The penetrating power (“hardness”) of these rays appears to be determined solely by the nature of the elements in the emitting substance. The velocity of the cathodic (or secondary Roentgen) rays seems to be quite independent of the matter exposed to the primary rays, but increases as the hardness (penetrating power) of the primary Roentgen rays increases. The _character_ of the emitted rays, in brief, appears to be quite unaffected by the chemical or physical condition of the element. Red-hot iron, for example, exhibits the same characteristic Roentgen radiation as iron at room temperature. But the _penetrating power_ (hardness) of this characteristic (emitting) radiation increases gradually and continuously with increasing atomic weight of the emitting elements. The complete independence of the penetrating power of the characteristic Roentgen radiation from external surroundings indicates strongly that it is closely connected with the nature of the nuclei (“cores”) of the atoms giving rise to it. FOOTNOTES: [2] When ultra-violet light is allowed to fall upon a metal it causes the metal to emit electrons and thus to acquire a positive charge, the velocity of the emitted electrons being exactly proportional to the frequency of the incident light. Or when light of X-ray type falls upon the surface of almost any substance, it takes hold of an electron in the atoms of that surface and hurls it out into space with a speed exactly proportional to the wave length of the light. This phenomenon is known as the photoelectric effect. CHAPTER V ULTRA-VIOLET LIGHT IN HEALTH AND DISEASE That both the compound rays of ordinary sunlight and ultra-violet rays (“artificial sunlight”) are highly effective in the treatment of a number of complaints is now well known. They are both in general use for the external treatment of rickets, tuberculosis, and a number of other diseases. Light-rays are also applied to hasten the healing of wounds. The use of the sun as a healing agent seems first to have been developed in a scientific way by Dr. Neils R. Finsen, a young Danish physician who was later awarded the Nobel Prize in Medicine. His original researches were undertaken toward the end of the 19th century. Then Dr. Rollier opened the first sunlight clinic in 1903, and in 1910 established his school at Leysin, in the Alps. Dr. Rollier is now treating about 1,000 patients, mostly afflicted with various forms of tuberculosis of the bone. The sun cure is also used to some extent for pulmonary tuberculosis, and with considerable success. (See my _Man’s Debt to the Sun_, Little Blue Book No. 808, Chapter IV.)