252 Mr. E. Rutherford en the Electrification 



The position of the bulb was so adjusted that the rate of 

 leak in each cylinder was exactly the same, so that since the 

 potential of each pair of quadrants fell at the same rate, the 

 needle of the electrometer remained at rest while the rays 

 were kept acting. If another gas was introduced into one of 

 the glass vessels BC it was found that the balance was dis- 

 turbed, owing to the variation of intensity of the rays in the 

 vessel AB, which was caused by the less or greater absorption 

 of the rays in their passage through the gas. In the experi- 

 ments the only rays which caused conduction in the lead 

 cylinders had to pass through the gas, and all stray radiation 

 was carefully screened off. 



If we assume that the absorption of energy in passing 

 through a thin layer of gas of thickness dl is proportional to 

 the intensity of the rays I at that point and to the length of 

 the gas dl traversed, the decrease of intensity of the rays due 

 to absorption of energy in the gas is equal to Xldl, where X is 

 a constant for any particular gas but varies for different gases, 

 and may be called the coefficient of absorption of the gas. 



Experimentally it was found that the rate of leak of a gas 

 is proportional to the intensity of radiation at any point. 



From these considerations it can readily be shown that the 

 ratio of the rate of leak when the rays pass through a length 

 I of the gas, to the rate of leak when the gas is removed and 

 a vacuum substituted is e~ Kl where e — %'1, and this result is 

 independent of any metal or insulators which the rays pass 

 through in both cases before reaching the testing vessels. 



The ratio of the rates of leak can be readily deduced from 

 the movement of the electrometer needle, and since the length 

 of the gas traversed is known, the coefficient X is thus 

 determined. 



Experiments were first made to see whether air absorbed any 

 appreciable amount of energy of the radiation. The balance 

 was obtained and then one of the glass vessels was exhausted 

 by an air-pump ; the electrometer slowly moved in one 

 direction while the rays were kept acting. If the other 

 vessel was also exhausted, the balance was again restored, and 

 if air was then let into the vessel first exhausted, the electro- 

 meter needle moved in the opposite direction. The variation 

 of the rate of leak after passing through 10 cm. of air was 

 about one per cent., but it was a difficult matter to determine 

 such a small variation with accuracy. It will be at once seen 

 that the approximate value of \l is 10 ~~ 2 , and therefore for air 

 \=10~ 3 , since Z = 10 cm. 



If we suppose ..r-radiation to be emitted by the sun, 

 assuming the radiation would have to pass through four miles 



