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be seething with scintillations or tiny flashes of light. 
These flashes are caused by the explosions of the atoms 
in the minute portion of real radium in the mixture. These 
atoms have been found to be so small that two hundred 
and fifty million of them would probably be required to 
cover one inch. As each atom explodes, a particle flies 
from it as a projectile from a gun. These particles are too 
small to be seen under the most powerful microscope. 
But scientists have found that when one of these particles 
is suddenly stopped by striking a crystal of zinc sulphide, 
the heat is sufficient to make a flash of light the eye can see. 
These are the flashes seen under a good reading glass, 
They occur at the rate of 200,000 a second on the average 
luminous dial on the average watch bought in commercial 
routine. It is the combined light of all the flashes of light 
seen under the reading glass that makes the light or glow 
that makes the dial visible in the dark without a reading 
glass. 
The brightness and durability of a Radium luminous 
dial depends on the number of these tiny flashes per seconds. 
The more Radium, the more flashes and the brighter the 
dial. But every flash means a blow upon a crystal of zine 
sulphide. These crystalscannotstand these blows indefinitely. 
They break down under them, and when this happens, 
there are no more flashes and the dial loses its glow. The 
zinc has failed, not the Radium. Only one twenty-fifth 
of one per cent of any quantity of Radium disintegrates 
or is lost in a year. From a gram of Radium, a small 
thimbleful, there are about 134 billion projectile-like 
particles every second. Crystals of zinc sulphide would 
break down very quickly if exposed to such a bombardment. 
By reducing the percentage of Radium until the number 
of these particles flying from the exploding atoms of the 
radium on each dial, was about 200,000 per second, it has 
been found that the dial would have a brightness easily 
visible in darkness and for a period of about five years. 
This means that the quantity of Radium on the average 
dial cannot be more than about one millionth of a gram, 
and it is only such a minute quantity that is on the average 
ial. 
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