ELECTRONS AND ATOMS 47 



ether, from which by the agency of energy, 

 giving to it stable forms of motion, particles 

 can be formed. Ordinary ponderable matter 

 consists of solutions, or suspensions, in this 

 continuous imponderable matter, and it has 

 been seen that the transmission of energy 

 through space has already made the postu- 

 lation of such a medium necessary for the 

 physicist. 



The study of radio-activity has disclosed 

 intermediate stages between the chemical 

 atom and the unformed pre-matter, or lumini- 

 ferous ether, which are of high interest from 

 the point of view of evolution. 



When the different types of radio-activity 

 produced either by an X-ray tube or a radio- 

 active substance, such as a salt of uranium 

 or radium, are examined it is found that these 

 can in each case be resolved into three types, 

 which are distinguished by letters of the Greek 

 alphabet as alpha (a), beta (0), and gamma (y), 

 radiations respectively. They are distinguished, 

 amongst other things, by their relative 

 penetrating powers for substances such as 

 the metals which are opaque to radiant light 

 or heat. The metal aluminium is used as a 

 standard, and it has been found that while 

 the alpha rays are readily stopped by a 

 piece of aluminium leaf of 0.05 of a milli- 



