of Gases exposed to Rontgen Rays. 253 



of homogeneous atmosphere, the intensity at the surface of 

 the earth would be approximately 10 -260 of the intensity of 

 the radiation before it reached the earth's atmosphere. This 

 is an excessively minute proportion, and it is not surprising 

 therefore that experiments, made even on the highest moun- 

 tains, to detect any Rontgen rays in solar radiation (Cajori, 

 Phil. Mag. Nov. 1896) should have been unsuccessful, even 

 if the intensity of the #- radiation at the Kmits of our atmosphere, 

 were greater than could be produced at the surface of a 

 Crookes tube. 



Gases like oxygen, coal-gas, carbon dioxide, whose leakage- 

 rates are about the same as that of air 7 absorb about the same 

 amount of energy. 



Sulphuretted hydrogen, which has a conductivity six times 

 as great as air, diminishes the intensity of the radiation by 

 about 4 per cent, in passing through 10 cm. of the gas. 

 Chlorine, whose conductivity is eighteen times that of air, dimi- 

 nishes the intensity about 12 per cent, for the same distance. 



The absorption of energy in these cases is not necessarily 

 selective, for the same results were obtained whatever gas 

 was used in the testing vessels. After the radiation had passed 

 through sulphuretted hydrogen the same diminution in 

 intensity was obtained whether air or sulphuretted hydrogen 

 was used in the testing vessels. 



Mercury vapour, which is one of the best conductors of 

 electricity under the ^-rays, is also one of the best absorbers of 

 the radiation. 



In the case of mercury it was not necessary to use a null 

 method. A glass bulb 7 cm. in diameter was taken and a 

 small amount of mercury introduced. The bulb was slowly 

 heated till it was filled with boiling mercury vapour, care 

 being taken that no mercury was allowed to condense on the 

 sides of the glass through which the rays passed. The rate 

 of leak was then taken in a conducting vessel for the radiation 

 which had passed through the mercury vapour. The mercury 

 was then removed and the bulb now filled with air was heated 

 to the same temperature. The rate of leak in the testing- 

 vessel with air in the bulb was found to be twice as great as 

 when the bulb wasfilled with mercury vapour at the tempera- 

 ture of boiling mercury. 



In the passage therefore through 7 cm. of mercury vapour 

 sufficient energy is absorbed to reduce the intensity of 

 radiation by one half. 



The vapour of methy iodide, which is even a better con- 

 ductor than mercury vapour, is also a powerful absorber of 

 the radiation. 



