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THE Greek philosophers Leucippus and Democritus (about 440—400 B.c.) were among the first to give definite shape to the conception that “the bodies which we see and handle, which we can set in motion or leave at rest, which we can break in pieces and destroy, are composed of smaller bodies, which we cannot see or handle, which are always in motion, and which can neither be stopped, nor broken in pieces, nor in any way destroyed or deprived of the least of their properties (Clerk Maxwell). This doctrine was developed by Epicurus (340—270 B.c.). In the poem De Rerum Natura, Lucretius gives what purports to be an account of the teaching of Epicurus on the subject. The conception of atoms is fully elucidated in this poem, and on it is based a theory of the physical universe, and to some extent also a theory of things moral and spiritual. Lucretius says that nothing exists except atoms and empty space, that the atoms are of different forms and different weights, and that the number of atoms of each form is infinite; that the atoms are in constant motion, and that all change consists in the separation and combination of atoms. According to Lucretius, every atom is indestructible, and its motion is indestructible likewise. Atoms unite to form different kinds of substances; the properties of the substances so formed depend on the mutual relations of the atoms—“it matters much with what others and in what positions the atoms of things are held in union, and what motions they mutually impart and receive."*

Nearly complete as it was in many respects, the Lucretian theory failed as a scientific conception; it did not work. It did not admit of accurate applications to the facts of nature. It was not a science-producing theory, but rather a speculation about the possible causes of natural events.

* Lucretius, De Rerum Natura, II. 1007—9 (Munro's translation).

The teachings of the atomists were opposed by the follow- 273 ers of Aristotle, for whom the names of things were as real or more real than the things themselves. As Aristotelianism prevailed during the middle ages, atomism declined. The atomic theory of the Greek philosophers was revived towards the end of the 16th century by Gassendi. Boyle and Newton were upholders of this theory. Newton's demonstration of the action of the force of gravitation made a science of atomic physics possible; but the great difficulty was, and still is, to form a clear image of the action of gravitation in terms of the atomic conception of the structure of matter.

Not much was done to advance the applications of the 274 atomic theory, after Newton, until in the early years of the present century Dalton made a serious attempt to determine the conditions under which the atoms of elementary bodies unite to form the atoms of compound bodies. Dalton said that it is possible to find the relative weights of the atoms of elements and compounds, and he indicated the method by which this could be done.

Dalton found that the mass of hydrogen which combined 275 with carbon to form a certain compound of these elements was twice as great as the mass of hydrogen which combined with the same quantity of carbon to form another compound of these elements He found also that a specified mass of carbon combined with a certain mass of oxygen to form one oxide of carbon, and with twice that mass of oxygen to form another oxide of carbon. He noticed similar regularities in the masses of oxygen which combined with a fixed mass of nitrogen. Meanwhile Dalton, led thereto by his physical experiments on the absorption of different gases by water, had been thinking a great deal about the ultimate structure of matter. He pictured to himself a quantity of carbonic acid gas as built up of innumerable minute particles, or atoms, each of which was itself composed of one atom of carbon and two atoms of oxygen ; a quantity of nitrous oxide gas as built up of a vast number of atoms, each of which was itself composed of yet smaller portions of matter, viz. of one atom of nitrogen and one atom of oxygen; and a quantity of hydrogen gas as built up of minute particles, which were single atoms each composed only of hydrogen. Fig. 20, copied from the original in the

New System of Chemical Philosophy, gives Dalton's pictorial

presentment of his conception of the atoms of these three




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Dalton's application of the term atom to compounds, such as water, carbonic acid, &c.,

, shews that he did not use the word atom in its strict etymological meaning as that which cannot be cut' but rather as signifying the smallest portion of a body which could exhibit the properties of the body. The atom of water, for instance, could be

separated into atoms of hydro8 8 8

gen and oxygen, but this act of separation into parts produced kinds of matter wholly different from the water. The properties of the parts of the atom of water were quite unlike those of the atom itself; whereas the properties of a quantity of water were garded by Dalton as the same as those of the atom of water.

The reason of the regularities in the compositions of the two oxides of carbon, or the two compounds of carbon and hydrogen, examined by Dalton, was to be found, according to him, in the nature

of the atoms of carbon, hydroFig. 20.

gen, and oxygen. It is only necessary to assume that the atoms of these elements do not separate into parts in chemical reactions, then the facts find a simple explanation. One atom of hydrogen combines with one atom of carbon to form one atom of a certain hydride of carbon ; if another compound of these elements is formed containing more hydrogen, relatively to the same



mass of carbon, the next smallest quantity of hydrogen which can combine with the atom of carbon is two atoms. Similarly with the oxides of carbon. One atom of carbon combines with one atom of oxygen to form an atom of an oxide of carbon : this is the simplest possible compound of the two elements. The next compound which can be formed by combining more oxygen with the same mass of carbon, must be that the atom of which is composed of one atom of carbon united with two atoms of oxygen.

Chemical action was thus conceived by Dalton to be an 278 action between atoms. A mass of any element, or compound, was regarded as constituted of a vast number of very small particles of matter, all alike, all having the same weight, but all unlike the atoms of any other element, or compound. The atoms of elements were supposed to be capable of combining together to form atoms of compounds. In some cases the atoms of compounds might combine together to form atoms of more complex compounds. An atom of one element might combine with one, two, three, &c. atoms of another element to form one, two, three, or more, distinct compounds; but the atoms of elements could not separate into parts. The atoms of compounds on the other hand separated into parts when compounds interacted to produce new kinds of matter, and these parts, which were elementary atoms, rearranged themselves to form atoms of the new compounds produced in the interactions. Dalton pictured to himself an atom of a compound as a structure, or building, formed of a definite number of elementary atonis arranged in a more or less definite manner. He used symbols to represent elementary atoms, and he grouped these symbols together to represent compound atoms. Thus the symbol o represented an atom of oxygen ; O, an atom of hydrogen ; 0, an atom of nitrogen ; , an atom of sulphur; è, an atom of aluminium; , an atom of potassium; and so on. In Fig. 21 are given a few of Dalton's symbols for the atoms of compounds. A represents an atom of potash alum; B an atom of aluminium nitrate; C an atoin of barium chloride ; and D an atom of barium nitrate.

Thus did Dalton's conception of the atom throw light on the laws of fixity of composition, multiple proportions, and reciprocal proportions.

“ It is one great object of this work” says Dalton in his 279 New System of Chemical Philosophy, "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



Fig. 21.

compound particle, and the number of less compound particles which enter into the formation of one more compound particle.” How then did he determine the relative weights of the ultimate particles of simple bodies ?

Let us take the case of oxygen. The atom of oxygen, said Dalton, is 8 times heavier than the atom of hydrogen: let us call the weight of the atom of hydrogen, or the atomic weight of hydrogen, one; then the atomic weight of oxygen is asserted to be 8. Masses of hydrogen and oxygen combine in the ratio 1:8, to form water; but an atom of water is formed by the union of atoms of hydrogen and oxygen; if it is assumed that an atom of water is formed by the union of one atom of hydrogen with one atom of oxygen, then the atomic weight of oxygen must be 8. In saying that the atomic weight of oxygen was 8, Dalton implicitly made this assumption. But it might be assumed that an atom of water is composed of one atom of oxygen united with two atoms of hydrogen; as an atom of hydrogen is the unit in terms of which the weights of the atoms of other elements are stated, it follows from this assumption that the atomic weight of oxygen is 16; because 2 : 16 =1:8. Or it might be assumed that an

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