548 MAGNETISM AND KLKCTRICITY that of the hydrogen ion. The fact, discovered by Thomson, that these particles travel with a velocity nearly a third that of light in a high vacuum when the proper voltage is used suggests that the mass of these particles is very small. Millikan's experiment showed that the charge on these cathode particles is exactly equal, though different in sign, to that of the nucleus of the hydrogen atom. It is now known that these small, negatively charged particles in the cathode rays are the identical electrons mentioned so often in previous sections. In the Crookes tube there is also a stream of positively charged ions, which travel from the anode to the cathode. These rays have little practical significance, although they have furnished some evidence concerning the structure of atoms, which will be discussed in the next Unit. X Rays Have Unusual Properties. Roentgen first observed the evidence of X rays when he shielded a Crookes tube with black paper in a darkened room. A piece of paper covered on one side wath a phosphorescent substance became luminous in spite of the fact that the light was covered. This experiment has been described as an accident, but such a combination of circumstances would not occur for many men. Fig. 271. A shockproof 200,000-volt X-ray tube used for cancer treatment. (Courtesy of the Westinghouse Electric and Manufacturing Company.) From this experiment Roentgen concluded that those rays would pass through opaque substances. He found that they would pass through a thousand-page book, two packs of cards, and thick blocks of wood. Thin sheets of metal also permitted the rays to pass, but thick layers prevented their passage. When the hand was held between the Crookes tube and the fluorescent screen, shadows of the bones were seen. He then tried taking pictures with X rays and was delighted to find that it could be done. On Christmas Eve, 1895, Roentgen showed his first X-ray pictures to an astonished group of German physicists. Among these pictures were photographs of the bones of his hand and of keys contained in a purse.
ELECTRICAL CONDUCTION IN GASES 549 These rays were unlike any previously studied, and their relation to cathode rays was for some time unknown, so they were called X rays, X standing for the unknown. Fig. 272. X-ray radiograph of a man shaving himself, taken with an exposure of 1/1,000,000 second. (Courtesy of the Westinghouse Electric and Manufacturing Company.) Early X-ray tubes were devised by Sir Herbert Jackson, of King's College, London, who refused to patent them and thus permitted a fortune to slip through his hands for the sake of Science. Such sacrifice of personal benefit to the development of Science is not uncommon; true scientists are more interested in the development of knowledge