This significant fact was not realized until much later, in the period when a radical change occurred in physics, which brought about a crisis, at the end of the 19th and the beginning of the 20th century. The crisis in physics was mainly the result of the attempts of some scientists to interpret the newly discovered quantum, electromagnetic, relativistic, and atomic phenomena in the light of classical mechanics. This crisis has been subjected by V. I. Lenin to a thoroughgoing philosophical analysis, in his well-known book "Materialism and Empiriocriticism".
One of the mechanistic constructions of a world picture, in the 17th and 18th centuries, originated with Descartes, who took as his point of departure the laws of collision and the contact interaction of bodies in conformance with the law he discovered of the conservation of momentum of colliding spheres. In cases when there was no visible contact between the bodies, Cartesian adherents tried providing a speculative explanation, as may be illustrated by the vortex hypothesis they applied to gravitation. Newton and his followers were opposed to such unfounded hypotheses. They took the law of gravitation as a description of facts and did not seek a model in terms of which it could be interpreted (this in fact could not be done even in the 19th century, before the advent of Einstein's theory). Even though many intellects of the 18th century, including Euler and Lomonosov, found the phenomenological approach of the Newtonians hard to accept, still it was no doubt progressive for its time.
Thus, in the 17th and 18th centuries, natural processes were interpreted in terms of the laws of classical mechanics. It was assumed that space and time exist independently of matter and of each other, space being taken to be Euclidean and time absolute and unique. Yet, space and time undoub tedly acted somehow on matter, since a free body, for instance, would move along a straight line rather than along any other. As is known, it subsequently became clear that the analysis of the concept of a perfectly "free" body, isolated from any other kind of matter, is a very complex question.
Now the problem of the structure of matter itself was the point of frequent discussions. The atomic hypothesis, set forth in the work of Gassendi and some other scientists, found many adherents. It was, as is known, fairly successfully developed by Bernoulli and Lomonosov, as applied to the treatment of thermal phenomena and the gas laws. However, owing to the impossibility of observing atoms and to the lack of understand ing of the chemical laws, these results seemed unwarranted and were eventually completely forgotten. Although Soviet historians of science have shown that the trends set by Lomonosov have been retained in the statements of some Moscow physicists, and in the works of Georgian and other scientists (cf. , for instance, V. D. Parkadze's investigations), these were not men of
distinction and thus their ideas did not carry sufficient authority to affect the course of development of physics to any appreciable extent.
In the second half of the 18th and the beginning of the 19th century, the investigation of magnetic, electrical, light, and thermal phenomena led to the phenomenological postulation of various "fluids", as special forms of matter. It was assumed that electricity is produced by an electrical fluid, heat by the flow of a caloric, light was interpreted as being vibrations of the ether, etc. Unity was achieved only by the classical-mechanical treat ment of any given motion or interaction.
The modern period in the evolution of physics and the creation of a unified world picture fits fairly closely within the span of a century, the 19th, culminating with the discovery of X-rays and radioactivity in 1895 1896, the discovery of the quantum laws in 1900, and the formulation of the theory of relativity in 1905. The 19th century is noted, first of all, for the discovery of a new form of matter, the electromagnetic field, which does not have any rest mass. Secondly, the 19th century saw the unification of different forms of matter; electrical and magnetic phenomena proved to be the manifestation of a single electromagnetic field, and thermal pheno mena were basically reduced to mechanical processes, i. e. , to the energy of the random motion of atomic particles. The electromagnetic wave also proved to be endowed with thermal properties (like any other kind of matter), viz., energy, temperature, entropy.
An important part in this unification was played by the universal law of conservation of energy, established in the middle of the 19th century (Meyer, Joule, and Helmholtz). When the great significance of energy was realized, attempts were even made to construct a unified world picture on the basis of the concept of energy alone. This picture, propounded by Ostwald, proves to be too restrictive; it blurred the distinctions between the individual properties of the elements, stressed nonatomistic conceptions, and ulti mately had an adverse effect on scientific thought. A fundamental discovery was that of one of the elementary carriers of electrical charge, i. e. , the negatively charged electron, which was predicted by Helmholtz. This discovery came relatively late (J. J. Thomson, 1897) and was already associated with the first probings into the atomic world.