Light Absorption and Fluorescence. 5 



they possess is the infra-red fundamental frequency, which 

 is an exact multiple of the true molecular frequency. In 

 the case of sulphur dioxide the infra-red fundamental is 

 14 times the true molecular frequency, and in the case of 

 water it is 8 times the true molecular frequency. It was 

 stated above that the smallest possible equal amounts of 

 energy which two or more atoms can evolve when com- 

 bining together is equal to one quantum measured at the 

 frequency which is the least common multiple of their 

 atomic frequencies. It does not follow, of course, that the 

 reacting atoms only evolve this smallest possible amount 

 of energy. They may evolve an amount of energy which 

 is 2, 3, 4, etc. times this smallest quantity, with the result 

 that the smallest frequency truly characteristic of the 

 molecule maybe a multiple of the true molecular frequency. 

 Indeed, it would seem that th^ infra-red fundamental is 

 the frequency which is truly characteristic of the freshly- 

 synthesized molecule. 



At the commencement the simplest possible case was 

 considered of the combination of two atoms, each cha- 

 racterized by a single elementary quantum. There is no 

 necessity to restrict the conditions in this way, and it 

 is to be expected that, at any rate in the atoms of some 

 elements, there will exist more than one possibility of 

 shift of the electrons, and that there will be elementary 

 quanta of different sizes associated with such atoms. It 

 has already been found that two different elementary 

 quanta are associated with the atom of oxygen in the 

 water molecule and with the atom of sulphur in the molecule 

 of sulphur dioxide. 



Whilst the establishment of molecular quanta, and hence 

 of molecular frequency, is a simple deduction from the 

 conception of elemental atomic quanta, it cannot be denied 

 that the molecule will also exhibit those frequencies which 

 are characteristic of its component atoms. Although these 

 atoms have united together to form the molecule, there 

 is no reason to expect that they have thereby lost their 

 individuality as far as their powers of absorbing or radiating 

 energy are concerned. The conception of the molecular 

 quantum is based on the assumption that the component 

 atoms can gain or lose elementary quanta when in com- 

 bination. In addition to this, there is definite evidence that 

 the molecule exhibits the specific frequencies of its atoms, 

 since, although these atomic frequencies have not yet been 

 observed in the long-wave infra-red, they are found in 

 combination with the molecular frequencies as subsidiary 



