August 27, 1909] 



SCIENCE 



275 



to make the uranium phosphoresce, that the 

 salts were just as active if they had been 

 kept in the dark. It thus appeared that 

 the property was due to the metal and not 

 to the phosphorescence, and that uranium 

 and its compounds possessed the power of 

 giving out rays which, like Rontgen rays, 

 affect a photographic plate, make certain 

 minerals phosphoresce, and make gases 

 through which they pass conductors of 

 electricity. 



Niepce de Saint-Victor had observed 

 some years before this discovery that paper 

 soaked in a solution of uranium nitrate 

 affected a photographic plate, but the ob- 

 servation excited but little interest. The 

 ground had not then been prepared, by the 

 discovery of the Rontgen rays, for its re- 

 ception, and it withered and was soon for- 

 gotten. 



Shortly after Becquerel's discovery of 

 uranium, Schmidt found that thorium 

 possessed similar properties. Then Mon- 

 sieur and Madame Curie, after a most diffi- 

 cult and laborious investigation, discov- 

 ered two new substances, radium and 

 polonium, possessing this property to an 

 enormously greater extent than either 

 thorium or uranium, and this was followed 

 by the discovery of actinium by Debierne. 

 Now the researches of Rutherford and 

 others have led to the discovery of so many 

 new radioactive substances that any at- 

 tempt at christening seems to have been 

 abandoned, and they are denoted, like po- 

 licemen, by the letters of the alphabet. 



Mr. Campbell has recently found that 

 potassium, though far inferior in this re- 

 spect to any of the substances I have 

 named, emits an appreciable amount of 

 radiation, the amount depending only on 

 the quantity of potassium, and being the 

 same whatever the source from which the 

 potassium is obtained or whatever the ele- 

 ments with which it may be in combination. 



The radiation emitted by these substances 

 is of three types known as a, ^ and y ray.s. 

 The a rays have been shown by Rutherford 

 to be positively electrified atoms of helium, 

 moving with speeds which reach up to 

 about one tenth of the velocity of light. 

 The /? Vays are negatively electrified cor- 

 puscles, moving in some eases with very 

 nearly the velocity of light itself, while the 

 y rays are unelectrified, and are analogous 

 to the Rontgen rays. 



The radioactivity of uranium was shown 

 by Crookes to arise from something mixed 

 with the uranium, and which differed suf- 

 ficiently in properties from the uranium 

 itself to enable it to be separated by chem- 

 ical analysis. He took some uranium, and 

 by chemical treatment separated it into 

 two portions, one of which was radioactive 

 and the other not. 



Next Becquerel found that if these two 

 portions were kept for several months, the 

 part which was not radioactive to begin 

 with regained radioactivity, while the part 

 which was radioactive to begin with had 

 lost its radioactivity. These effects and 

 many others receive a complete explana- 

 tion by the theory of radioactive change 

 which we owe to Rutherford and Soddy. 



According to this theory, the radioactive 

 elements are not permanent, but are grad- 

 ually breaking up into elements of lower 

 atomic weight; uranium, for example, is 

 slowly breaking up, one of the products be- 

 ing radium, while radium breaks up into a 

 radioactive gas called radium emanation, 

 the emanation into another radioactive sub- 

 stance, and so on, and that the radiations 

 are a kind of swan's song emitted by the 

 atoms when they pass from one form to 

 another; that, for example, it is when a 

 radium atom breaks up and an atom of the 

 emanation appears that the rays which con- 

 stitute the radioactivity are produced. 



Thus, on this view the atoms of the radio- 



