THE GROWTH OF CHEMICAL IDEAS 135 



neon, krypton, and xenon. For a little time it looked as if 

 there were no place for them in the periodic table, and then 

 it ^\as realized that they formed a ne^v group of elements of 

 zero valency unable to form compounds. Instead of casting 

 doubt on the classification, they extended and enhanced its 

 validity. 



An even more important discovery of hitherto unknown 

 elements was made when Pierre Curie and his wife isolated 

 from the residues of uraniimi ore the strongly radioactive 

 radium, of which the atoms ^vere found to be decomposing 

 and chans^ino^ into atoms of lower atomic ^veioht. Stimulated 

 by Roentgen's discovery of the x-rays in 1895, a number of 

 observers tested various fluorescent materials under the im- 

 pression that the origin of the x-rays might be connected ^vith 

 the fluorescence that the cathode stream excited in the glass. 

 Among these observers, Henri Becquerel used some beautiful 

 yellow-green crystals of uranium salts and found that when 

 these ^v ere wrapped in black paper and left in contact ^vith 

 a photographic film, they produced a blackening of the film 

 ^vhen it was developed. This observation excited a good 

 deal of interest. Madame Curie and her husband studied 

 salts of other elements and discovered that thorium ^\'ould 

 also produce an effect on a film in the same ^vay that uranium 

 did and that the activity of different thorium and uranium 

 ores differed, some of them producing four or five times as 

 much effect as another ore containing the same amount of 

 metal. The tests finally indicated that the natural uranium 

 ore kno^vn as pitchblende contains something highly active. 

 Monsieur and Madame Curie undertook to analyze systemati- 

 cally about a ton of pitchblende ore, testing all the products 

 at each step for their activity as sho^vn in the production of 

 ionization in an electroscope, an eff^ect that proved to be 

 parallel to the exposure of a photographic plate. This re- 

 sulted in the isolation of two residues, in one of which the 

 barium of the pitchblende was isolated and in the other, the 

 bismuth; these residues ^vere forty to sixty times more ac- 

 tive than uraniimi. Ho^vever, normal barium and bismuth 



