RADIOACTIVITY 273 



This disengagement of energy is essentially connected 

 with the atom of the substance; it constitutes an atomic 

 phenomenon ; moreover, it is spontaneous. These two char- 

 acteristics are essential. 



We have actual knowledge of bodies feebly radioactive: 

 uranium and thorium ; and of many bodies strongly radio- 

 active: radium, polonium, actinium, radiothorium, ionium. 



These bodies are found in nature in an extreme state of 

 dilution ; and this is not the effect of chance. 



Among the strongly radioactive bodies, radium alone has 

 been isolated in the state of a pure salt ; in the richest min- 

 erals this body is found in the proportion of a few? 

 decigrams per ton of mineral. 



Radioactive substances emit rays which have the faculty 

 of impressing sensitive plates, of exciting phosphorescence, 

 and of rendering gases conductors of electricity ; but which 

 do not exhibit refraction, polarization, or regular reflection. 



These rays offer, therefore, analogies to cathodic, positive, 

 and Rontgen rays. An attentive examination has proved 

 that the ray-emission of radioactive bodies can be divided 

 into three groups, /?, a, y, respectively analogous to the 

 three groups of rays which have just been named, and 

 which are formed in a Crookes tube. 



The /? rays are constituted by an emission of negative 

 electrons, and the a rays by an emission of particles posi- 

 tively charged, while the y rays are not charged. The 

 emission of the rays and the ft rays correspond to a spon- 

 taneous disengagement of electricity by the radioactive 

 bodies. 



The rays of these bodies produce numerous effects of vari- 

 ous nature: chemical effects, of which the most important 

 is the decomposition of water ; physiological effects, such as 

 the action upon the epidermis and other tissues an action 

 which is currently employed for medical applications. 

 Certain radioactive substances are spontaneously luminous. 



The radioactive bodies are sources of heat. Radium 

 gives rise to a disengagement of heat of 118 cal. per gram 

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