174 OUR PHYSICAL WORLD 



beyond the control of man. They persist in going on under any 

 and all conditions. He cannot stop them or start them. 



Here is a possible source of energy that may some day be 

 under man's control. If we could start an element to giving off 

 this energy of decomposition and check it at will, it might put 

 at our disposal the greatest source of energy available. We do 

 use the emanations of radium now. When the alpha rays strike 

 certain chemicals like the sulphate of zinc they make a visible 

 splash of light. So we coat the hands of a watch with a paint 

 in which there is such a chemical and a very tiny amount of 

 a salt of radium, and the hands are then visible in the dark. 

 Radium salts are used in the treatment of cancer and other 

 pathological conditions. But they must be handled with extreme 

 care for the radiant energy shot off causes the death and rapid 

 decomposition of living tissue, making bad " burns, " and they 

 go through most any substance, penetrating the armor plate of 

 a battleship as if there were nothing in their way. Lead seems 

 to be relatively impervious to them. 



The chief source of these radioactive substances is a mineral 

 called carnotite. It is found in this country abundantly in 

 Colorado and in less quantity elsewhere. Radium forms a very 

 small part of it, so that it takes a trainload of the ore to make a 

 thimbleful of the radium salt. Yet the energy given off by this 

 amount is very great. It would make enough luminous paint 

 to cover the state of Illinois. 



These radioactive substances are not the only sources of 

 streams of electrons and of X-rays. These were produced by elec- 

 trical discharges through tubes from which the air or other gases 

 had been largely exhausted (vacuum tubes) for some time before 

 radioactive substances were discovered. The streams of elec- 

 trons were known as cathode rays. The X-rays have been used 

 in medical diagnosis for many years now. They penetrate flesh 

 but are stopped in part by bone, metal, and other foreign sub- 

 stances so that it is possible to get pictures of broken or deformed 

 bones, foreign substances such as bullets or pins that have lodged 



