THE MYSTERY OF MATTER. 185 



are slightly bent by magnetism and travel at the approximate 

 rate of 12,000 miles a second. They consist of particles about the 

 size of an atom of ordinary matter ; indeed, they are generally 

 considered to be free atoms of helium. They carry a small 

 charge of positive electricity, and are considered to be identical 

 with the positive ions described before. The heating effect of 

 radium seems to be mainly due to the a rays, and is produced 

 by the constant bombardment of the molecules of radium 

 by its own a particles. The fB rays are very different. They 

 are composed of exceedingly minute particles about toV o part 

 the size of a hydrogen atom, and are emitted with the extreme 

 velocity of over 120,000 miles a second. At this rate they 

 would travel from the earth to the moon in 2 seconds, or from 

 the earth to the sun, 93 millions of miles, in something like 

 13 minutes. They travel no less than half a million times as 

 fast as a cannon-ball. They are easily bent or deflected by 

 magnetism in the opposite direction to the a rays ; they carry 

 a charge of negative electricity ; and they are much more 

 penetrating than the a rays. The photographic action of radium 

 is principally due to the y3 corpuscles, which appear to be 

 identical with the negative ions before mentioned. 



To give some idea of the exceeding minuteness of these tiny 

 bodies, let us imagine a collection of a particles and y3 corpuscles 

 the size of a drop of water, and let us suppose this collection to 

 be magnified to about the size of the earth, a globe having 

 a diameter of 8,000 miles ; we should then see the a particles 

 about the size of small shot, while the /3 corpuscles would still 

 remain much too small for the naked eye to distinguish. 



The 7 rays are much the most penetrating of all. They are 

 not composed of particles, but are due to peculiar vibrations or 

 pulses in the ether. They never occur when /B rays are 

 absent, and are believed to be caiised by the sudden arrest of 

 the motion of the /3 corpuscles. 



The radium rays have the property of exciting fluorescence or 

 phosphorescence in various substances (such as sulphate of zinc, 

 for example) which may be placed close to a radium salt. This 

 is produced by the bombardment of the a particles. A little 

 instrument called a spinthariscope has been devised to show this. 

 It is also well seen when a radium salt is mixed with sulphide 

 of zinc and painted on a cardboard screen, when it will glow 

 perpetually in the dark. If such a screen be examined through 

 a lens in a darkened room, an immense number of tiny scintilla- 

 tions can constantly be seen. Radium has also the property of 



