PRESIDENTIAL ADDRESS. 433 
molecular breaking, and consequent metastasis or polymerisation. ‘though doubt- 
less the hypothesis of Hantzsch and Werner could be invoked, or the modified 
hypotheses of Manchot or Anselmino, I think there should be some simpler 
explanation. Someone suggests polymorphism. Now polymorphism means that 
a change of crystalline form takes place, which might doubtless connote change 
of colour. If one watches phototropic crystals changing colour under the influence 
of lght from yellow to red, and notices that after remaining in the dark 
the same crystals have changed back to the original colour—and, remember, 
these changes can be repeated with the same crystals apparently without 
limit—it will not be considered likely that this phenomenon depends on a rever- 
sible change of crystalline form. In a communication to the Chemical Society 
some three years ago Mr. Shepheard and I put forward the following suggestion : 
‘Evidence is accumulating of reversible isomeric reactions, like those described 
in this paper, which are indicated by physical differences, such as changes of 
colour. It is possible that these may be explained by hypotheses, similar to 
that of Hantzsch and Werner assuming intra-molecular rearrangement; but in 
the case of phototropy and thermotropy it should not be forgotten that the 
substances exhibiting these phenomena are solids. No one will doubt, however, 
that these differences of colour depend on isomeric change of some kind, but 
in the case of solids we know practically nothing of their molecules, not even 
of their relative molecular weights. The molecules of solids are probably far 
more complex than those of liquids or gases; indeed, they may be rather complex 
groups or aggregates of ordinary gaseous molecules, which would give rise to 
far more numerous possibilities of isomerism. It appears to us that phototropic 
and thermotropic reactions are more probably due to isomeric changes affecting 
the aggregation of molecules in solids than to intra-molecular change of molecules 
derived from a study of gases.’ 
It seems to me that just as atoms may be structures built of sub-atoms of some 
kind, and just as molecules of gases are built of atoms variously linked together, 
it is reasonable to conceive that molecules might combine to form aggregates, 
particularly when constituting solids; that as the sub-atoms may be conceived 
to have a combining valency, and the atoms are already accredited with this 
property, and in addition, as is supposed with Thiele’s partial or Werner’s 
auxiliary valencies, molecules may have valencies also whereby to combine 
into molecular aggregates. It may be presumed that such aggregates are more 
complicated in structure, and thus may give rise to greater variety of isomerides, 
and be more readily transmutable than gaseous molecules. If such aggregates 
of gaseous molecules exist they might explain not only the easily changed 
isomerides recently studied, but also the large class of ‘molecular compounds’ 
of the older chemists. I imagine someone saying that in suggesting this hypo- 
thesis—which by the way is not new, for it is mentioned in Ostwald’s ‘ Outlines’ 
—I am violating the canon to which I have myself subscribed, as a condition 
of a scientific hypothesis, that it should be verifiable. Perhaps we carry our 
critical faculty sometimes too far. It is most highly scientific to doubt, but 
doubt which is merely destructive has little value; rather, with Descartes, it 
should lead on to construction, for he who builds even imperfectly is better 
than he who simply destroys. And I do not doubt that some way will be 
found to study solids and obtain data that will lead to the determination of their 
molecular aggregate weights. The study of molecular volumes of solid solutions ; 
the remarkable results obtained by Pope and Barlow; Tutton’s work on crystal- 
lography, and much besides, induce the hope that some day solids like gases will 
find their Avogadro. 
Part III. 
In the pursuit of all this abstract theory, and still more so in the bewildering 
multitude of undigested individual facts, there is danger that important and 
fundamental, even moral, considerations may be lost sight of. For 
ee example take the fundamental question, Why should we pursue 
; na chemistry? No doubt it is considered by its votaries, those who 
ee seek in our laboratories to advance the science, that they are entitled 
sd 7 cathe to have provided for them, and will be rewarded by the provision 
ee: of, the ordinary means of livelihood; but these, it will scarcely be 
denied, could generally be far better assured by other pursuits. It is suggested 
that intellectual discipline is a reason; but, I ask, for what purpose? Will 
‘1912, Fr 
