they increase in frequency from the infra-red, which are the slowest, to the ultra-violet, which are the most rapid.
As we have already said, it has recently been shown that the odorous vapours absorb certain ultra-violet rays. That is to say, when the beam of light is directed through a chamber containing the odorous vapour before entering the prism, what are known as absorption-bands—vertical blacklines in the white—appear in the photograph of the spectrum.
Similar lines are seen, as a matter of fact, in the visible spectrum of sunlight, and as these correspond in position with the spectrum given by chemical elements in an incandescent gaseous state, it is supposed that they are produced by the absorption of the corresponding light-rays by these gases in the solar atmosphere.
The physical explanation given of this phenomenon is that the molecules of the gas in the sun absorb such light-rays as are equal in rate of vibration to the rate of their own vibrating molecule.
In the same way, Heyninx and others argue that the odorous vapour is composed of molecules which are vibrating with a period equal to that of the light-rays they absorb.
Moreover, since the position of the absorption-band in the photograph varies, lying in some cases
