"Such, then," writes Dr Wilson, "were the steps by which Dalton was conducted to the discovery of the laws of combining proportions. He was testing, by experiment, the truth of a hypothesis as to the cause of the specific solubility of gases in water, which proved in the end to be quite untenable; but, like Columbus, who missed an El Dorado but found an America, he discovered something better. From what Dr Thomson tells us, he was struck by observing that the quantity of hydrogen in fire-damp is exactly twice that in heavy carburetted hydrogen, the quantity of carbon being the same in both. His constant reference of the properties of masses to those of their smallest molecules, led him at once to connect these proportions in which the carbon and hydrogen occurred, with the relative weights of their attracted particles." Dr Wilson supposes that Dalton reasoned thus: "Hydrogen and carbon are made up of particles which have different weights, the carbon atoms being all six times heavier than the hydrogen ones; but if hydrogen and carbon have atoms differing in relative weight, oxygen, nitrogen, and every other elementary substance will have atoms differing in relative weight also; and these may be ascertained by finding the relative weights according to which the masses made up of them combine with each other. To Dalton's mind, fitted, as it were, already with the conception of everything consisting of atoms, it was only necessary to introduce the additional idea of those atoms differing in relative weight, and all the laws of combining proportion rose at once into view. He was gifted with a bold, self-reliant, farglancing, generalising spirit, and the researches he had long been prosecuting had doubtless strengthened greatly that faith in the uniformity of Nature's laws, which we all inherit as an essential part of our mental constitution. We may believe that, without an effort, and almost instinctively, he would infer that if hydrogen followed a law of multiple proportion in its higher combinations with carbon, a similar relation would be found to hold in every case where the same elements united to form more than one compound." Dalton's views of chemical combination, including both the facts and the hypothesis which expressed and explained them, are generally known as his "Atomic Theory." CHAPTER IX. A SKETCH OF THE ATOMIC THEORY FROM THALES TO SIR ISAAC NEWTON. “For hot, cold, moist, and dry, four champions fierce, OEVAL with the manifestation of the reasoning faculties-that far and pre-historic past of man's development-the constitution of the earth and its different aspects and surroundings would hardly fail to excite the imagination of the denizens of the wilde; and, in the course of time, natural phenomena would offer large discussion to the better endowed of the race. A recognition, however faint, of the external world and its living beings, would gratify the dawning mind, and vie in interest with the contemplation of the gods, whose attributes were not seldom associated with both human and terrestrial agencies. In scanning these agencies endless speculations arose, as fanciful in tone as they were ephemeral in duration; each new thought proving as baffling as its predecessor in the attempted solution of the great problem :-What is the constitution, the ultimate composition, or real nature of matter itself? The bold expounders of the cosmogony undertook an arduous task, and as their efforts to unravel the in L tricacies of nature often failed, they found it convenient to follow the example of the theogonists, and shelter their ignorance under the shadows of the mythological altars of their age and race—an orthodox mode of treating scientific difficulties that is not without its counterpart in the history of the state churches of these latter days. The progress of knowledge might well be dilatory amid the almost impenetrable mists of superstition in the past, when to-day the cry of "more light" is still so audible among the adepts of science; and this, be it remembered, after eighteen centuries of Christian indoctrination, itself ushered into the world upon a large substratum of man's intellectual gains. History has made us cognisant of the varied acquirements of the Eastern nations; of the marvellous skill and aptitude of the Egyptians; of the nobler forms of art, the noblest ever vouchsafed to man, being developed pari passu with the grandly philosophic aims of the Greeks; of the world-wide dominion and scope of the Romans; of the subtle grasp of the Arabian physicians, and the erudite lore of the schoolmen ; all operating more or less in the direction of enlightenment and civilisation. Yet the science of these modern days culling its data from the great stores of evidence of the past, and favoured by novel experimental appliances and methods of inquiry strictly inductive, can lay claim to little more than a firm step on the threshold of discovery. As of yore, so do enthusiastic minds now look hopefully for still higher revelations in science; and assuredly, if there was ever an epoch in human history marked by bold and progressive lines, and A Sketch of the Atomic Theory. 163 powers to fathom the arcana of the cosmos, it is the present age that has been blessed, beyond all precedent, by discoveries of overpowering brilliancy and magnitude. Man looks upon the Earth, its waters and dry land, and admires its meres, its meadows, and its mountains; he soars for miles in its circumambient air; he mines its superficial crust, and fathoms its ocean depths; and everywhere marks a marvellous diversity of form and substance in the stratified rock, the tidal wave, and transparent ether. His admiration is enhanced by contemplating the myriads of organisms in active life, taking their start from the primitive organic cell that in its timely growth and maturity may become shaped into the umbrageous palm or gnarled oak; or find its nidus in the higher organisation of the chimpanzee or cetacea. Yet the organic and inorganic worlds in all their entirety; the blood and the life thereof, as well as the adamantine conditions of inert matter, when subjected to chemical analysis, become resolved into a few primary or elementary substances. They are designated simple or elementary bodies, because they can be shown to exhibit one kind of ponderable matter only, be it light as air, or heavy as lead; for instance, the gases oxygen and hydrogen, or the metals gold and silver, which the chemist has hitherto failed to resolve into more parts or constituents than one. To-day the chemist assures us of sixty-three elementary bodies-some of them being little heard of, others in vast proportion to the mass; thus four well-known elements in their various compounds, |