288 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 31 



pression as it sank to lower levels, and tlie clouds evaporated into 

 thin air. 



It was while watching such clouds in his native hills of Scotland 

 that C. T. R. Wilson conceived his beautiful laboratory experiments 

 on clouds. Of course he couldn't bring the mountains into his 

 laboratory, but he could expand his moist air in a cylinder with a 

 piston at one end. He made his cylinder of glass in order to see 

 what was going on. I have one patterned after his design here in 

 my hand. Here are the glass top and sides, with the whole vessel 

 partially filled with inky water. There is a lamp beside the glass 

 cylinder so we can see better what is going on. I can compress the 

 air in the glass chamber by squeezing the bulb. We let the air 

 remain under this pressure for a moment, until it becomes saturated 

 with moisture, and then allow it to expand. As it expands the air 

 cools and a cloud forms in the chamber just as it did on the moun- 

 tain top. 



Did it ever occur to you that, when a cloud forms, each little drop 

 of moisture in the cloud must condense on something? Usually it 

 condenses on a speck of dust floating in the air, and after a rain- 

 storm these dust particles are carried to the ground and the air is 

 beautifully clear. But when the dust has been removed, what can 

 the drops condense on? There are always in the air some broken 

 bits of atoms and molecules, which we call ions. These ions are 

 produced by rays from radioactive substances in the ground and 

 other sources. So, Mr. Wilson tried the experiment of placing a 

 speck of radium in his expansion chamber, to see what kind of 

 clouds would be formed. Let's see what happens when we repeat 

 his experiment. Those of you who are near enough will see the little 

 white lines radiating out from the tip of the glass rod which carries 

 the radium. These little white lines are tiny clouds of water drops, 

 condensed on the ions left along the paths of particles shot out by 

 the radium. It is clear that particles of some kind are coming from 

 the radium. What are they? 



A series of photographs will illustrate what is happening in this 

 chamber. A picture taken from above (pi. 1, fig. 1) shows the 

 glass walls of the chamber, and the rod on which the speck of 

 radium is placed. The more or less diffuse lines are the clouds of 

 water drops that mark the paths of the particles ejected from the 

 radium. 



What are these particles? Let us call them alpha particles, in 

 order not to imply anything about what they are, and look into 

 their properties. Plate 1, Figure 2, shows a sharper photograph, 

 each line a thin straight cloud, marking the path of an alpha par- 

 ticle. Rutherford (recently made Lord Rutherford in recognition 



