306 Mr. C. S. Wright on Variations in the 



will be seen that there is no evidence of any appreciable 

 regular variation in the conductivity. It is to be noted, too, 

 that the extreme values obtained did not differ from the mean 

 conductivity by more than 3 per cent. of the latter. 



In the results which will be given later, it will be seen 

 that variations amounting to as much as 50 per cent, were 

 obtained in the conductivity, by a change in the observing 

 station ; and from the results obtained and given in Tables IX. r 

 X., and XI., itwill be seen that any variation in conductivity 

 due to daily changes in the value of the penetrating radiation 

 from the earth which might have existed, were negligible in 

 comparison with the variation in the conductivity due to a 

 change in the point of observation. 



(c) Secondary Radiation from the Walls of a Room. 



In view of the existence of a penetrating radiation at the 

 surface of the earth, having its origin either in the atmosphere 

 or in the soil, and in view of the production by such radia- 

 tions of secondary rays at the surface of substances traversed 

 by them, it was thought advisable, before going on with the 

 main part of the investigation, to see how far the influence 

 of a secondary radiation excited in the walls of a room could 

 be detected from those walls. 



To obtain some information on this point, two plans 

 suggested themselves : (1) to place the conductivity chamber 

 at a selected distance from a wall, and to study the secondary 

 rays excited at that wall by a quantity of radium placed at 

 points on a circle with the chamber at its centre. (2) To 

 study the variation in the ionization in a metal receiver with 

 the radium at a fixed distance from the wall, and the chamber 

 placed at points on a circle with the radium as centre. A 

 set of measurements was made by following the first plan, 

 but time has not permitted the carrying out of a series of 

 observations with the second arrangement. 



The electroscope provided with a zinc receiver in these 

 measurements was placed at a distance of about one metre 

 from the wall of a large room, and the radium enclosed in a 

 lead box with walls 2 cms. in thickness was moved around 

 the circumference of a circle, having as centre the cylinder 

 and as radius the distance of the same from the wall. 



The values for " n," the number of ions per c.c. per sec, 

 due to the primary radiations from the radium as well as the 

 radiation emitted by the brick wall, together with the cor- 

 responding secondary rays excited in the metallic cylinder, 

 are given in Table VII., and show a regular decrease in the 

 ionization as the distance of the radium from the wall was 



