Radiation from Hot Gases, 257 



great, and the pulses emitted during collision are very 

 " thin " because the duration of collision is very short. 

 When the velocity is much less the duration of collision is- 

 greater and the pulses emitted are broader, giving rise to 

 ultra-violet, luminous, or infra-red radiation, according to 

 the magnitude of the velocity in the free-path. If the law 

 o£ equipartition of energy applies approximately to systems 

 consisting of atoms and free corpuscles, this theory accounts- 

 very well for the known laws of radiation. It accounts, for 

 instance, for the shift of the maximum point in the continuous- 

 spectrum energy curves towards the shorter wave-lengths- 

 with rise in temperature. 



Line and band spectra in the visible part of the spectrum 

 are not emitted as the result of heat alone. It seems pro- 

 bable that an element can only emit its characteristic spectrum 

 when it is in the state of a gas, and its molecules more or 

 less free from the influence of their neighbours. The spectrum 

 of an element which is solid at ordinary temperatures is 

 observed by placing it in a non-luminous flame, or by ex- 

 amining the light from an electric spark or arc between 

 terminals made of that element. It was at one time thought 

 that the emission of the spectrum was caused by the high 

 temperature to which the element was raised by these means, 

 but it seems probable that the only influence of temperature- 

 is to vaporize the element. Whenever a gas is emitting its 

 characteristic spectrum it is ionized, and it seems fairly 

 certain that the emission of the spectrum is in some way 

 dependent upon ionization. It is, however, possible that a 

 gas would emit its characteristic spectrum in virtue of its- 

 temperature alone, if it were possible to heat it to a tempe- 

 rature much higher than that which can be commanded in a 

 laboratory *. When an atom loses a corpuscle (in other 

 words, when it is being ionized) the remaining corpuscles 

 vibrate about new positions of equilibrium, and again when 

 the atom is restored to electrical neutrality, the corpuscles- 

 vibrate about their old positions of equilibrium. These 

 corpuscles grip the pether in virtue of their electrical charge,. 

 and when vibrating they send out trains of waves which 

 travel through the sether with the velocity of light. The- 

 corpuscles in an atom of a particular element have certain 

 definite periods of vibration which correspond to the wave- 

 lengths of the lines in its characteristic spectrum. The 

 corpuscles in an atom may also be set in vibration if the 

 atom is bombarded by the free corpuscles which move with 

 high velocities in the heated ionized gas. The collisions- 



* See p. 258. 



