“ In the fourth essay he remarks, “I see no sufficient reason why we may not conclude that all elastic fluids under the same pressure expand equally by heat and that for any given expansion of mercury, the corresponding expansion of air is proportionally something less, the higher the temperature. . . . It seems, therefore, that general laws respecting the absolute quantity and the nature of heat are more likely to be derived from elastic fluids than from other substances.” He thus enunciated the law of the expansion of gases, stated some months later by Gay-Lussac. ”

“ But from a study of Dalton’s own MS. laboratory notebooks, discovered in the rooms of the Manchester society, Roscoe and Harden (A New View of the Origin of Dalton’s Atomic Theory, 1896) conclude that so far from Dalton being led to the idea that chemical combination consists in the approximation of atoms of definite and characteristic weight by his search for an explanation of the law of combination in multiple proportions, the idea of atomic structure arose in his mind as a purely physical conception, forced upon him by study of the physical properties of the atmosphere and other gases. ”

“ In 1794 he was elected a member of the Manchester Literary and Philosophical Society, and a few weeks after election he communicated his first paper on “Extraordinary facts relating to the vision of colours,” in which he gave the earliest account of the optical peculiarity known as Daltonism or colour-blindness, and summed up its characteristics as observed in himself and others. ”