CONSTITUTION OF THE STARS RUSSELL 157 



from atomic nuclei, represents indeed one such gigantic store. But 

 the amount of energy which must once have been stored in each 

 gram of the sun's mass, to account for its past radiation of heat, is 

 even greater than that contained in uranium. We can not do more 

 than guess where it may have been hidden ; but one very recent piece 

 of work affords a possible clue. 



Aston, in one of the brilliant researches which we have learned 

 to associate with the Cavandish Laboratory at Cambridge, has in- 

 vented a beautiful apparatus which sorts atoms, by giving them elec- 

 trical charges and shooting them through a vacuum under the in- 

 fluence of electric and magnetic fields. The resulting forces deflect 

 atoms of different weights in different directions, and bring each 

 kind to a separate focus upon a photographic plate, producing 

 images when the plate is developed. By measuring these plates the 

 atomic weights may be determined; and Aston has found that, in 

 every case but one, the true atomic weights are exact integers, 

 within the accuracy of measurement, which is about 1 part in 1,000. 

 When the chemist finds an atomic weight which is not an integer, 

 such as 35.46 for chlorine, this is really the average for two different 

 kinds of atoms of the same chemical properties, but different weights, 

 both of which are integers — 35 and 37 in this case. The one ex- 

 ception is hydrogen, for which the chemist's determination 1.008 is 

 exactly confirmed. 



Now it is more than a century since Prout suggested that, since 

 the atomic weights are so nearly integers, the atoms themselves are 

 built up out of simple units. We now transfer this idea to the 

 atomic nuclei, which contain practically all the mass, and Aston's 

 beautiful researches practically compel belief. The hydrogen nucleus, 

 or " proton," is the lightest of all, and we would naturally look 

 to it as the fundamental unit. Kutherford's success in knocking 

 the nuclei of elements such as oxygen, nitrogen, and sodium to bits, 

 by collision with fast-moving alpha particles, has furnished a 

 definite proof that protons, and alpha particles as well, are actual 

 constituents of these nuclei. Many nuclei must also contain elec- 

 trons, which prevent the net electric charge from getting too high. 

 It looks, for example, as if an alpha particle was built of four 

 protons and two electrons, held together by forces of whose nature 

 we are ignorant. This would give exactly the right electric charge ; 

 but the mass of the four protons would be greater than that of 

 the alpha particle by 1 part in 130. (The electrons weigh next to 

 nothing.) This seems to spoil the explanation altogether, but an 

 escape is found in that great resolver of otherwise intractable dif- 

 ficulties, the principle of relativity. According to this, all energy 

 has mass, and all mass is equivalent to energy. The loss of mass in 



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