (Isted.) 2, 891. Oxford, 

 1873. 



A. Bartoli, Sopra i movementi prodotti della luce et dal calorie, Florence, Le 

 Monnier, 1876; also Nuovo Cimento, 15, 193, 1884. 



