THE NA TURE OF MA TTER 1 73 



weight, valence, and other physical and chemical properties of the 

 elements. It has been, too, a valuable aid in the discovery of 

 new elements. For instance, when Mendeleeff first stated it the 

 element scandium was unknown, as indeed were several others 

 now known. He was able to predict the discovery of this ele- 

 ment and to give its probable atomic weight, valence, and many 

 of its physical and chemical properties. Chemists were therefore 

 on the lookout for it, and it was only a few years after the predic- 

 tion of its discovery before this was accomplished. The properties 

 of the new element agreed remarkably well with the predictions. 

 One of the most startling discoveries of modern chemistry is 

 that the elements which the old chemists thought were the sim- 

 plest forms of matter and could not be resolved into still simpler 

 things are capable of such resolution. The more complex ones 

 like uranium and radium are giving off emanations by which 

 they change to other so-called elements. Three things seem to 

 be emitted from such decomposing substances: (i) what are 

 known as alpha rays which seem to be streams of helium mole- 

 cules, moving at about 18,000 miles per second; (2) beta rays or 

 streams of electrons, moving with a very high velocity, about 

 that of light, 186,000,000 miles per second; and (3) gamma 

 rays or X-rays, a form of vibratory impulse. Bacquerel first 

 discovered radioactive substances when he found that uranium 

 would make a shadow picture on a photographic plate even 

 through a protecting layer of black paper, and this in a perfectly 

 dark place. This was in 1896. Professor and Madame Curie 

 discovered polonium and radium, much more active substances, 

 two years later. Now we know the uranium decomposes in 

 time to form radium, which passes through several stages and 

 gives rise to niton and this to polonium, which in turn by loss 

 of these emanations becomes lead. The time consumed in these 

 transitional changes varies greatly with the different substances. 

 Thus, it takes some 5,000,000 years for half of a given mass of 

 uranium to change to radium, but only about 136 days for po- 

 lonium to change similarly to lead. These changes are as yet 



