SIR J. J. THOMSON'S NEW METHOD OF 

 CHEMICAL ANALYSIS 



By F. W. ASTON, B.A., B.Sc, A.I.C. 



No observer of the progress of " Molecular " Physics and 

 Chemistry during the past decade or so can fail to have been 

 struck by the extraordinarily intimate knowledge we have 

 acquired, especially recently, of Atoms and Molecules — the 

 individual units of complex matter. The results serve to 

 confirm the shrewd estimate made by a great scientific thinker 

 like the late Lord Kelvin that molecules are indeed almost incon- 

 ceivably small compared with the masses of matter affecting our 

 senses in everyday life. Thus the consensus of a variety of 

 methods shows that a thimbleful of the air we breathe contains 

 about a thousand million million million molecules, the average 

 diameter of each of these being one hundred-millionth of an inch ; 

 or to give a more practical illustration, a molecule of carbon in 

 the paper upon which this article is printed subtends to the 

 reader's eye the same angle as would a normal human being 

 at the distance of the moon. 



To hope that an effect appreciable to our senses could be 

 produced by a body so minute as a molecule would therefore at 

 first sight seem absurd, yet this has been done in several notable 

 instances in a most convincing manner. Thus in the spinthari- 

 scope of Sir William Crookes we actually see the flash of light 

 caused by the impact of a single a ray (which is a charged 

 molecule of helium) upon a screen of zinc blende. Rutherford 

 and Geiger have shown the measurable " kick " of a delicate 

 electrometer due to the ionisation produced by a similar a ray. 

 Whilst C. T. R. Wilson, with the aid of an apparatus recently 

 exhibited at the Royal Society, has been able, in the most 

 beautiful manner possible, both to see and to photograph the 

 track of a single charged molecule. 



The explanation of such large effects as these lies in the fact 

 that the charged molecule constituting an a ray is moving at 

 so prodigious a velocity that in its collision with other material 



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