Dr R. A. Millikan 165 



Cameron to help him, continued his researches. He 

 began a series of new experiments by climbing mountains 

 and sinking electroscopes in deep, clear lakes at great 

 heights. The first lake visited was Lake Muir, which lies 

 at a height of eleven thousand eight hundred feet, and 

 here the sealed electroscope was sunk to a depth of no 

 less than sixty feet before all signs of ionization (dis- 

 turbance by rays) ceased. 

 Dr Millikan says : 



This was the first time the zero of an electroscope — the 

 reading with all external radiations, both local and cosmic, 

 completely cut out — had been definitely determined, and the 

 results accordingly began to show that it was possible to make 

 with certainty determinations of the absolute amount of the 

 penetrating radiation. 



It must be explained that most water is radio-active, 

 and therefore affects the electroscope. From Dr Milli- 

 kan's point of view, the beauty of these deep, snow-fed 

 lakes is that their water has hardly any radio-activity, 

 actually less than one-hundredth of that of ordinary tap 

 water. 



Next, readings were taken in another snow-fed lake 

 three hundred miles to the south, at a height of six 

 thousand seven hundred feet, and the readings of the 

 electroscope were found to have a similar curve, but with 

 each reading displaced just six feet upward. But six feet 

 of water is exactly equal, in absorbing power, to a layer 

 of atmosphere five thousand one hundred feet thick — in 

 other words, to the difference in the height of the two 

 lakes. Here then was proof of three things : 



1. That the effects in Lake Muir had not been due to 

 any radio-activity in the water. 



2. That the source of the rays affecting the electroscope 

 was not in the layer of atmosphere between the two 

 altitudes. 



