Air in Closed Vessels and its Caw y. 561 



emission and the absorption for the material, and on the 

 thickness of the screen. On this view. as the thickness ot' 

 the screen i< increased, the amount by which the radiation 

 passing through it is altered approaches a constant value, 

 and after a certain thickness has been reached, any further 

 increase in thickness will not affect the quantity of radiation 

 which pas-es through the screen. The result obtained with, 

 iron seems to confirm this view, as one screen five times the 

 thickness of another produced a diminution of the ionization 

 only a little more than twice as great. It is further borne 

 out by the result obtained by Cooke, who found that, no matter 

 how thick his lead screen, it was impossible to reduce the 

 ionization by more than about 30 per cent. The increase 

 found in the case of the wooden screen also seems at first 

 sight to confirm this view, but the fact that the radiation 

 producing this increase i> entirely cut off by 2 mm. of lead 

 shows it to be of quite a different character from that which 



we have been considering. This much may be regarded as 



. . . 



certain, however, that the ionization in a closed vessel is in 



part at least due to the action of a very penetrating radia- 

 tion from an external source. This leads naturally to the 

 question of the source. Cooke tried, unsuccessfully, to obtain 

 some evidence as to its direction by measuring the ionization 

 in his vessel with a thick lead screen in different relative 

 positions. The conclusion he came to was that the radiation 

 came equally from all quarters. It seemed possible, however, 

 that the more sensitive apparatus used in these experiments 

 might reveal differences which his apparatus had failed to 

 detect, and in any case the result was worth confirming. The 

 apparatus was placed on the lower side of a strong vertical 

 rectangular wooden framework, the two extremities of the 

 lower fide being mounted on two wooden blocks. The frame- 

 work was so arranged that a rectangular screen of lead, about 

 ;')<> cm. square and about 3'5 or 4*0 cm. thick, could be slipped 

 under the lower side, or rested on the upper side, and iu those 

 two positions was equidistant from the centre of the apparatus, 

 and -o in each case -ubtended the same angle there. Thus 

 radiation coming equally from above and below would lie 

 equally absorbed by the screen in the two position-. The 

 method of measuring the ionization in the vessel was slightly 

 altered, with some gain in accuracy. Between the earth and the 

 earthed terminal of the battery was inserted a potentiometer 

 arrangement by mean- of which the voltage ot' the battery 

 could be opposed by any fraction of 4 volts. At the end of 

 i of time over which the leak was being measured, 

 the position of the gold-leaf was carefully noted. The 



