256 Mr. M. X. Shaha on Maxwell's Stresses. 



possessing kinetic energy above a certain limiting value,, 

 they emit a single-line spectrum of determinate wave-length. 



10. It is interesting to note that low-pressure effects on 

 spectra have not engaged the attention of physicists to the 

 same extent. Considerable attention has, however, been 

 devoted in recent years to the investigation of the spectra 

 of metals in vacuo. 



11. Coming, finally, to the case of discharge in vacuum- 

 tubes, reference should be made to the work of R. W. Lawson, 

 who found that variation of the current density and voltage in 

 the discharge-tube did not produce any change in the rela- 

 tive intensity of the nitrogen bands, but considerable differ- 

 ence resulted from pressure change?. In the case of air, 

 oxygen lines were not discernible, the spectrum being the 

 same as that of nitrogen. It appears, accordingly, that no 

 previous worker has (so far as we have been able to discover) 

 noticed the simplification of the spectrum of air in a discharge- 

 tube with which the present paper deals. 



Our thanks are due to the authorities of the Presidency 

 College for offering us facilities for carrying on these 

 researches in the Presidency College Laboratory, Calcutta. 



XXIV. On Maxwell's Stresses. By Megh Nad Shaha^ 

 M.Sc, Research Scholar in Mathematical Physics, Sir T* 

 N.Palit College of Science, Calcutta*. 



1. 1% /fAXWELL f has shown that the mechanical action 

 XyjL between two electrical systems at rest can be 

 accounted for by assuming the existence of certain stresses 

 distributed over a surface completely enclosing one of the 

 svstems. If ^r be the potential at any point due to the 

 whole system, the X-component of the mechanical force on 

 one of the systems can be shown to be 



^-sJjBJ^**^ 



(i) 



where the integration extends over the space occupied by 

 the first system. 



2. If the force is really due to the presence of stresses on 

 a surface enclosing the first system, we have 



¥ x = ( fx^S = (({lX x + mX ¥ + nX 2 )dS, . 



(2) 



where X x , X y , X z &c. . . . are the various surface-tractions, 



* Communicated by Prof. D. N. Mallik. 



t Maxwell, ' Electricity and Magnetism/ vol. i. chap. v. 



