ABSORPTION. When white light falls upon any material substance there is always a certain amount scattered from the surface, and also as a rule sonic scattered from the interior portions of the body if light enters. The light scattered from the surface is white, but if the body is a piece of polished metal or has a 'metallic lustre.' there is mixed with the white light some that is colored. This colored light is due to the fact that there is a certain amount of absorption of waves of a particular wave-length at the surface. Thus gold appears yellow. because out of the incident white light the bluish-green light enters at the surface, and the yellow light is re flected. If a sheet of gold is hammered out ex tremely thin, the transmitted light is bluish. If the light penetrates into a substance, and is either transmitted through or relleeted out by small solid particles serving as mirrors, the color is generally the same, being due to the absorption of certain waves in the body of the substance. (The energy of these absorbed waves goes generally into heat-effects. hut may be spent in producing fluorescence, q.v.) Some substances absorb two or more colors to different extents; so a thin plate of it will be of a different color from a thick one.
As explained in the article on Coton, the par ticular color ordinarily attributed to a substance is that perceived by a normal eye when viewing the substance in white light. There are two methods for determining the cause of the color of an object. One method is to put it, if it is not epaque, between the slit and prism of a spectro scope, the slit being illuminated by white and to notice the change in the spectrum. The resulting spectrum is called the 'absorption spec trum' of the body, and the colors which are trans mitted are the ones which. when combined in the eye, produce the color of the object. The other method is to throw a pure spectrum of white light on a screen, and to move the object slowly along through the spectrum; if it appears black when held in any color, it shows that the waves of this color are absorbed by the object. If two pieces of colored glass are superimposed, or if two colored paints are mixed, the resulting color is that due to the waves which are lett after each glass or each paint has subtracted its colors.
Absorption must be due to the fact that the 'atoms' of matter can vibrate in definite la-Hods; and if ether-waves of a proper period enter the substance they will by resonance set the portions of matter in vibration :tint so lose their own energy. If the energy thus gained by the atoms is dissipated among the molecules. increas ing their energy, heat-elfeets are prod need. It may happen, however, that atom: are set vibrat ing in definite periods by the absorption of ether waves of shorter period, and, instead of having their energy spent in heat-effects, emit ether wa ves of their own. This is called 'fluorescence' (q.v.). Thus a solution of quinine absorbs
ether-waves whose period is sit short that they are invisible, and in return for them emits violet and blue light. If the fluorescent body continues to emit light for some time after the incident light is cut off, it is said to be 'phosphorescent.' Tsang 1TioN. As explained in the article I LID1A all material bodies are emitting ether waves: that is, by reason of the vibrations of the atoms, disturbances are produced in the ether. The nature of these waves emitted by any body may be determined by analyzing the radiation, using a suitable dispersing apparatus —prism or grating or interferometer—and study ing the resulting spectrums. The difficulty comes in obtaining instruments which detect the pres ence of ether-waves of all lengths. These waves carry energy; and if an instrument absorbs them, some change in it will he produced. provided it is sensitive enough. the kind of change depending on the nature of the instrument and the wave length of the waves. Thus. if waves of wave length extending from about 0.000015 em. to 0.00007 eta. fall upon suitable photographic plates. there is chemical action Iris need: if waves of wavelength between about 0.000030 cm. and 0.000075 em. enter the human eye. color is perceived: if waves of wave-lengths from 0.0000-1 em. to 0.007 em. are absorbed by suitable ther mometers (q.v.). change in temperature is no ticed]. Waves longer than 0.007 cm. have not yet been observed as being emitted by ordinary mat ter; waves longer than 0.2 cm. may. however, be produced by electrical means. Using proper in struments, it has been proved that solids and liquids emit continuous spectra; that is, waves of all wavelengths between certain limits can be shown to he present. t:ases. however, when rendered luminous by the passage of an electric discharge through them or when produced by vaporizing solids in the electric are. give di•won tinuons spectra ; that is. only isolated trains of waves arc present. This is what would he ex pected.
In a gas the molecules have great freedom of motion, and as one dashes to and fro having successive encounters with other molecules, its atoms have time during the intervals of free motion to emit long trains of waves of definite periods. But the molecules of solids and liquids are so close together and so hamper each other that there is no opportunity for the atoms to vibrate freely. Since the 'emission spectrum' of a gas is due to the natural frequencies of the atoms, and since the 'absorption spectrum' of any body when light of a continuous spectrum falls upon it is due to the absorption of the energy of those incident waves whose frequencies are the same as those of the atoms, it might he expected that those waves present in the former are absent in the latter. This is observed to be true, with certain limitations. See RADIA TION and SPECTROSCOPY.