126 KADIOACTIVITY. 



has been cooled in liquid air and the emanation is no longer lumin- 

 ous, but the walls of the cold tube emit strong light. I conse- 

 quently decide that the emanation is a gaseous body which, as more 

 exact research has proved, condenses at 150°. It emits a rays, which 

 the air renders luminous and electrically conductive. Without these 

 properties in the emitted rays we should never have discovered the 

 gas, for our supj)ly of radium is almost invisibly small. Ramsay and 

 Soddy announce that from CO mg. of radium they obtained in four 

 days one-fiftieth of a mm.^ of the pure emanation, unmixed w^ith air; 

 that is, an amount wdiich would fill a recej)tacle scarcely so large as 

 the head of a small pin. Imagine this amount to be diffused 

 through the laboratory of a physicist, and he would hardly be aware 

 of it, but perhaps not because it was so little, but because it was so 

 much that his apparatus would refuse to work, its isolation being 

 destroyed. This illustration gives an idea of the sensitiveness of 

 the methods through which we can discover the minutest traces of 

 the radioactive substances by estimating the conductivity of the 

 atmosphere. 



If we succeed in collecting a like amount of the emanation in a 

 glass vacuum tube, it will emit a bright light, which in a few days 

 will become noticeably weaker, and in a few weeks will disappear, 

 while we find in its place, by exact spectroscopic experiment, traces 

 of a gas which certainly was not present before, namely, helium. 

 These experiments have been conducted in the most diverse circum- 

 stances by many investigators, a part of whom approached the task 

 with no expectation of finding this result confirmed, so that there is 

 no room for doubt that radium, a chemical element, as positively 

 defined as gold or iron, or as any emanation which has been secured, 

 transforms itself into helium, an equally sharply defined .element. 



We have thus seen one element transformed into another, an occur- 

 rence never before witnessed and long held impossible. In such a 

 process, which is possible onh^ by a change in the atom itself by the 

 separation of a part of it, an enormous amount of energy must be set 

 free, and when such a process actually takes place in radium an 

 appreciable amount of warmth must be obtained; but in spite of this 

 we can discover no change in the weight of the preparation itself. 

 One difficulty in bringing the phenomena of radium into accord with 

 the principle of energy is now removed. Instead of a failure of the 

 principles of the natural sciences, held unchangeable by the timid 

 doubter of the phenomena of the radioactive substances, the view of 

 an entirely new world, the world of atoms, has been revealed to us. 

 We need no longer stop with the chemical elements, but may study 

 the origin and dissolution of these forms also, which have hitherto 

 been held as preexistent and unchangeable. 



In fact, gratifying advance has already been made, especially by 



