Atomism [ a privative and temnein to cut, i.e. indivisible] is the system of those who hold that all bodies are composed of minute, indivisible particles of matter called atoms. We must distinguish between

• atomism as a philosophy and
• atomism as a theory of science.

Atomism as a philosophy originated with Leucippus. Democritus (b. 460 B.C.), his disciple, generally considered the father of atomism, as practically nothing is known of Leucippus. The theory of Democritus may be summed up in the following propositions:

All bodies are composed of atoms and spaces between the atoms.

Atoms are eternal, indivisible, infinite in number, and homogeneous in nature ; all differences in bodies are due to a difference in the size, shape or location of the atoms.

There is no purpose or design in nature, and in this sense all is ruled by chance.

All activity is reduced to local motion.

The formation of the universe is due to the fact that the larger atoms fall faster, and by striking against the smaller ones combine with them; thus the whole universe is the result of the fortuitous concourse of atoms. Countless worlds are formed simultaneously and successively. Epicurus (342-270 B. C.) adopted the theory of Democritus, but corrected the blunder, pointed out by Aristotle, that larger atoms fall faster than smaller ones in vacuo . He substituted a power in the atoms to decline a little from the line of fall. Atomism is defended by Lucretius Carus (95-51 B.C. ) in his poem, "De Rerum Naturâ." With the exception of a few alchemists in the Middle Ages, we find no representatives of atomism until Gassendi (1592-1655) renewed the atomism of Epicurus. Gassendi tried to harmonize atomism with Christian teaching by postulating atoms finite in number and created by God. With the application of atomism to the sciences, philosophic atomism also revived, and became for a time the most popular philosophy. Present-day philosophic atomic regards matter as homogeneous and explains all physical and chemical properties of bodies by a difference in mass of matter and local motion. The atom itself is inert and devoid of all activity. The molecule, taken over from the sciences, is but an edifice of unchangeable atoms. Philosophic atomism stands entirely on the basis of materialism, and, though it invokes the necessary laws of matter, its exclusion of final causes makes it in the last analysis a philosophy of chance.

The atomic theory was first applied to chemistry by Dalton (1808), but with him it meant little more than an expression of proportions in chemical composition. The theory supplied a simple explanation of the facts observed before him: that elements combine in definite and multiple proportions. The discovery in the same year by Gay-Lussac of the law that gases under the same pressure and temperature have equal volumes was at the same time a confirmation and an aid in determining atomic weights. Avogadro's law (1811) that gases under the same conditions of pressure and temperature have an equal number of molecules, and the law of Petit and Dulong that the product of the specific heat and atomic weight of an element gives a constant number were further confirmations and aids. The atomic theory was soon applied to physics, and is today the basis of most of the sciences. Its main outlines are: Matter is not continuous but atomically constituted. An atom is the smallest particle of matter that can enter a chemical reaction. Atoms of like nature constitute elements, those of unlike nature constitute compounds. The elements known today are about 76 in number and differ from one another in weight and physical and chemical properties. Atoms combine to form molecules, which are the smallest quantities of matter that can exist in a free state, whether of an element or a compound. Some believe that the atom retains its individuality in the molecule, whilst others consider the molecule homogeneous throughout. The theoretic formulas of structure of Frankland suppose them to remain. The spaces between the atoms are filled with an imponderable matter called ether. Upon the nature of ether the greatest differences of opinion exist. The adoption by scientists of Maxwell's theory of light seems to render the ether-hypothesis with its many contradictions superfluous. At all events it is quite independent of the atomic theory.

The results obtained by the Hungarian Lenard, the English physicist J.J. Thomson, and many others, by means of electric discharges in ratified gases, the discovery of Hertzian waves a better understanding of electrolysis and the discovery of radium by Madame Curie have made necessary a modification of the atomic theory of matter. The atom, hitherto considered solid and indivisible, is now believed to break up into ions or electrons. This new theory, however, must not be considered as opposed to the atomic theory; it comes rather as an extension of it. In chemistry, the principal field of the atomic theory, the atom will still remain as the chemically indivisible unit. The hypothesis of subatoms is, moreover, not entirely new; it was proposed by Spencer as early as 1872 ("Contemporary Rev.", June, 1872) and defended by Crookes in 1886.

The physico-chemical theory of atomism, though is not a demonstrated truth, offers a satisfactory explanation of a great number of phenomena, and will, no doubt, remain essentially the same, no matter how it may be modified in its details. In chemistry, it does not stop arbitrarily in the division of matter, but stops at chemical division. If another science demands further division, or if philosophy must postulate a division of the atom into essential principles, that is not the concern of chemistry. Science has no interest in defending the indivisible atom of Democritus.

Scholastic philosophy finds nothing in the scientific theory of atomism which it cannot harmonize with its principles, though it must reject the mechanical explanation, often proposed in the name of science, which looks upon the atom as an absolutely inert mass, devoid of all activities and properties. Schlolastic philosophers find in the different physical and chemical properties of the elements an indication of specificlly different natures. Chemical changes are for them substantial changes, and chemical formulas indicate the mode in which the elements react on one another in the production of the compound. They are not a representation of the molecular edifice built up of unchangeable atoms. Some would accept even this latter view and admit that there are no substantial changes in inanimate nature (Gutberlet). This view can also be harmonized more easily with the facts of stereo-chemistry. As regards the phenomena observed in radlo-activity, a generalization, either in the materialistic sense, that all matter is homogeneous, or in the scholastic sense, that all elements can be changed into one another, is in the present state of science premature.

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