Page:Elementary Text-book of Physics (Anthony, 1897).djvu/468

454 temperature, and that to these are added shorter and shorter wave lengths as the temperature rises. Draper showed that the spectrum of a red-hot body exhibits no rays of shorter wave length than the red, but that as the temperature rises the spectrum is extended in the direction of the violet, the additions occurring in the order of the wave lengths. At the same time the colors previously existing increase in brightness, indicating an increase in energy of the vibrations of longer wave length as those of shorter wave length become visible. Experiments by Nichols on the radiations from glowing platinum show that vibrations of shorter wave length are not altogether absent from the radiations of a body of comparatively low temperature, and he was led to believe that all wave lengths are present in the radiations from even the coldest bodies, but are too feeble to be detected.

With gases, as has been seen, the radiations are apparently confined to a few definite wave lengths, but careful observations of the spectra of gases show that the lines are not defined with absolute sharpness, but fade away, although very rapidly, into the dark background. In many cases the existence of radiations may be traced throughout the spectrum, and it is a question whether the spectra of gases are not after all continuous, only showing strongly marked and sharply defined maxima where the lines occur. In general, increase of temperature does not alter the spectra of gases except to increase their intensity, but there are some cases in which additional lines appear as the temperature rises, and a few cases in which the spectrum undergoes a complete chafige at a certain temperature. This occurs with those compound gases which suffer dissociation at a certain temperature, and at higher temperatures give the spectra of their elements. When it occurs with gases supposed to be elements it suggests the question whether they are not really compounds, the molecules of which at the high temperature are divided, giving new molecules of which the rates of vibration are entirely different from those of the original body.

372. Fluorescence and Phosphorescence.—A few substances