X-rays were discovered by accident. Professor Roentgen was working at his desk with one of the glass tubes when he was called to lunch. He laid the tube with the electric discharge passing through it on a book. Returning from lunch he took a photographic plate-holder which was under the book and made some outdoor exposures with his camera. On developing the plates a picture of a key appeared on one of them. He was greatly puzzled at first, but after a search for the key found it between the leaves of the book. The strange light from the electric discharge in the glass tube had passed through the book and the hard-rubber slide of the plate-holder and made a shadow-picture of the key on the photographic plate. He tried the strange light in many ways, and found that it would go through many objects. It would even go through the human body, so that shadow-pictures of the bones and organs of the body could be obtained. In Fig. 117 is shown a physician using X-rays. Fig. 118 is an X-ray photograph of the eye. [Illustration: FIG. 117--A PHYSICIAN EXAMINING THE BONES OF THE ARM BY MEANS OF X-RAYS] [Illustration: FIG. 118--X-RAY PHOTOGRAPH OF THE EYE The eye is above and to the left of the larger black circle. The smaller black circle is a shot which has lodged back of the eye.] Not long after the discovery of X-rays it was discovered that light very much like the X-rays is given out by certain minerals. One of the most interesting and the best known of these is radium. Radium gives out a light somewhat like X-rays that will go through copper and other metals. It does many other strange things. It gives out heat as well as light; so much heat, in fact, that it is always about five degrees warmer than the air around it. It continues to give out heat at such a rate that a pound of radium will melt a pound of ice every hour. It can probably keep this up for at least a thousand years. If this heat could be used in running an engine, a hundred pounds of radium would run a one-horse-power engine without stopping for many hundred years. The power of Niagara might be replaced by the power of radium if an engine that could use this power were invented. Fig. 119 is from a photograph made with radium. [Illustration: FIG. 119--PHOTOGRAPH MADE WITH RADIUM A purse containing a coin. The strange light from the radium goes through the purse and the slide of the plate-holder and makes a shadow-picture.] The great inventor of the future may be able to use the heat of radium or some new power now unknown. We have seen how, through the toil of many years and the labors of many men, the great inventions of our age have come into being. It may be that we are now witnessing other great inventions in the making. APPENDIX BRIEF NOTES ON IMPORTANT INVENTIONS Aerial Navigation First air balloon--Montgolfier Brothers, France, 1783. First balloon ascension--Rozier, France, 1783. First gas balloon--Charles, France, 1783.