[Illustration: _Painting by Ford Madox Brown_ _By permission of the Town Hall Committee of the Manchester Corporation_
During the following months he returned to the study of those cases in which the same elements combine to form more than one compound. We have seen that oxygen unites with nitric oxide to form two compounds, and that into the one compound twice as much nitric oxide (by weight) enters as into the other. A like relation was found in the weight of oxygen combining with carbon in the two compounds carbon monoxide and carbonic acid. In the summer of 1804 he investigated the composition of two compounds of hydrogen and carbon, marsh gas (methane) and olefiant gas (ethylene), and found that the first contained just twice as much hydrogen in relation to the carbon as the second compound contained. In a series of compounds of the same two elements one atom of one unites with one, two, three, or more atoms of the other; that is, a simple ratio exists between the weights in which the second element enters into combination with the first. This law of multiple proportions afforded confirmation of Dalton's atomic theory, or chemical theory of definite proportions.
"Without such a theory," says Sir Henry Roscoe, "modern chemistry would be a chaos; with it, order reigns supreme, and every apparently contradictory discovery only marks out more distinctly the value and importance of Dalton's work." In 1826 Sir Humphry Davy recognized Dalton's services to science in the following terms: "Finding that in certain compounds of gaseous bodies the same elements always combined in the same proportions, and that when there was more than one combination the quantity of the elements always had a constant relation,--such as 1 to 2, or 1 to 3, or 1 to 4,--he explained this fact on the Newtonian doctrine of indivisible atoms; and contended that, the relative weight of one atom to that of any other atom being known, its proportions or weight in all its combinations might be ascertained, thus making the statics of chemistry depend upon simple questions in subtraction or multiplication and enabling the student to deduce an immense number of facts from a few well-authenticated experimental results. Mr. Dalton's permanent reputation will rest upon his having discovered a simple principle universally applicable to the facts of chemistry, in fixing the proportions in which bodies combine, and thus laying the foundation for future labors respecting the sublime and transcendental parts of the science of corpuscular motion. His merits in this respect resemble those of Kepler in astronomy."