TRANSACTIONS OF SECTION B. 509 
compounds, ‘That such conclusions can be drawn is evident from the above 
example. 
The only study of related substances which satisfies all the requirements noted 
above is to be found in the brilliant work of Tutton on_the sulphates and selenates; 
this relates to the comparatively small changes which result from the replacement 
of K by Cs, Rb and NH, in so-called ‘isomorphous compounds.’ In spite of the 
smallness of the changes produced, Tutton has been able to establish definite 
relationships between the atomic weight of the metal concerned and the crystal 
structure, and his work therefore stands as a sample of the way in which chemico- 
crystallographical investigations must in the future be carried out for the purpose 
of determining the mutual relationship of the crystal structure and the chemical 
substance. 
As is well known, isomorphous substances possess the power of forming 
homogeneous mixtures, and, since in these mixtures the properties continuously 
change with the composition, they have been described as ‘ solid solutions.’ Such 
a miscibility often exists, however, within certain limits between substances 
possessing totally different crystal structures, so that the solidification curves of 
the molten mixtures of such substances show just the same relationships as if the 
mixtures had been made from two truly isomorphous materials. It is therefore 
incorrect to conclude that two substances are isomorphous from an examination of 
the melting and solidification curves of their mixtures. 
Ouly those substances which exhibit corresponding crystal structures as a 
result of their chemical analogy can be regarded as isomorphous; mixed solutions 
of two such substances may be caused to deposit apparently homogeneous crystals 
in which the atoms of one element are partially replaced by atoms of another 
without overthrowing the equilibrium of the crystalline structure. ‘The preserva- 
tion of the equilibrium will clearly be the more easy the less the differences between 
the forces determining the crystal structure in the two isomorphous substances, 
and the less therefore also the difference between the two crystal structures. The 
properties of the mixture must be immediately deducible additively from those of 
the components ; thus, for instance, each component must preserve its individual 
specific gravity in the mixture, as has been shown by the careful investigations of 
Retgers to be the case. 
Lastly, the facts which Kipping and Pope have brought to light in connection 
with optically active and racemic compounds can be brought into unison with the 
above discussion on crystal structure. The crystal structure of a racemic com- 
pound contains a regular-point system of carbon atoms, one half of which is the 
mirror image of the other half—the other atoms present in the compound form 
point systems similarly arranged and constituted—whilst the pseudoracemic 
substances stand to the active components in the same relation as polysymmetric 
substances. 
The object of the author is to state clearly the point of view in accordance 
with which previous investigations must be developed in order that generally 
applicable conclusions can be deduced therefrom. 
2. On Dynamic Isomerism. By T. M. Lowry, D.Sc.—See Reports, p. 193. 
3. The Constitution of Phthalein Salts. By Professor Ricuarp Meyer, 
The author gave an account of his experimental work on the constitution 
of phthalein salts at the last Versammlung deutscher Naturforscher und Aerzte 
at Cassel.! 
Phenolphthalein itself is colourless and is generally regarded as a lactone, 
whilst the quinonoid formula is assigned to its red alkali salts by the majority of 
' Ber. d. deutsch. Chem, Ges., 36, 2949 (1903). 
