[WRIGHT] VARIATIONS IN THE CONDUCTIVITY OF AIR 107 



which it will be remembered was found to be inappreciable, and which 

 is exactly what should be expected if only some two or three of the ion& 

 generated in the cylinder per c.c. per second were due to a penetrating 

 radiation from the earth. 



VIII. — Summary of Eesults. 



The results obtained in the preceding investigation may be summed 

 up as follows: — 



(1) No evidence of a regular daily variation was noted. 



(2) It has been shown that there is a penetrating secondary radia- 

 tion set up by penetrating rays such as those from radium, in the brick 

 wall of a room. 



(3) Proofs have been adduced to show that the water of Lake 

 Ontario acts as a perfect screen, both for the earth's radiation and if 

 a sufficient depth be taken, for the y rays from radium. On this 

 account, and owing to the fact that the water of Lake Ontario contains 

 no active impurity, it has been possible to determine what portion of 

 the ionization in the receivers ,used in this investigation was due to 

 residual active impurities and to intrinsic activity in the metals of the 

 receivers. 



(4) Based on this fact, a determination has been made of the 

 ionization in free air due to radioactive impurities in a clay soil, and 

 this value, 0.9 ions per c.c. per second, has been found to be in close 

 agreement with a value deduced by Strong from Strutt's determination 

 of the radium content of thé earth. 



(5) The ratio of the ionization in cylinders of lead, zinc, and 

 aluminium due to the radiations from the earth has been determined 

 and has been found different from the ratio for the ionization due to 

 the gamma rays from radium, — a result which needs confirmation, but 

 which points to a difference in the penetrability of the two radiations. 



(6) The values obtained in the open for the ionization in well 

 cleaned receivers of lead, zinc, and aluminium, are lower than any 

 hitherto recorded, the numbers 8.6, 6.0 and 6.5 respectively being ob^- 

 tained over the water of Lake Ontario. 



Considered as a whole, the experiments described above are inter- 

 esting from the light which they throw on the question of the radio- 

 activity of metals and substances generally. The values obtained for 

 " q " for the three cylinders at Station 2, Table XI, differ from each 

 other but little. They are, moreover, of the order of magnitude of 

 effects which might easily be accounted for by active impurities in the 

 metals, since differences as large as these values of " q " may easily be 



