258 THE ORIGIN OF LIFE 



somie temperature. It is therefore a case of change 

 of molecular structure. 



On the other hand, he records a number of cases 

 of coloured flames which give emission bands and 

 are yet quite transparent to their own radiations. 

 It is true for several bands of calcium, strontium, 

 barium, copper, and carbon, and, within the errors 

 of observation, the brightness of these bands has 

 been found to double with the thickness of the 

 flame. The same result has been found in the case 

 of the luminosity produced by the electric discharge 

 in hydrogen in high vacua, but these may all be 

 due to the fact that the coefiicient of absorption 

 is in these cases small, thus requiring a great thick- 

 ness of gas to produce a sensible efi"ect, whilst it 

 indicates that the number of luminous molecules 

 is small relatively to that in substances in which 

 the absorption during luminosity is great. 



Tyndall found that many gases have .emissive 

 and absorptive powers in the infra-red which are 

 proportional, in which case the radiation must be 

 due to molecular aggregations, according to the 

 theory we have put forward. 



He also found that these powers of radiation 

 and absorption are very difierent for diflferent gases, 

 that gases formed of complicated molecules give far 

 higher values than simple gases. 



Ethylene, for instance, emits and absorbs much 

 more strongly than air, and the emission from a 

 polished plate of metal can be considerably increased 

 by coating it with a film of this gas. 



