664 POPULAR SCIENCE MOXTHLl. 



makes a noise resembling the crack of a pistol. Kow, this dis- 

 charge can be used in a variety of ways to excite various trans- 

 formers in order to produce the best conditions for exciting the X 

 rays. The method of using this powerful discharge to excite a 

 transformer seems at present the most promising one in seeking 

 the best conditions for obtaining rays of high penetrating power. 



There is still another method of obtaining the rays yet in 

 its infancy — the simplest method of all, for no apparatus is 

 required. 



It has been discovered that certain substances, like the salts of 

 uranium, have the power of emitting rays which have all the prop- 

 erties of the X rays. The list of such substances is constantly 

 increasing, and they arc called radio-active substances. It is pos- 



FiG. 3. — The burniriL,' of an iron wire l»y the nmst powerful electric discharge yet produced. 



sible to take a shadow picture of the hand through a board by 

 placing the hand on a covered sensitive plate, resting the board on 

 the back of the hand, and strewing the board with one of these 

 radio-active substances in the form of a powder. Can it be that 

 all the skill and industry which has been employed to perfect X-ray 

 apparatus is to be supplanted by a powder? The peculiar prop- 

 erty shown by the radio-active substances leads investigators to 

 surmise that we have evidence of new substances, and we have the 

 waves radium and polonium. 



The methods by which the X rays are detected in practical em- 

 ployment in surgery have not been essentially changed. The ordi- 

 nary photographic plate, shielded in a plate holder, is still used to 

 receive the shadow cast by the bones, and salts of barium or of 

 calcinm strewn on pastel)oard serve as fluorescent screens to receive 



