ON ABSORPTION SPECTRA OF ORGANIC COMPOUNDS. 297 



of the fundamental absorption band characteristic of the molecule in the short 

 wave infra-red, is either the least common integral multiple of all three fre- 

 quencies, Ai, A2, and A3, or a small multiple of tliat least common multiple. 

 Finally, the central frequency of any absorption band shown by the substance 

 in Ihe visible or ultra-violet is an exact multiple of the frequency F. 



The importance of these relationships becomes manifest when it is remem- 

 bered tli.it the frequencies Ri and R2 are those which Cohlentz found to be 

 characteristic of definite groups of atoms within the molecule. Another very 

 important fact is that the absorption band shown by a molecule in the short 

 wave infra-rtd, the central frequency of which has been denoted by F, is 

 exhibited by that molecule under all conditions, whatever changes in the visible 

 or ultra-violet absorption bands may be induced by change of solvent, etc. The 

 conclusion, therefore, inevitably follows that the characteristic absorption band 

 in the short wave infra-red is due to the molecule as a whole, that the long 

 wave infra-red frequencies, Ri and Ro, are due to groups of atoms forming part 

 of the molecule, and that the frequencies in the very long wave infra-red, Ai, 

 A2, and A3, are due to the atoms composing the molecule, whilst the visible and 

 ultra-violet bands 'are dependent on the conditions under which the molecule 

 exists. 



There is no question but that the enunciation of the energy quantum theory 

 by Planck marks a most important stage in the development of our knowledge 

 of absorption spectra, and tliat this theory must necessarily govern all interpreta- 

 tion of absorption phenomena. Since all processes of ab.sorption or emission of 

 energy must be looked upon as being discontinuous and not continuous as pre- 

 viously supposed, it is very obvious that absorption spectra become placed on a 

 quantitative basis. It was the vagueness of the old qualitative conception of 

 absorption that permitted many theories to escape an early demise. The litera- 

 ture of the past twenty years abounds with such theories, which at best were 

 only ad hoc proposals invented to explain isolated observations. The quantitative 

 basis afforded by the conception of energy quanta shows up all the observations 

 in true perspective, and enhances the value of the arithmetical relationships de- 

 tailed above, for they can only be explained by energy quanta. One very simple 

 example will illustrate this point, and the case of pyridine may be considered. 



Pyridine in the liquid state exhibits an absorption band in the ultra-violet 

 with a central wave-length of 258 ///i and a central frequency of 1.1628x10''. 

 When pyridine is absorbing light of this frequency the process is not continuous, 

 but consists in the absorption of fixed amounts or quanta of energy, each 

 quantum being the product of the frequency into the universal constant 

 6.57x10--'. Each molecule of pyridine, therefore, absorbs a series of quanta 

 of energy, each quantum being 1.1628 x 10'^ X6.57x 10-" or 7.64xl0-'=erg. Now- 

 let one molecule of pyridine absorb one quantum of 7.64 xlO''" erg. This mole- 

 cule will no longer be in equilibrium with its surroundings, and will, therefore, 

 proceed to lose this energy in the form of heat — that is to say, the energy is 

 radiated at an infra-red frequency. This radiation must also be emitted as 

 quanta; and, further, it must be emitted as an exact whole number of quanta, 

 since no energy can be destroyed. It follows that the one quantum absorbed 

 must exactly equal the sum of the quanta radiated, and, therefore, 



7.64 X 10-'= = xx F X 6..57 X 10"" 



where F is the radiating frequency. Hence we have 



1.1628xI0" = a;xF, 



and therefore the ultra-violet frequency must be an integral multiple of an 

 infra-red frequency. This, however, is known to be the case, for pyridine has 

 an infra-red band, with central wave-length of 6.192 u and a central frequency 

 of 4.845 xlO>^ 

 and 1.1628x10" = 24x4.845x10". 



The same will be true at whatever of its characteristic frequencies in the 

 ultra-violet pyridine absorbs energy, all these being integral multiples of the 

 infra-red frequency, 4.845x10". 



This argument may be applied to any absorption band, and thus the central 

 frequency of every absorption band shown by a substance must always be an 



