42 "M RAYS 



sulphide, their effect must increase slowly, and 

 that when they are suppressed, their effect can 

 only disappear slowly. 1 



Pebbles picked up at about four o'clock p.m., 

 in a yard where they had been exposed to the 

 sun, spontaneously emitted " N " rays ; bringing 

 them near a small mass of phosphorescent 

 sulphide was sufficient to increase its luminosity. 

 Fragments of calcareous stone, brick, etc., 

 picked up in the same yard, produced analogous 

 actions. 



The activity of all these bodies still persisted 

 after four days, without any sensible diminution. 

 It is, however, necessary for the manifestation 

 of such actions that the surface of these bodies 

 should be quite dry ; for we know that the 

 thinnest layer of moisture is sufficient to arrest 

 " N " rays. Vegetable earth was found to be 

 inactive, doubtless on account of its moisture ; 

 pebbles taken from several centimetres under- 

 neath the surface of the soil were inactive, 

 even after being dried. 



1 I repeat here that, as a rule, when experimenting with 

 " N " rays, it is advantageous to replace the Auer burner by a 

 Nernst lamp absorbing about 200 watts. 



