Dalton represented his atoms by symbols, as, for instance, oxygen by a circle, hydrogen by a circle with a dot, and other elements by similar simple figures. He considers that all bodies are composed of atoms, which, however small, have a definite size and weight. The symbols of oxygen and hydrogen placed together represented water, which he supposed to be composed of an atom of each ; and other symbols were used in a similar manner. The atomic weights are the relative or combining weights, not the absolute weights. Assuming hydrogen to be the lightest body he called it 1 ; oxygen he determined to be 7, but it has since been ascertained to be 8. Dr. Wollaston has called oxygen 1; hydrogen then becomes -125. Dalton not only stated the general principle of combination in definite proportions, but be stated the chief laws of combination on which modern chemical analysis is based : 1, that the same compound consists invariably of the came constituents ; 2, that the elements of every compound always unite in the same proportion by weight (and Ouy-Luesac, in 1809, proved that gases unite in the same proportion by volume as well as by weight); 3, that, when any element combines in more pro portions than one, those proportions are multiples—I, 2, 3, 4; 6, 12, 18, 24 ; 8, 16, 24, 32, and so on ; 4, that, if two substances combine in a certain proportion with a third, they combine in exactly the same proportion with each other ; 5, that the combining proportion of a compound is the sum of its constituents, as hydrogen 1 oxygen 8 = water 9. Davy substituted the word proportion for that of atom, and Wollaston that of equivalent, which is now generally used ; but whatever be the term, the meaning is the same, and in proportion as analyses have become more accurate, the laws which Dalton laid down have been more remarkably confirmed.
The third volume of Dalton's 'New System of Chemical Philosophy' was not published till 1827, but the greater part of it bad been printed nearly ten years before. He treats of the metallie oxides, the aul phurets, phosphurets, carburets, and of the alloys. In the interval between the printing and publication, many of the facts had been anticipated by others, and some of them carried much farther. The most important part of the volume is the appendix, of about 90 pages, in which he discusses with his usual sagacity various important matters connected with heat and vapour. lie gives a new table of atomic weights, much more copious than those contained in the two preceding volumes, and in which he introduces the corrections rendered necessary by the numerous correct analyses which had been made since the publication of the second volume.
Dr. Dalton'a other works, which are tolerably numerous, are inserted in the Manchester Transactions," Nicholson's; Journal,' the Philo sophical Transactions,' and the 'Philosophical Magazine,' and consist of experiments and observations on heat, vapour, evaporation, rain, wind, the aurora borealis, dew, and a variety of other phyaical subjects.
Dr. Dalton was of middle stature and strongly made. His face is said to have resembled the portraits of Newton. His power of mind was naturally strong ; he was a patient observer, and an independent thinker, with the moat perfect self-reliance, and with an extraordinary power of tracing the relations of physical phenomena; his experiments had rarely an insulated character, but were steps in some process of wide generalisation. By such a process, comparing the results of numerous experiments and numerous facts which had been estab lished, he elicited order out of seeming confusion, and may truly be said to have become the legislator of chemistry, which before his time was little better than an experimental art—an accumulated mass of unconnected and imperfectly-developed facts. Dalton laid down the laws of the combination of substances, and at once advanced chemistry to the rank of a science. His moral character was worthy of his intel lectual. lie was a man of the strictest truth and honesty ; inde pendent, grave, reserved, but not austere; frugal, but not parsimonious. Unassuming, the honours which he received were voluntarily bestowed upon him by those who were beat able to estimate the value of his services to science and manufacturing industry ; unostentatious, it was some time before his townsmen in Manchester were aware of his merits, but a small circle of friends appreciated him highly, aud long before he received his pension offered to provide him with an inde pendence that he might devote the whole of his time to science, but he declined to accept it, observing "that teaching was a kind of recreation, and that if richer he would probably not spend more time in his investigations than he was accustomed to do.' His mode of life was singularly uniform. He was rarely from home except when he went to some place to lecture ; he attended the meeting of the Society of Friends, of which he was a member, twice every Sunday ; be went daily to his laboratory, of which the apparatus was of the simplest end indeed rudest kind; ho played at bowls on the afternoon of every Thursday; and he paid an annual visit to his friends and the mountains in Cumberland and Westmoreland. He was never married.
Dalton was elected a Fellow of the Royal Society about 1821; he was elected a corresponding-member of the Institute of France, and a few years later was enrolled a Foreigu Fellow. He was also a member of the Royal Academies of Science of Berlin and Munich, and of the Natural History Society of Moscow.
(Thomson, history of Chemistry, vol. ii.; Pharmaceutical Journal, Oct. 1841; Life and Discoveries of Dalton, in British Quarterly Review, No. 1 ; British Association Reports.)