LIGHT AND RADIATION IN RELATION TO HEALTH. The phenomena of life result from the reactions of living substance to radiant energy, and depend primarily on the sun.
All radiations are conceived of as electromagnetic waves con ducted in a hypothetical medium, aether, and travelling with the velocity of light, 186,000m. a second. According to theory each of the atoms (see ATOM) of which a chemical element is composed consists of a central nucleus positively charged (protons and elec trons densely packed, and electrons negatively charged, revolv ing round the nucleus in a variety of orbits with enormous speed, comparable to the planets revolving round the sun). The more complex the atom the greater the number of its electrons. When an electron is displaced from an outer to an inner orbit, radiant energy is given off. When an electron jumps from an inner to an outer orbit, energy is absorbed. Only a certain variety of orbits are possible for an electron to revolve in; a certain quantum of energy is required to effect each jump.
To have any effect on matter radiant energy must be absorbed. There then takes place a change of energy, e.g., into electrical ef fect, heat, fluorescence, chemical action. All radiations when ab sorbed by a perfect black body are transformed into heat. A ther mopile coated with lamp black is used for measuring in calories per sq.cm. per second, or in ergs per sq.cm. per second, intensity of radiant energy emitted by a source or received by a surface. This is the final standard of measurement. The body is composed of an infinite number of living cells. The living cell is the seat of an infinitely intricate play of energy, containing millions upon mil lions of molecules. We have to conceive of electrons being dis placed by radiations in the atoms of these molecules, of molecular change enhanced thereby, provoking reactions which manifest themselves as signs of life, of the spirit of man evolving out of such transformations of energy.
Radiations are classified by frequency of vibration, and by wave length ; they include first, the Hertzian waves used in radio with wave lengths extending to a thousand metres or more; to these waves the body is transparent; to be heard, they must be trans mitted, received and transformed by suitable electrical apparatus.
Then come the infra-red rays with wave lengths from 600,000 to 7,000 A.U. (Angstrom Unit = one ten-millionth of a millimetre). Absorbed largely by water and converted into heat, these rays are caught as they pass to, or come off, the earth, by the vapour charged atmosphere. Thus is the earth kept warm to a temperate degree and life made possible. (See RADIATION.) Absorbed by the wet substance of the skin, these rays heat par ticularly the outer layers, whence heat is conducted to deeper parts and distributed by the blood circulating through the skin.
Acting on the cool dry skin. these rays when intense provoke a dry prickling and not very agreeable sensation of heat. Next to the infra-red come the visible rays, a very narrow part of the vast radiation spectrum, with wave lengths from 8,000 to 4,00o A.U. The media of the eyes have been evolved transparent to these rays. The skin is much less transparent ; while a Dart is reflected.
a part of these rays penetrates to the cutaneous blood vessels, and absorbed therein, is changed into heat ; in a cool dry skin these rays provoke transudation of water, and give thus a more agree able sensation of heat.
The transformation of energy of visible rays taking place in the blood may have important effects as yet unknown. It is in this respect that sources of visible radiation have a superiority over in fra-red rays as means of providing artificial heat ; infra-red rays from hot water pipes heat rather the surface of the skin, while vis ible rays from sun, fire or lamp penetrate and heat the blood and tissues below the surface. Red rays are not wholly absorbed by the blood in the skin and so penetrate deeper still. By means of pow erful incandescent lamps and red glass screens an agreeable and easily controlled method of applying heat to the body and provoking hyperaemia is obtained.