The agitation of the electrons is thus produced in a different way in gases, and we must not apply Kirchoff's law to them, although at first sight they appear to conform to it. We have seen that the particular waves which an incandescent gas radiates are also absorbed by it. This we should expect, because the particular electron which has such a period of vibration that it sends out a certain wave-length will naturally be in tune to exactly similar waves which fall on it, and will so resound to them, and absorb their energy. The quantitative law, however, that the absorbing power is exactly equal to the radiating power, is not true for gases. {43} +Emission of Polarised Light.+--One very interesting result of Kirchoff's law is the emission of polarized light by glowing tourmaline and by one or two other crystal when they are heated to incandescence. In ordinary light the vibrations are in all directions perpendicular to the line along winch the light travels, that is, the vibrations at any point are in a plane perpendicular to this line. Now any vibration in a plane may be expressed as the sum of two component vibrations, one component in one direction and the other in a perpendicular direction. If we divide up the vibrations all along the wave in this way we shall have two waves, one of which has its vibrations all in one direction and the other in a perpendicular direction. Such waves, in which the vibrations all lie in one plane, are said to be plane polarised. Tourmaline is possessed of the curious property of absorbing vibrations in one direction of the crystal much more rapidly than it does those vibrations perpendicular to this direction, and therefore light which passes through it emerges partially, or in some cases wholly, plane polarised. Since the absorbing power of tourmaline is different for the two components, the emissive power should also be different, and that component which was most absorbed should be radiated most strongly. This was found to be true by Kirchoff himself, who detected and roughly measured the polarised light emitted. Subsequently in 1902, Pflüger carried out exact experiments which gave a beautiful confirmation of the law. {44} CHAPTER V FULL RADIATION