258 Mr. W. T. David on Thermal 



between molecules in a gas are apparently too soft to set the 

 intra-atomic corpuscles into vibration, even though the gas 

 be heated to the highest temperature we can command. 

 According to Mr. Jeans, the intra-molecular vibrations are 

 not appreciably excited if the duration of molecular collision 

 (i. e., the time during which two molecules remain in contact 

 or within each other's sphere of influence during collision) 

 is long in comparison with the periods of the intra-molecular 

 vibrations, though if the duration of collision is comparable 

 with the periods of these vibrations the intra-atomic cor- 

 puscles will be set into vibration by these molecular collisions'*. 

 The duration of molecular collisions is dependent upon the 

 velocity with which the molecules approach one another ; 

 the higher the velocity the less time they remain in contact 

 during collision. The mean velocity of the molecules is 

 approximately proportional to the square root of the absolute 

 temperature, and the duration of molecular collisions there- 

 fore decreases as the temperature of the gas is increased. 

 Therefore, according to this theory, at extremely high tem- 

 peratures the duration of molecular collisions may be short 

 enough to excite the intra-molecular vibrations which give 

 rise to the characteristic spectrum, and the emission of the 

 •characteristic spectrum would then be thermal in origin. 

 In the case of an ionized gas, however, the free corpuscles, 

 which possess a very small mass, would have a much higher 

 velocity than the heavier molecules, and the duration of 

 •collision between a molecule and free corpuscle seems to be 

 short enough to shake the intra-atomic corpuscles into 

 vibration, at any rate when the gas temperature is in the 

 neighbourhood of 1000° C. 



The Emission of Infra-red Radiation by Gases, 



The molecules of some gases possess vibrations of much 

 lower frequencies than those corresponding to luminous 

 radiation (which is of wave-length from *3 /a to *7/i,). Car- 

 bonic acid gas and steam emit radiation of much greater 

 wave-length than that of luminous radiation. Carbonic acid 

 gas has emission and absorption bands whose maxima are at 

 2'8 /J,, 4z'4z fju, and 14*1 //, approximately, and water-vapour has 

 a series of bands throughout the whole of the infra-red 

 region, the first band (and the one possessing most energy) 

 being in the neighbourhood of 2'8 fi. Paschen t in 1894 



* Jeans, ' Dynamical Theory of Gases/ Camb. Univ. Press, chapt. ix. 

 t ' Die Emission der Gase,' Wied. Ann. 1. p. 409 (1893) ; li. p. 1 

 <1894) ; and lii. p. 209 (1894). 



