ASSAULT ON ATOMS — COMPTON" 289 



of his work with atoms) caught a large number of these particles 

 to find out what they were Avhen there are enough of them to handle. 

 Niton is a radio-active gas, a hundred thousand times as active as 

 radium. He compressed some of this gas into a fine glass tube with 

 walls so thin that the alpha particles would pass right through. 

 After a few days he noticed gas collecting in the space surrounding 

 this tube, and this gas he forced into a fine tube above. On passing 

 an electric discharge through the tube and looking through a spectro- 

 scope at the light emitted, he saw the brilliant spectrum characteris- 

 tic of the gas helium. 



Many of you know the romance of helium. Observed many years 

 ago by Lockyer in the spectrum of the sun, it remained unknown 

 on the earth for a generation until Rayleigh and Ramsay, making 

 a i^recise measurement of the density of the nitrogen in the air, 

 found it different from the nitrogen prepared in the laboratory. 

 Search for the cause of the discrepancy revealed a whole series of 

 new gases — argon with which our incandescent lamps are filled, neon 

 with which we advertise our wares in blazing red, helium with which 

 we now fill our dirigibles, and two others, krypton and xenon, which 

 are now of great value in certain laboratory experiments. Thus was 

 helium found, and here we see it being formed — the birth of helium 

 atoms. For these alpha particles are none other than atoms of 

 helium gas. 



We can count these atoms one by one as they come from a prepara- 

 tion of radium. It might be done using an expansion chamber of 

 this type, and counting the tracks as they appear. A better method 

 is to allow the atoms to enter an electrical counting chamber. Each 

 particle then can make its record on a moving film, as we see in 

 Plate 1, Figure 3. Every little peak here marks the birth of a helium 

 atom from its parent radium. 



Imagine that we have thus counted all the atoms of helium that 

 come through the walls of Rutherford's glass tube, and make the 

 gas that he observed in his spectroscope. How many atoms would 

 we have? In a little glass bulb the size of a large pea, filled with 

 helium at atmosphere pressure, the number of atoms is about 1 

 with 19 ciphers after it. Perhaps that doesn't mean much to you. 

 Let me put it this way. Two thousand years ago Julius Caesar gave 

 a dying gasp, " Et tu Brute?" In the intervening millenniums the 

 molecules of air that he breathed out with that cry have been blown 

 around the world in ocean storms, washed with rains, warmed by 

 the sunshine, and dispersed to the ends of the earth. Of course only 

 a very small fraction of these molecules are now in this room ; but 

 at your next breath each of you will probably inhale half a dozen or 

 so of the molecules of Caesar's last breath. 



