MOLECULAR THEORY, in physics and chemistry, the theory which teaches that mat ter, however homogeneous it may appear to be, is in reality a heterogeneous aggregate of ultra microscopic particles called "molecules." In a crude form this idea was entertained and de fended, even before the Christian era, by cer tain of the philosophers of Greece and Rome, among whom Democritus and Lucretius may be specially mentioned. The writings of these ancient authorities are of great interest to the historian, and are highly creditable when allow ance is made for the state of science at the time they were produced; but they are too nebulous to be of practical value to the modern physicist, who demands that every theory shall be tested by a critical comparison of its necessary conse quences with the facts of observation.
The distinction between the atomic theory and the molecular theory, though still conven ient for many purposes, is nevertheless an arti ficial one, which will doubtless disappear in time. The atomic theory relates to the view that the chemist takes with regard to the con stitution of matter, while the molecular theory relates to the point of view of the physicist. The atom has been the ultimate unit in which the chemist has been interested, while the mole cule has been, until the very recent past, the ultimate unit with which the physicist has con cerned himself. The chemist has dealt mainly
with the phenomena of chemical combination, as observed when two or more definite sub stances unite so as to produce one or more new substances, and to him it has not been essential to speculate with regard to the physical nature of the tiny particles that constitute his funda mental units, nor with regard to their behavior except as it concerns the chemical phenomena that he observes in his test-tubes and other apparatus. The physicist, on the other hand, has paid but little attention to the facts of chemical combination, hut has confined his at tention chiefly to the mechanical, thermal, elec trical and magnetic phenomena that are ob served in connection with definite substances whose chemical constitution remains unchanged, and to him the molecule has been a small, phys ical body, possessing mass and extension, mov chanics. The modern corpuscular theory of treatment in accordance with the laws of me chanics. The modern corpuscular theory of matter, however, promises to do a great deal to bring the physical and chemical viewpoints far closer together, and perhaps even to bridge wholly the gap between them. (See ELEcraot