The difficulty of examining very small things by reason of the perturbations introduced by our measuring instruments can be illustrated in a simple manner. To take the temperature of a hot bath is easy enough, but a thimbleful would be chilled or other wise altered by the thermometer employed. It is in fact difficult to observe any very small quantity without introducing pertur bations. Heisenberg has actually formulated a law of uncertainty, in a singularly definite manner, showing that Planck's quantum is involved. There is an uncertainty in the position of an electron, and an uncertainty about its speed or momentum. The product of the two uncertainties is equal to Planck's constant Ii.
There are still more recent developments. Speculation is active, the revolution is in progress and fresh discoveries are constantly being made. The whole constitutes an elaborate network of inter lockings and coincidences, which must have a deep-seated mean ing when we can unravel the tangled skein. All we can do now is to hint at the stages that are being reached, to realize that nothing like the last word has been spoken, to wonder at the genius which has so greatly illuminated and yet partly confused us and to have faith in the advent of a great generalization. (0. J. L.) PHYSICS, ARTICLES ON. The astonishing advances which the science of physics has made since the Eleventh Edition of the Encyclopaedia Britannica was prepared have been met by a complete and drastic revision of the physics articles. How far the researches of the present century have changed the outlook is evidenced by the fact that the Eleventh Edition contains noth ing on relativity, on atomic structure or on the quantum theory, three questions which dominate modern physics, and influence every branch. Not only have z'rticles by the highest authorities on the new topics been introduced, and the less fundamental articles on older subjects been cut down in size in such a way as to make the balance accord with actuality, but every article has been either completely rewritten or revised so as to make it part of a proportioned and organic body of information on modern physics.
The articles SCIENCE and PHYSICS give a general view of the scope of physics, and its place among the sciences. The funda mental nature of space and time in the physical world, and of gravitation, is discussed under RELATIVITY and SPACE-TIME, while the history of the growth, and another aspect of the nature of, the relativity problem is given under ETHER. An account of i experiments and observations which played a great part in pre paring the way for relativity will be found under MICHELSON MORLEY EXPERIMENT and ABERRATION OF LIGHT. Measurements of the VELOCITY OF LIGHT, the constancy of which is fundamental, are described under the article so entitled. The historical, ex perimental and theoretical aspects of the subject are thus all readily accessible.
The subject-matter of physics can be broadly classed as matter and radiation. The physical nature of matter is expounded under KINETIC THEORY OF MATTER, which deals mainly with gases. Supplementary to this article is BROWNIAN MOVEMENT. The tran sition from gas to liquid is handled from the general theoretical point of view in the article LIQUEFACTION OF GASES, which also deals with technically important aspects of the subject. The mod ern view of solids comes under SOLID STATE, THEORY OF. For the arrangement of atoms and molecules in crystalline solids the arti cle X-RAYS AND CRYSTAL STRUCTURE should be consulted. Crys tals have a regular ordered structure : in many ways the antithe sis of the crystalline state is the colloidal state, dealt with under COLLOIDS. Important general properties of liquids have special
articles SURFACE TENSION and VISCOSITY devoted to them. These articles, thefi, deal with the general properties of matter : the field of radiation is covered in RADIATION, RAYS, which deals broadly with the various kinds of waves comprised in the electro magnetic spectrum.
The peculiar task of physics in the present century has been to effect a synthesis of the properties of matter and radiation in terms of units of electric charge, of matter and of radiation. Modern atomic physics, which comprises this field, is well repre sented in the Encyclopaedia. The unit of electricity is the ELEC TRON, to which an article is devoted. A vast body of work that established and developed the electron theory is described under ELECTRICITY, CONDUCTION OF, IN GASES. The connecting link between matter and radiation is furnished by the nuclear theory of the atom, which asserts that an atom consists of a minute, positively-charged massive nucleus, surrounded by group ings of electrons in motion. The general nuclear theory is ex plained under ATOM, the properties of the nucleus, and their pro found consequences for physics and chemistry, under NUCLEUS, ISOTOPES, and POSITIVE RAYS. TRANSMUTATION OF THE ELE MENTS presents another aspect of nuclear theory. RADIOACTIVITY is an article of first importance for the understanding of modern physics. The theory of emission of light and X-rays by an atom, described under ATOM, is based upon the QUANTUM THE ORY, discussed in a key article which leads up to the new develop ments known as wave mechanics. X-RAYS, NATURE OF; SPECTROS COPY; BAND SPECTRUM ; RESONANCE POTENTIALS ; THERMIONICS ; PHOTOELECTRICITY; COMPTON EFFECT; RAMAN EFFECT deal with the experimental and theoretical side of radiation phenomena which have a prominent place in atomic physics, while WILSON CLOUD CHAMBER describes a method of experimenting on single atoms and electrons which has had far-reaching results.
The general body of physics is dealt with under comprehensive articles devoted to the main conventional divisions—HEAT, LIGHT, SOUND, ELECTRICITY, MAGNETISM, with important supple mentary articles, among which may be mentioned: under heat, THERMODYNAMICS ; CALORIMETRY ; THERMOMETRY ; under light, OPTICS; SPECTROSCOPY; FLUORESCENCE AND PHOSPHORESCENCE; under electricity, ELECTRICITY, CONDUCTION OF ; ELECTROLYSIS. Important instruments and their uses are described under : CON DENSER; MICROSCOPE; MICROSCOPY; MIRROR; BINOCULAR IN STRUMENT; INTERFEROMETER; INSTRUMENTS, ELECTRICAL; ACCUMULATOR; INDUCTION COIL ; COMPASS ; SELENIUM CELLS; VACCUM.
Applied physics is another aspect of the subject which receives full representation. The meteorological side is represented by ELECTRICITY, ATMOSPHERIC and the geophysical by TERRESTRIAL MAGNETISM; EARTH CURRENTS and GRAVITATION. Some of the chief heads under which the bearing of physics in everyday life and industry is described in detail are : ACOUSTICS ; AERODYNAM ICS ; ELECTRIC LAMPS AND VALVES, MANUFACTURE OF ; ELECTRO MAGNET; GRAMOPHONE; LENS; PHOTOGRAPHY; PHOTOMETRY; TELEGRAPH ; TELEPHONE ; TELEVISION ; X-RAYS, Applications Of. Every aspect of wireless is included in the scheme: ELECTRIC WAVES deals with Hertz's original discovery and the subsequent laboratory developments ; WIRELESS TELEGRAPHY deals, in two divisions, with the theoretical and practical aspect of the subject, while THERMIONIC VALVE; MICROPHONE; LOUD SPEAKER; RADIO RECEIVER describe apparatus vital to BROADCASTING, all aspects of which are discussed under that title. (E. N. DA C. A.)