93 CHEMISTRY AND THE ATOMIC THEORY By experiments on falling bodies, Galileo disproved Aristotle's conception of bodies intrinsically light, and thus made possible a consistent theory of dynamics. The corresponding simplification in chemistry was made when Lavoisier impressed on the world the fact that there was no need to explain combustion by the conception of a body with properties fundamentally unlike those of other material substances. The unity of outlook which followed made modern chemistry possible. Many facts already known fell into a reasonable scheme of knowledge, and innumerable fresh facts came to light under the impetus given to chemistry by the new and simpler views. It was inevitable that the fundamental question of the nature of matter should come up for reconsideration. The atomic theory of the Greeks, which we have seen mirrored in the poetry of Lucretius, was held as a probable speculation by many seventeenth and eighteenth century philosophers, and used by physicists such as Newton and Boyle. But no real and considerable advance was made in the theory itself till Lavoisier's work had simplified and defined the problem, and the development of experimental chemistry had given firm ground from which to start. The theory then ceased to be a philosophic speculation, and became a definite scientific hypothesis, framed to explain certain quantitative measurements, and itself of a nature to be put into a quantitative form. In other words, the measurement of the relative weights in which chemical elements combine gave the relative weights of their constituent atoms. This result is due to John Dalton (17661844), who was born of a Quaker family in Cumberland, and worked and died at Manchester. A NEW SYSTEM OF CHEMICAL PHILOSOPHY By JOHN DALTON, 1808. PART I. CHAP. II. On the Constitution of Bodies. THERE are three distinctions in the kinds of bodies, or three states, which have more especially claimed the attention of philosophical chemists; namely, those which are marked by the terms elastic fluids, liquids, and solids. A very famous instance is exhibited to us in water, of a body, which, in certain circum stances, is capable of assuming all the three states. In steam we recognise a perfectly elastic fluid, in water a perfect liquid, and in ice a complete solid. These observations have tacitly led to the conclusion which seems universally adopted, that all bodies of sensible magnitude, whether liquid or solid, are constituted of a vast number of extremely small particles, or atoms of matter bound together by a force of attraction, which is more or less powerful according to circumstances.... Whether the ultimate particles of a body, such as water, are all alike, that is, of the same figure, weight, &c. is a question of some importance. From what is known, we have no reason to apprehend a diversity in these particulars: if it does exist in water, it must equally exist in the elements constituting water, namely, hydrogen and oxygen. Now it is scarcely possible to conceive how the aggregates of dissimilar particles should be so uniformly the same. If some of the particles of water were heavier than others, if a parcel of the liquid on any occasion were constituted principally of these heavier particles, it must be supposed to affect the specific gravity of the mass, a circumstance not known. Similar observations may be made on other substances. Therefore we may conclude that the ultimate particles of all homogeneous bodies are perfectly alike in weight, figure, &c. In other words, every particle of water is like every other particle of water; every particle of hydrogen is like every other particle of hydrogen, &c. PART I. CHAP III. On Chemical Synthesis. When any body exists in the elastic state, its ultimate particles are separated from each other to a much greater distance than in any other state; each particle occupies the centre of a comparatively large sphere, and supports its dignity by keeping all the rest, which by their gravity, or otherwise, are disposed to encroach upon it, at a respectful distance. When we attempt to conceive the number of particles in an atmosphere, it is somewhat like attempting to conceive the number of stars in the universe: we are confounded with the thought. But if we limit the subject, by taking a given volume of any gas, we seem persuaded that, let the divisions be ever so minute, the number of particles must be finite; just as in a given space of the universe, the number of stars and planets cannot be infinite. Chemical analysis and synthesis go no farther than to the separation of particles one from another, and to their reunion. No new creation or destruction of matter is within the reach of chemical agency. We might as well attempt to introduce a new planet into the solar system, or to annihilate one already in existence, as to create or destroy a particle of hydrogen. All the changes we can produce, consist in separating particles that are in a state of cohesion or combination, and joining those that were previously at a distance. In all chemical investigations, it has justly been considered an important object to ascertain the relative weights of the simples which constitute a compound. But unfortunately the enquiry has terminated here; whereas from the relative weights in the mass, the relative weights of the ultimate particles or atoms of the bodies might have been inferred, from which their number and weight in various other compounds would appear, in order to assist and to guide future investigations, and to correct their results. Now it is one great object of this work, to shew the importance and advantage of ascertaining the relative weights of the ultimate particles, both of simple and compound bodies, the number of simple elementary particles which constitute one compound particle, and the number of less compound particles which enter into the formation of one more compound particle. If there are two bodies, A and B, which are disposed to combine, the following is the order in which the combinations may take place, beginning with the most simple: namely, I atom of A+ 1 atom of B = I atom of C, binary. |