t> Prof. E. C. C. Baly on 



frequencies within the absorption-band groups in the 

 infra-red, visible, and ultra-violet regions. The question 

 then arises as to the course of events when a molecule 

 is exposed to radiation of a frequency that is the same 

 as one of its characteristic atomic frequencies which must 

 be active in the extreme infra-red. Let it be supposed 

 that the molecule formed by the combination of two 

 elementary atoms having the characteristic Irequencies 

 9 x 10 10 and 1*5 xlQ 11 is exposed to monochromatic radia- 

 tion of the frequency 9 x 10 10 . The atom having this 

 frequency will absorb this energy in elementary quanta 

 of 9x 6 - 56 x 10~ 17 erg; and, further, let it be supposed 

 that this atom absorbs five such quanta. The total quantity 

 of energy now absorbed is equal to the minimum quantity 

 of energy which that atom evolved when combining with 

 the atom with characteristic frequency 1*5 X 10 11 , and is 

 equal to one molecular quantum at the true molecular 

 frequency. If the postulate made at the beginning as to 

 the combination of atoms be accepted, then it would seem 

 to follow as a natural consequence that the total energy 

 absorbed by the atom can be transferred to or taken over by 

 the whole molecule as exactly one true molecular quantum. 

 In fact the molecule can obtain one true molecular quantum 

 by the absorption of a whole number of elementary quanta 

 by its atoms, tho whole number being, of course, determined 

 by the frequencies of the other atoms in the molecule and 

 the least common multiple of all the atomic frequencies. 

 Further, there is no reason against this process being con- 

 tinuous in the sense that a molecule will be able to gain 

 more true molecular quanta than the single one by absorption 

 of the specified number of elementary quanta by its atoms. 



Again, this process will be reversible : that is to say, 

 a molecule will be able to radiate one or more true mole- 

 cular quanta in the form of the specified number of 

 elementary quanta characteristic of one of its atoms. 



It will be seen that this leads to the conception of critical 

 amounts of energy associated with elementary atoms in 

 combination, the critical amount of energy of an atom 

 being a whole number of elementary quanta, characteristic 

 of that atom, which in their sum equal one true molecular 

 quantum characteristic of the molecule of which that atom 

 forms a part. When an atom is exposed to radiation of 

 a frequency equal to its own frequency, it can absorb its 

 elementary quanta until its critical quantity is reached 

 when this critical quantity becomes merged into the mole- 

 cular energy content as one true molecular quantum. 



