312 



NATURE 



[November 3, 192 1 



hibited by one and the same substance under dif- 

 ferent conditions are restricted to the visible and 

 ultra-violet, the fundamental short-wave infra-red 

 frequency remaining the same. 



Ihere is one point about absorption spectra 

 which has been strangely neglected in all theories 

 - — namely, the ultimate destination of the energy 

 that is absorbed. It is obvious that when a sub- 

 stance exhibits an absorption band energy is being 

 absorbed, and if no photochemical change is 

 thereby produced the whole of the absorbed energy 

 is again radiated in the infra-red. The integral 

 relationship between the frequencies is in harmony 

 with an energy quantum theory, since, if the quan- 

 tum of energy is the product of the frequency into 

 a constant, one single quantum absorbed at one 

 frequency can be radiated as an exact number of 

 quanta at a smaller frequency if the first fre- 

 quency is an integral multiple of the second. 



The usually accepted basis of all theories of 

 absorption is the assumption that a molecule is 

 characterised by certain definite frequencies or free 

 periods of vibration, and that absorption of energy 

 takes place as the result of these. There are, 

 however, certain objections to this assumption, 

 such, for instance, as the fact that the frequencies 

 of a molecule are far larger than those of the 

 atoms which it contains, Ihese objections can at 

 once be met by making an entirely different 

 assumption— -namely, that a molecule is character- 

 ised by an amount of energy which determines its 

 frequency. The cardinal assumption may be made 

 that each elementary atom is characterised by a 

 fixed amount of energy or elementary quantum 

 which is associated with a definite physical pro- 

 cess such as the shift of an electron from one 

 stationary orbit to another. These elementary 

 quanta are related together in that they are multi- 

 ples of a fundamental unit, possibly the elementary 

 quantum of the hydrogen atom. . On this hypo- 

 thesis an atom can only absorb or radiate one or 

 more of its elementary quanta. 



It may readily be shown, on the grounds that 

 when two or more atoms combine they each lose 

 an equal amount of energy, that the resulting 

 molecule is endowed with a molecular quantum 

 which is a multiple of the least common integral 

 multiple of the elementary atomic quanta of its 

 atoms. If the physical process in the atom occu- 

 pies a definite time, assumed the same for all 

 atoms, then all atoms and molecules will have the 

 power of absorbing or radiating energy of a de- 

 finite frequency. In all probability the molecular 

 quantum establishes the fundamental molecular 

 frequency in the short-wave infra-red of which 

 the visible and ultra-violet frequencies are exact 

 multiples. On this theory, therefore, a molecule, 

 like an atom, can lose or gain energy as a whole 

 only in terms of its molecular quantum. 



There is little doubt that the origin of the 

 affinity between atoms which causes them to com- 

 bine is to be found in their electromagnetic-force 

 fields, and when the combination has taken place 

 the external faces of the atoms must come into 

 play. These cannot exist in any molecule without 

 XO. 2714, VOL. 108] 



mutual influence, and, indeed, the force lines must 

 condense with the escape of energy to form a 

 molecular-force field on which the reactivity of 

 the molecule will depend. Obviously, this energy 

 loss is a process in which the molecule as a whole 

 takes part, and consequently the energy will be 

 lost in molecular quanta. A freshly synthesised 

 molecule, therefore, must pass into one of a 

 number of possible phases according to the number 

 of quanta that have been lost. It is a matter of 

 simple proof that when a freshly synthesised 

 molecule loses x molecular quanta in this way it 

 becomes endowed with a new quantum which is 

 x^ \ times the molecular quantum. The molecular 

 phase, therefore, will exhibit its characteristic fre- 

 quency together with a phase frequency which is 

 an integral multiple of the molecular frequency. 



The total number of molecular quanta that are 

 (n olved in the force-field condensation will depend 

 on the nature of the external fields of the atoms. 

 The more nearly balanced these are, the greater 

 the nomber of molecular quanta that will be lost. 

 The great majority of organic compounds have a 

 molecular frequency of the order of i x lo^*, so 

 that if four quanta are lost in the force-field con- 

 densation, the phase frequency will be 5 x 10^*, 

 which is situated in the red.; but if 10 quanta are 

 lost, the phase frequency will be i-i x 10^^, which is 

 in the ultra-violet. Again, if 17 quanta are lost, 

 the phase frequency will be i-8xio^^, which is 

 situated in the extreme ultra-violet beyond the 

 limit of the quartz spectrograph. 



It is perfectly possible to change the phase in 

 which a molecule exists by supplying to it or 

 taking from it energy in an amount equal to one 

 or more molecular quanta. This can be done in 

 many cases by use of a suitable solvent, or even 

 by a change of physical state, such as from liquid 

 to gas. The change in phase is indicated by a 

 change in the position of the absorption band in 

 the visible or ultra-violet, and this phenomenon is 

 frequently observed when different solvents are 

 used. The change of molecular phase with change 

 in physical state is well instanced by piperidine 

 and pyridine. Liquid piperidine is diactinic to all 

 the visible and ultra-violet rays transmitted by a 

 quartz spectrograph, because its absorption band 

 lies in the very extreme ultra-violet. Piperidine 

 vapour, on the other hand, exhibits a strong 

 absorption band in the near ultra-violet. The 

 absorption bands of liquid and gaseous pyridine 

 are also quite different. 



The molecular-phase hypothesis clearly has a 

 quantitative basis, since the molecular quantum 

 evolved in the phase change is given in ergs by 

 the product of the frequency into the time con- 

 stant 6-57 X 10--". It applies, moreover, to in^j 

 organic substances as well as to organic, and of 

 this a typical instance is given by sulphur. It is 

 now accepted that the allotropes of sulphur are 

 equilibrium mixtures of four different molecular 

 species of sulphur, Sa, S„ S^,, S^, and there is 

 little doubt that these are in reality four molecular 

 phases, for they exhibit absorption frequencies 

 which are multiples of the fundamental molecular 



