306 REPORTS ON THE STATE OF SCIENCE, ETC. 



the molecule with the power of forming addition compounds with other mole- 

 cules, it forms the basis of what is recognised by chemists as residual affinity. 

 Owing to the existence of two types of affinity with their origin in the two faces 

 of the atoms, there will be two types of residual affinity which evidence them- 

 selves as acid and basic respectively. If two molecules which possess basic and 

 acid residual affinity respectively come together they will tend to form a com- 

 plex molecule, and this complex can be stabilised in one of two ways, depending 

 on the relation between the atomic fields of the two component molecules. 



In the first phice the complex may lose energy as a whole to the surroundings, 

 with the result that it cannot be dissociated until this amount of energy has been 

 supplied to it. In the formation of this type of complex both components pass 

 into a more condensed phase, and it forms the basis of salt formation, such as 

 aniline hydrochloride, etc. 



In the second place, if the tendency of tlie atomic fields to undergo condensa- 

 tion is greater in one molecule than it is in the other, then, when tlie complex 

 is formed, the first component molecule will give up one or more molecular 

 quanta to the second component molecule. Again will the complex molecule 

 be stabilised, since, although no energy has been evolved to the surroundings 

 during its formation, the complex cannot be dissociated unless the second com- 

 ponent molecule render up to the first the one or more molecular quanta. It 

 is, of course, necessary, in order that such transference of molecular quanta 

 can take place, that the two molecules be characterised by molecular quanta of 

 exactly the same size. This identity is, however, secured, because it has been 

 proved by absorption spectra observations in the short wave infra-red that, 

 when a complex of two or more molecules is formed, it is characterised by its 

 own single molecular quantum. The conditions, therefore, for the transference 

 of energy from one component to the other are perfect (Balv and Tryhorn, 

 Phil. Mag. 31, 417 (1916)). 



It is evident from this that both components, in forming such a complex, 

 must undergo a change in phase, since the first component which gives up one 

 or more molecular quanta must pass into a more condensed phase, whilst the 

 second component which accepts these quanta must pass into a less condensed 

 phase. This explanation, therefore, is capable of experimental proof, since 

 the frequency of the first molecule in the free state must shift towards the 

 ultra-violet, whilst the frequency of the second molecule in the free state must 

 shift towards the red. The absorption spectra of several of these complexes 

 have been observed, and. without entering into the experimental details, it may 

 be stated that in each case the change in frequency in opposite directions on the 

 part of the two molecules in forming the complex has been established. Com- 

 plexes of this type are familiar to the organic chemist, and the type may be 

 exemplified by those formed by picric acid with the aromatic hydrocarbons, 

 diphenylaniine with chlorobenzaldehyde, etc. 



The effect of a solvent m changing the phase of a substance may, therefore, 

 be readily understood, since it is the solvent molecules which supply energy to 

 the solute molecules. Indeed, this energy transference is the essential charac- 

 teristic of the phenomenon of solution. Moreover, the existence of the same 

 compound in different phases when in solution in different solvents is also 

 explained, the particular phase formed depending on the number of molecular 

 quanta supplied to each solute molecule by the solvent molecules. There are 

 many instances known of this phenomenon, which is one of the commonest 

 observed in the study of the absorption spectra of organic compounds. One 

 of the most striking cases is that of trinitrobenzene, which exists in four different 

 molecular phases when in solution in solvents of different degrees of basicity. 

 Other examples were given in an early part of this report when the structure- 

 absorption theory was criticised. . The real explanation of these changes in 

 absorption, and in some cases of changes in visible colour, observed with different 

 solvents is now to hand, because they are due to the formation of different 

 phases of one and the same molecule by the supply to it of one or more molecular 

 quanta by the solvent. It cannot be denied thiit in certain specific instances, 

 such as the nitrophenols. it is possible to write a different structure when a 

 molecule exhiliits different absorption bands, but these instances are very few 

 ni number compared with those iii wliich structural changes cannot be written, 



