Radiation from Hot Gases. 265 



-duration of collision between pairs of molecules decreases as 

 the velocity with which they approach one another increases, 

 and since the velocities of the molecules of a gas at any 

 temperature have widely different values (grouped about the 

 mean according to the law of errors), the collisions between 

 slowly moving pairs may be too soft to excite the vibrations 

 within those molecules, but the collisions between the fast 

 moving pairs may be short enough to excite their vibrations. 

 As the temperature of the gas is raised the molecules will all 

 move with greater velocities, with the result that the pro- 

 portion of collisions soft as regards this type of vibration 

 will be decreased, and the number of molecules which have 

 their vibrations excited will therefore be increased. The 

 specific heat will therefore increase, not suddenly, but 

 gradually with the temperature, though, of course, the rate 

 of increase will not be constant. It will be greatest at the 

 temperature at which the mean molecular velocity is just 

 great enough to make the duration of collisions short enough 

 to excite the vibrations, for the proportion of molecules 

 moving with the mean velocity is far greater than the pro- 

 portion of molecules moving with other velocities. 



Infra-red Spectrum of the Bunsen Flame. 



Julius made a very complete analysis of the infra-red 

 emission from various kinds of flame. He examined the 

 spectrum of the flame by means of a rock-salt prism, and 

 found that in all flames producing both C0 2 and water-vapour 

 that the infra-red spectrum consisted of two emission bands 

 whose maxima are at 2'S fi and 4*4 /jl *. Of these the 4'4 /i 

 band was considerably stronger than the 2*8 /jl band. The 

 4'4 /i band is absent from the spectrum of a pure hydrogen 

 flame, but it contains the 2*8 /jl band ; while the spectrum of 

 a pure CO flame contains both the 2'8 /jl and 4*4//, bands, 

 but the 2*8 yLt band is very weak. Infra-red flame spectra 

 seem to be independent of the nature of the combustible 

 gas ; they depend solely on the products of combustion. 



Paschen found that the 4*4 /jl emission band of C0 2 has its 

 maximum shifted towards the long wave-lengths with rise of 

 temperature t, the maximum being at 4*27 /jl at 17° C. 



* There is also a small emission band in the neighbourhood of 14 /* due 

 to C0 2 (corresponding to the absorption band of cold C0 2 in the neigh- 

 bourhood of 15 /x), but the energy in this band at the temperature of 

 the Bunsen flame is small in comparison with the energy in the 2*8 ft and 

 4'4/z bands. 



t The emission band of C0 2 in the neighbourhood of 14// shift 

 towards the short wave-lengths with rise of temperature, as is nsual. 



Phil. Mag. S. 6. Yol. 25. No. 146. Feb. 1913. T 



