EXPLORING THE ATOM 



impact of a single alpha particle. He can thus esti- 

 mate accurately the number of particles given off by 

 a known quantity of radium in a certain time as be- 

 fore. The result coincides with the other method of 

 counting within the limits of errors of observation. 

 Thus there is a check on the method of counting and 

 measuring the atoms, and we may feel fairly sure 

 that the bewildering result already given represents 

 substantially the facts. 



Although this method of counting the atoms de- 

 pends upon direct vision, it must not be inferred that 

 the observer actually sees the atom itself. What he 

 sees is the commotion created among the particles 

 of the sensitized screen when the atom dashes into 

 their midst. When you fire a rifle ball into the 

 smooth surface of a lake half a mile away, you see 

 the splash of the water clearly enough, but of course 

 you do not see the rifle ball itself. The effect is 

 precisely similar when the splash of light caused by 

 the alpha particle is viewed. 



The alpha particle itself is as far beyond the range 

 of vision even aided by the most powerful micro- 

 scope, as a rifle ball would be at the distance of sev- 

 eral miles. 



To give a tangible idea of the hopeless invisibility 

 of an atom, we may note that the smallest particle of 

 matter visible under the magnifying influence of the 

 most powerful microscope is of such dimensions that 

 50,000 such particles placed in line would be required 

 to extend across the space of one centimeter (about 

 two-fifths of an inch). If we calculate the cube of 

 this number, we find that 125 thousand billion such 



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