6 THE PRESIDENTIAL ADDRESS 



the stored energy of the atom is drawn upon, as we believe, and 

 has been drawn upon for ages, to keep up that blessed radiation 

 which makes man's life possible and is the source of all his power. 



In the middle nineties there set in an astonishing renaissance of 

 physical science which has centred in the study of the atom and 

 extends by inevitable logic to the stars. In quick succession came 

 three great discoveries : the X-rays by Rontgen in 1895, radio- 

 activity by Becquerel in 1896, and the electron by J. J. Thomson in 

 1897. Sensational, puzzling, upsetting, these events inspired every 

 physicist to new activities of thought and equipped every laboratory 

 with no less novel methods of research. A flood of further dis- 

 covery followed, the flow of which continues unabated. Within the 

 last few months notable items have been announced that well 

 deserve our attention. It may not be inappropriate if I try for 

 a few minutes to touch — however lightly — on one or two aspects of 

 this subject, as it is seen through the eyes of an engineer. 



Thanks mainly to J. J. Thomson, Rutherford and Bohr, we 

 now recognise the atom of any substance to be a highly com- 

 plex structure, built up, so to speak, of two sorts of blocks or 

 brickbats — the electrons, which are indivisible units of negative 

 electricity, and the protons, which are indivisible units of positive 

 electricity. It is strangely simple to be taken back, as it were, to 

 the nursery floor and the childish game, and given just two 

 sorts of blocks, exactly alike in each sort, and exactly the 

 same number of each sort, with which to build the universe 

 of material things. The blocks are unbreakable : we cannot 

 produce them or destroy them or change them. In respect of 

 electrical quality the two kinds are equal and opposite, but they 

 contribute very unequally to the atom's mass, each proton (for some 

 reason not yet understood) contributing about 1,840 times more 

 than each electron. Every substance is made up of blocks of the 

 same two sorts. If you compare different substances you find 

 that the diversity of their chemical and other properties arises 

 solely from differences in the number and arrangement of the 

 blocks which compose their atoms. Any atom, in its normal or 

 electrically neutral state, must contain an equal number of protons 

 and electrons. All the protons in any atom are gathered close 

 together at the centre, along with some of the electrons, forming 

 a compact, dense portion which is called the nucleus. Although 

 the nucleus accounts for nearly the whole of the atom's mass, it 

 occupies no more than a very minute fraction of the atom's volume. 

 Those of the electrons which are within the nucleus doubtless 

 serve to bind the protons together ; the other electrons constitute, 

 as it were, a voluminous crinoline, or rather a series of crinolines, 

 extending relatively far away frorn the centre and giving the whole 



