On the effect of X-rays of different wave-lengths upon some animal tissues.—Proof of differential action

In a paper (1) upon the Germicidal Action of Ultra-Violet rays, it is shewn that a marked differential action exists in this part of the spectrum,i. e ., electromagnetic disturbances have a different effect upon the same kind of organism according to the wave-length operating. It was found that when microorganisms were exposed to the ultra-violet and visible radiation from a tungsten arc for a considerable period of time, germicidal action stopped abruptly atλ = 2960 Å. U. It was possible in this particular instance to correlate the differential action with a selective absorption by the micro-organisms of just those wave-lengths which had germicidal action upon them. In attempting to extend this kind of experiment into the region of X-rays, it soon becomes certain from ordinary electroscopic absorption measurements that this correlation, even if it exists, will no the found, because the absorptive power exhibited by a medium upon a beam of X-rays is dependent in so many cases solely upon the mean density of that medium. Hence if we take a section of some animal tissue the density of which is about the same as water, the absorption which it will exert upon a beam of X-rays will be practically the same as that of a layer of water of equal thickness, in spite of the fact that in the layer of tissue the composition varies from point to point according to the cellular structure involved. In order to reveal this point-to-point absorption it would be necessary to construct electroscopic instruments on a microscopic scale.