1922] The Structure of Organic Crystals 581 



WEEKLY EVENING MEETING, 



Friday, May 19, 1922. 



Sir J. J. Thomson, O.M. M.A. LL.D. D.Sc. F.R.S., 



Honorary Professor of Natural Philosophy R.I., 



Vice-President, in the Chair. 



Sir William Bragg, K.B.E. D.Sc. F.R.S. M.R.I. 



The Structure of Organic Crystals. 



It may be said with truth that modern advances in physical science 

 are due in the main to the acquisition of the power to handle the 

 individual atom. UDtil the present time we have always attacked 

 the problems of matter by examining the behaviour of atoms or 

 molecules in groups. The new powers arise in two ways : — 



In the first the individual atom is endowed with excessive speed 

 and energy, and is able to make its individuality felt on this account. 

 The a-particle of the radioactive radiations is a helium atom moving 

 with a speed of the order of one-tenth of that of light. While in 

 possession of the relatively tremendous energy which the speed 

 implies it can, unaided, make a visible impression on a fluorescent 

 screen. It can pass through thousands of other atoms without 

 sensible deviation, and if occasionally it suffers violent deflection it 

 has penetrated to the very core of the atom which has deflected it. 

 Rutherford has shown us what important deductions can be drawn 

 as to the construction of the atom by examining these rare and sharp 

 deviations, and is going even further in examining the shattering 

 effect which the deflecting atom may itself experience. So also, the 

 electron endowed with sufficient speed can transverse matter and 

 bring about ionisation and other effects of great interest, but if its 

 velocity becomes less that one million metres per second this free 

 existence disappears. It is attached to the first atom it meets. 



The second method of attack upon the individual atom proceeds 

 on very different lines. It is by way of the mutual action of X-rays 

 and crystals. When we are examining things by eyesight we follow 

 the influence of the objects that we look at upon the waves of light. 

 If we wish to penetrate deeper into the minute, we take advantage of 

 the optical effects of lenses and build microscopes : but, even then 

 we cannot attack individual objects containing less than many 

 thousands of individual atoms. A limit is set by the difficulty that 

 light cannot show us the form of things which are much smaller than 

 the wave length of the light itself. With the aid of the very short 



