John Dalton, the son of a Quaker hand-loom weaver, was born at Eaglesfield, in Cumberland, in 1766. While still a boy he took to school-teaching, and acquired, in his leisure and by his own exertions, a competent knowledge of mathematics and physical science. In 1793 he was called to give instruction in mathematics, natural philosophy, and chemistry at the Manchester New College, the Nonconformist academy—now moved from Warrington—in which Priestley had formerly lectured. Here he remained six years, leaving the college to take up an independent position as a private tutor, so as to enable him the more freely to pursue his scientific inquiries. In 1800 he became Secretary of the Philosophical Society of Manchester, and remained connected, as an official, with that institution until his death in 1844. The greater number of his scientific communications were published by that society. In the outset of his scientific career he was attracted to meteorology; and it was probably its problems which led him in the first place to experiment, and to speculate on the physical constitution of gases. In the course of these observations he was led to the discovery of the law of thermal expansion of gases, with which his name is now generally associated. His speculations concerning the physical constitution of gaseous substances, arising from the contemplation of gaseous phenomena, led him to the conception that a gas is composed of particles that repel one another with a force decreasing as the distance of their centres from each other; and it is probable that in this manner he familiarised himself with the idea of the existence of atoms. His first insight into the laws of the chemical combination of these atoms seems to have originated from his discovery that, when two substances unite in different proportions, these proportions may be expressed in simple multiples of whole numbers. Thus he found, on examining the composition of marsh gas and of ethylene, both hydrocarbons, that for the same weight of hydrogen there was twice the amount of carbon in ethylene that there was in marsh gas. He then examined the oxides of nitrogen, and found a similar regularity to hold good in these compounds. Some time prior to the autumn of 1803 Dalton was led to the supposition that these regularities could be satisfactorily explained by the assumption that matter is composed of atoms having sizes and weights differing with each substance, but of identical weight and size for any particular substance, and that chemical combination consists in the approximation of these atoms. This simple hypothesis explained all the facts then known. It explained the constancy in the chemical composition of substances, which may be said to have been established by Proust, and which is now formulated as the Law of Constant Proportion—that the same body is invariably composed of the same elements, united in the same proportion. It explained also the fact discovered by Dalton that, when an element unites with another in different proportions, the higher proportions are multiples of the lowest—now formulated as the Law of Multiple Proportion. It further explained the fact, which may be said to have been foreshadowed by Richter, that when two bodies, A and B, separately combine with a third body, C, the proportions of A and B which unite with C are measures or multiples of the proportions in which A and B combine together. This is known as the Law of Reciprocal Proportion.
John Dalton.
From a painting by B. R. Faulkner in the possession of the Royal Society
Dalton’s theory was first made generally known by Thomas Thomson, in the third edition of his System of Chemistry, published in 1807, and was employed by Thomson in his paper on “The Oxalates of Strontium,” published the same year in the Philosophical Transactions. The first printed account by Dalton himself is contained in Part I. of his New System of Chemical Philosophy, published in 1808, the substance of which had been previously given in a course of lectures at the Royal Institution, London, and subsequently repeated in Edinburgh and Glasgow.
The statement of his theory is contained in chapter iii. of this work, under the heading “Of Chemical Synthesis,” and is accompanied by a plate and explanation, of which a facsimile is given on pp. 130–1.
The facts upon which Dalton based his theory are incontrovertible; but Dalton’s explanation of them was not universally accepted at the time he gave it. Davy, who, of course, was familiar with the conception of atoms as part of the Newtonian philosophy, objected to the term “atomic weight” introduced by Dalton, and suggested the expression “combining proportion”; and Wollaston, for similar reasons, proposed the term “equivalent,” as denoting the constant quantity with which bodies went in and out of combination. There is no doubt that the use of these terms retarded the general acceptance of Dalton’s doctrine, and, moreover, brought into the science a confusion which was not finally dispelled, as we shall see, until during the second half of the century.
ELEMENTS
Simple
Binary