432 TRANSACTIONS OF SECTION B. 
previously presented difficulties can be explained. There are, however, others 
long known to chemists, such as double salts and the combination of water with 
salts, formerly called ‘ molecular compounds,’ definite and individual, in which 
these views do not assist us. These compounds do not exist as gases, and 
unless they admit of experimental study by the methods of dilute solution, even 
their gaseous molecular weights cannot be ascertained. 
It is noteworthy that in most of the instances recently investigated where 
variable valency has been assumed the compounds studied have been easily 
decomposable solids or liquids, and for one reason or another their gaseous 
molecular weights could not be determined. Many of these compounds, indeed, 
only exist at low temperatures. As instances of work of this kind I may 
mention Collie and Tickle on quadrivalent oxygen in dimethylpyrone derivatives ; 
Gomberg on triphenylmethyl; Landolf on acetone di-hydrofluoride; Thiele and 
Peter on methyl-iodo-dichloride; and similar studies by Kehrmann, Willstatter 
and Iglauer, Bilow and Sicherer, Baeyer and Villiger, Archibald and McIntosh, 
Chattaway, Pfeiffer and Truskier, and others. j 
Another most interesting class of solids which are capable of existing in two 
isomeric forms distinguished from each other by such physical properties as 
density or colour are the Schiff’s bases or anils. Some of these were studied 
by Hantzsch, who proposed to explain their existence by the Hantzsch-Werner 
stereo hypothesis :— 
HO.C,H,.CH ~ HC.C,H,OH 
II 
NR’ NR’ 
But since only a few, and these not very satisfactory compounds, show this 
isomerism, which do not contain the hydroxyl group, other suggestions have 
been put forward to account for the isomerism, by Anselmino and by Manchot. 
In my own laboratory, associated with Mr. F. G. Shepheard and also with 
Miss Rosalind Clarke, I have made a study of various Schifi’s bases for the purpose 
of investigating the remarkable property which some of these bases exhibit of 
phototropy. By phototropy is meant the capability of reversible change of colour 
in solids depending upon the presence or absence of light. Incidentally, too, I 
wished to study another physical property which many Schiff’s bases possess, in 
common with other substances, of reversible change of colour with raising or 
lowering of temperature. This property we have called thermotropy, and many 
old instances will be remembered of substances of simpler constitution which 
exhibit it : thus, when subjected to the temperature of solid carbon dioxide, 
ordinary sulphur becomes colourless, red oxide of mercury becomes yellow, ver- 
milion becomes scarlet, and on return to the ordinary temperature the original 
colours reappear. 
As has been pointed out in a recent communication by Biilman, it is most 
important in these discussions that we should be perfectly clear in the use of 
terms. I take it for granted that isomerism is a general term for compounds 
differing in some respect but having the same composition. If the molecules 
(gaseous) have the same weights they are metamerides; if of different weights 
they are polymeridcs. When solids crystallise in more than one form they are 
polymorphous. Now it does not seem reasonable to suppose that reversible 
colour changes such as those exhibited by phototropes or thermotropes involve 
such violent intra-molecular changes as the breaking and reconnecting of atomic 
linkages. For example, take the three bases, salicylidene-m-toluidine, 
which in the dark or immediately it is exposed to light is yellow, but on con- 
tinued exposure to light quickly becomes orange, and changes back again to 
its original colour in the dark; salicylidene-m-aminophenol, which at ordinary 
temperatures is orange, but is much paler at the temperature of solid carbon 
dioxide, on raising the temperature to nearly the melting-point (128-9°) becomes 
orange red, and these changes take place in the reverse order again on cooling; 
salicylidene-p-aminobenzoic acid, studied by ourselves and by Manchot and 
Furlong independently, shows a wider range of thermotropic change between 
bright yellow and blood-red, and is also phototropic. 
To explain such changes as these and the others of a similar nature previously 
referred to, I think some less drastic hypothesis should be sought than intra- 
