BATTLE OP THE ALCHEMISTS — COMPTON 271 



ing molecules individually, nioasurinjr their separate velocities and 

 the energy and force re(iiiired individually to pull them apart into 

 their constituent atoms. The puzzles of the old alchemists were 

 solved by the recognition of two classes of substances, elements and 

 compounds, of which the former retain their identity throughout 

 all action of earth, water, air, fire — or any other physical or chem- 

 ical agent. Thus alchemy, which sought to transmute the elements, 

 became supplanted by chemistry, which occupied itself with the 

 various combinations of these elements to form chemical compounds. 

 "Alchemy was dead ! Long live chemistry ! " But is this the end 

 of the story? 



The textbook in which I first studied chemistry in 1904 defined an 

 atom as " an indivisible, indestructible, and unchangeable unit of mat- 

 ter." Yet 5 years earlier J. J. Thomson and his colleagues had split 

 up atoms into electrons and positive ions, and within 20 years it had 

 come to be realized that the atom could be very changeable — could, 

 in fact, exist in any one of an infinite variety of conditions commonly 

 termed " excited states." Thus the atom is not indivisible and is not 

 unchangeable. But these changes do not really affect the identity 

 of the atom. The electrons which it loses come back to it or others 

 take their places; it does not stay in its excited states very long, but 

 reverts to its normal state, usually within a hundred millionth of a 

 second. So, after all, the atom is still the same old atom, and its 

 new attributes which have been discovered by the physicists, while 

 they add to its versatility, do not undermine its fundamental charac- 

 ter of good old-fashioned chemical respectability. 



In its ionization and its excited states the identity of the atom is 

 like that of a man. You may cut off his hair or his nails, but they 

 come back. You may even amputate a finger or a leg, but he is 

 still the same man. Or you may excite him to a fit of anger or 

 activity, but he cools down again. Through it all he retains his 

 identity through that mysterious something that we call his soul. 



Now the soul of an atom is its nucleus. 'J'hrough ionization and 

 excited states this nucleus remains, so far as we know, unchanged. 

 Until we know the nucleus of the atom we no more know the atom 

 than do we know a man by his hair, nails, fingers, or legs. What do 

 we know about the nucleus ? 



Beyond a doubt we know very exactly the mass of every kind of 

 atomic nucleus and that it is composed of a definite number of pro- 

 tons and electrons, which we Imow, and that it has a positive electric 

 charge, which we know accurately. Thus the hydrogen nucleus con- 

 sists of a single proton ; the helium nucleus consists of 4 protons and 

 2 electrons and has a mass which is 0.77 percent less than the sum 

 of 4 hydrogen nuclei; the uranium nucleus consists of 238 protons 



