144 ESSAYS BIOGRAPHICAL AND CHEMICAL 



of such particles can be determined by a device which 

 may be illustrated thus : Imagine a bullet fired from a 

 rifle placed horizontally, at some little distance above the 

 ground ; the bullet will be attracted to the earth, and will 

 fall to the ground after it has gone a certain distance. 

 The factors which determine the spot at which it will 

 strike the ground (excluding the retarding influence of 

 air) are its speed, and the attraction of the earth. If the 

 attraction is known, the speed can be calculated. This 

 analogy illustrates, although imperfectly, the method of 

 arriving at the speed of these impelled particles. They 

 are deviated by a magnetic field, and have a trajectory 

 just as a rifle-bullet has; and their speed has been cal- 

 culated by Becquerel at 160,000 kilometres or 100,000 

 miles per second. This estimate differs greatly from the 

 one previously mentioned for cathode rays. 



In conclusion, it has been suggested that the existence 

 of such radiations and emanations may be attributed to 

 the existence of ' electrons ' in the free state. An electron, 

 it may be explained, is an electric charge which attaches 

 itself to an atom of an element, thereby converting it into 

 an ion. The act of solution in water of such a substance 

 as common salt is now currently held to cause the atom 

 of sodium to separate from the chlorine atom, while each 

 acquires an electric charge, the sodium combining with a 

 positive electron, the chlorine with a negative one, thus : 

 NaCl + water + = Na + water + Cl + 

 water; the neutral molecule of electricity, consisting of 

 two oppositely charged electrons being thus dissociated. 

 Now it is conceivable that such a substance as pitchblende 

 or its radio-active constituents may combine with one of 

 the electrons, liberating the other. It has, indeed, been 

 shown by the Curies that radium rays charge negatively 

 the bodies which receive them, while the radium prepara- 

 tion acquires a positive charge. 



