PRESIDENTIAL ADDRESS. 335 
whilst the other exhibits the structure of the symmetrical triketohexamethylene, 
CO .CH, 
HC’ “\co. 
CO. CH, 
A new suggestion is thus made to the effect that tautomerism consists in the 
possibility of geometrically partitioning the close-packed assemblage in two or 
more alternative ways, each giving molecular units of the same composition but 
of different constitutions. The idea that in the occurrence of tautomerism 
some component atom wanders from one position to another in the molecule 
is thus rejected; the change in constitution arises from the transference of atoms 
as between two or more molecules. As the older conceptions of the mechanism 
of tautomerism do not provide a satisfactory explanation of the experimental 
facts, the suggestion now made is perhaps worthy of consideration. 
The new line of work has many bearings upon the subject of chemical change}; 
thus, the assemblage which is assigned to acetylene (or methylacetylene) is con- 
vertible, by symmetrical distortion, into that representing benzene (or the 
1:3: 5-trimethylbenzene, mesitylene). Further, the great change in chemical 
behaviour which accompanies many types of chemical substitution is possibly 
connected with the manner in which the actual atomic volumes are affected by 
the replacement ; on converting benzene, in which the atomic volumes of carbon 
and hydrogen are as 4:1, into bromobenzene, a considerable increase in 
molecular volume occurs. The atomic volumes of carbon and hydrogen still, 
presumably, preserve the 4:1 ratio, and the volume appropriated by the entering 
bromine atom is approximately the same as that occupied by each hydrogen 
atom already present; the actual atomic volumes of carbon and hydrogen must 
thus be supposed to have increased during the production of bromobenzene. It 
can hardly be supposed that this fundamental volume change, even apart from 
a distortion of the aromatic ring arising from slight inequality of hydrogen and 
bromine atomic domains in the molecule, could occur without the exhibition 
of considerable differences in chemical properties as between benzene and 
bromobenzene. 
Whatever view may be taken as to the accuracy of the conclusions concerning 
the relation between crystal structure and chemical constitution which are 
briefly discussed in the present Address, no critic will be disposed to doubt 
that wide developments in chemical science will result from the cultivation of 
crystal study; it seems clear that any satisfactory theory of the solid state 
must be largely crystallographic in character. The chief hindrance to progress 
at present consists in the lack of chemists trained in modern crystallographic 
methods; in my own country the only school in which chemical students were 
trained in Crystallography, dissociated from Mineralogy, was founded by 
Dr. Henry E. Armstrong and Sir Henry A. Miers in 1886. After doing a vast 
amount of valuable educational work this school has recently been allowed 
to become extinct. 
In a Presidential Address to the Mineralogical Society in 1888, Mr. Lazarus 
Fletcher remarked that ‘a knowledge of the elements of Crystallography, includ- 
ing the mechanics of crystal measurements, ought to be made a sine qua non for 
a degree in Chemistry at every University.’ Twenty-five years later we find 
that no European University has applied this principle, and in consequence the 
chemical crystallographer has the greatest difficulty in making himself intelligible 
to his purely chemical colleagues. May I, in concluding, express the hope that 
the Colonial Universities, less fettered by tradition than their older sisters, may 
lead in the work of placing the subject of crystal structure in its legitimate 
position as one of the most important branches of modern Physical Chemistry? 
The following Papers were then read :— 
1. Residual Affinity and Co-ordination. By Professor Guperr T. 
Moraan and Henry Wesster Moss. 
The molecular structures now known as co-ordination compounds owe their 
stability not merely to the forces (principal valency and residual affinity) 
