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 other atmospheric gases. A second edition of the ‘Meteorological Essays’ was published in 1834, with the addition of some notes collected into an appendix, but with no alteration in the text. A catalogue of auroræ observed between 1796 and 1834 was added (p. 218).

Dalton was admitted a member of the Literary and Philosophical Society of Manchester 3 Oct. 1794, and read 31 Oct. a paper on ‘Extraordinary Facts relating to the Vision of Colours’ (Manchester Memoirs, v. 28). In it he gave the first detailed description of the peculiarity now known as ‘colour-blindness,’ discovered in himself through the attention paid by him in 1792, in the course of his botanical studies, to the hues of flowers. The defect was shared by his brother, and was studied on the continent under the name of ‘Daltonism.’ A post-mortem examination in his own case showed his explanation, by a supposed blue tinge in one of the humours of the eye, to have no foundation in fact.

He communicated to the same society on 1 March 1799 ‘Experiments and Observations to determine whether the Quantity of Rain and Dew is equal to the quantity of Water carried off by the Rivers and raised by Evaporation; with an Enquiry into the Origin of Springs’ (ib. v. 346). The last point, then much debated, was practically settled by Dalton's conclusion that springs are fed by rain. The same paper contained a further development of his theory of aqueous vapour, with the earliest definition of the ‘dew-point.’ It was followed on 12 April 1799 by an essay on the ‘Power of Fluids to conduct Heat’ (ib. v. 373), in which he combated Count Rumford's view that the circulation of heat in fluids is by convection solely. That entitled ‘Experiments and Observations on the Heat and Cold produced by the Mechanical Condensation and Rarefaction of Air,’ read on 27 June 1800 (ib. v. 515), contained the understated but important result that the temperature of air compressed to one-half its volume is raised 50° Fahrenheit.

Dalton's next communication gave him at once a European reputation. It consisted of four distinct essays comprised under a single heading, and was read on 2, 16, and 30 Oct. 1801 (ib. v. 535). The first was ‘On the Constitution of Mixed Gases,’ and expounded the doctrine of their mechanical diffusion, further developed in a paper read on 28 Jan. 1803. His inquiries into the relations of aqueous vapour and atmospheric air had convinced him that each follows its own laws of equilibrium, as if the other were absent. In 1801 he hit upon the explanatory idea, verified by numerous experiments, that the particles of every kind of elastic fluid are elastic only with regard to those of their own kind. This now discarded theorem rested on the fact (first observed by Dalton) that the quantity of aqueous vapour suspended in a given space depends upon temperature alone, and is unaffected by the pressure of air. Hence his generalisation that the maximum density of a vapour in contact with its liquid remains the same whether other gases be present or not. A further corollary was the extension of Boyle's law to a mixture of gases. In consonance with these views was Dalton's theory of the atmosphere, by which he regarded each of its constituents as forming a distinct envelope with its own proper limit of altitude (Phil. Trans. cxvi. 174). Observation, however, has shown no corresponding decrease in the proportion of oxygen at great heights.

The second essay of the set, ‘On the Force of Steam,’ gave the first table of its varying elasticity at temperatures from 32° to 212°, and described the ‘dew-point hygrometer’ (p. 582). The issue of some recent experiments was remarkably anticipated in the following sentence: ‘There can scarcely be a doubt entertained respecting the reducibility of all elastic fluids of whatever kind into liquids; and we ought not to despair of effecting it in low temperatures, and by strong pressure exerted upon the unmixed gases’ (p. 550). The third essay, ‘On Evaporation,’ showed the quantity of water evaporated in a given time to be strictly proportional to the force of aqueous vapour at the same temperature, and to be the same in air as in vacuo. The fourth, ‘On the Expansion of Gases by Heat,’ announced the law (arrived at almost simultaneously by Gay-Lussac) ‘that all elastic fluids expand the same quantity by heat’ (p. 537). This is known as ‘Dalton's law of the equality of gaseous dilatation.’ The fraction of their original volume, by which gases expand, under constant pressure, between 32° and 212°, was fixed by Dalton at 0.376 (since reduced to 0.367).

By these discoveries meteorology was constituted a science. They excited a strong interest, were immediately and widely discussed, and, with some minor deductions, made good their footing. From meteorology Dalton progressed naturally to chemistry. One of his leading mental characteristics was his proneness and power to realise distinctly what he thought about. His meditations on the atmospheric gases had led him to conceive them as composed of atoms, each surrounded by a very diffuse envelope of heat. That he should seek to follow them in their combinations was but an inevitable further