ULTRAVIOLET RADIATION— MEIER 379 



there is nothing unusual about them, become gems of glowing beauty 

 when placed in the ultraviolet rays. In the fluorescent minerals 

 some slight impurity exists which is the source of the fluorescent 

 property of the mineral. Pale, minute microscopic organisms such 

 as protozoans and bacteria assume brilliant colors in the ultra- 

 violet rays. As they color differently in the separate rays, and as 

 different species behave differently, their more minute outlines and 

 details becoming more evident, the ultraviolet provides a new method 

 for identification of the different species. 



This magic power of the ultraviolet is called fluorescence. When 

 a system is excited by absorbing radiation, photochemists teach us 

 that some of the excited molecules may return to their normal state 

 with the emission of radiation of a different wave length from that 

 which produced the excitation. Teeth, various parts of animal tis- 

 sue, and certain chemical substances are also commonly known to 

 fluoresce in the energy-rich ultraviolet light. 



Fluorescence has been adapted to numerous practical uses with 

 which we are all familiar. Brilliant stage effects are produced by 

 tinting the costumes or scenery with fluorescent substances. Many 

 oils and chemicals exhibit fluorescence, and ultraviolet light is used 

 to detect them, as in testing cloth suspected of being contaminated 

 by traces of lubricating oil from the machines and in detecting 

 forgeries of paintings and documents by exposing differences in the 

 chemical composition of the material used in making the original 

 and the forged copy. 



In photochemistry, a complicated and difficult field too involved 

 for discussion here, knowledge of ultraviolet rays has proved useful 

 because the light of shorter wave length which possesses greater 

 energy, is more likely to produce chemical reaction. The decompo- 

 sition of many complex chemical compounds by ultraviolet rays 

 has been studied by photochemists. 



The layman is often confused by the terms ultraviolet light. 

 X-rays, and cosmic radiation. He knows that there is a difference 

 between them, but his idea of the relative position of each in re- 

 lation to the spectrum is not clear. The X-rays are shorter waves 

 of energy than the ultraviolet, and cosmic rays are even shorter than 

 the X-rays. Ultraviolet rays, as we have seen, are present in sun- 

 light or are produced artificially by a quartz mercuiy lamp. X-rays 

 are produced chiefly by projecting streams of electrons (the smallest 

 known particles of matter) against blocks or targets of metal. 

 X-rays were discovered in 1895 by Prof. Wilhelm Conrad Roentgen, 

 a Bavarian physicist. When investigating electrical discharges 

 through a vacuum tube covered with black paper, he observed that 

 a fluorescent screen at a distance glowed when the current was 



