176 Prof. A. Stanley Mackenzie on Secondary Radiation 



degrees Fahrenheit. I£ the whole mass of air be ^ of the 

 Earth's, while gravity is *38 o£ ours, the pressure is 

 M l9l = -09 o£ the Earth's, 



whence the boiling-point is 44° C. or 79 + 32 = 111° F. 



For the same reason, sublimation takes place more freely 

 at identical temperatures there. Proportionally, therefore, 

 there would be more water-vapour in the air. 



Results. — In conclusion we may summarize the results for 

 the more probable values of the following quantities for 

 Mars : — 



Mean temperature 48° F. or 9° C. 



Boiling-point of water 111° F. or 44° C. 



Amount of air per unit surface. . 7 in. or 177 mm. ; 2/9 of the Earth's. 



Density of air at surface 2'5 in. or 63 mm. ; 1/12 „ „ 



The look of the surface entirely corroborates the tempe- 

 rature results of this investigation. 



XI. Secondary Radiation from a Plate exposed to Rays 

 from Radium. By A. Stanley Mackenzie, Ph. Z>., Munro 

 Professor of Physics, Dalhousie University, Halifax, N.S. * 



THE following experiments were made to examine more 

 carefully than has been done the secondary radiation 

 from the back side of a plate bombarded by the rays from 

 radium, and to see what light a comparison of this radiation 

 with that from the front side would throw on the mechanism 

 involved in the production of secondary rays ; and, further, 

 to see what evidence it would give of the secondary radiation 

 of penetrating type, of which other experiments seem to show 

 the existence. 



In order to restrict to a definite bundle the beam of radium 

 rays employed, and to shield as far as possible the rest of the 

 apparatus from rays leaving the radium in other directions, 

 the radium w T as put at the apex of a conical opening of 19° 

 half-angle in a massive block of lead. The accompanying 

 diagram (fig. 1) will make the arrangement clear. A hole 

 was bored through the lead block along the axis of the cone, 

 and the radium (5 nog. in a glass tube) was put in its place 

 by being inserted in the end of a brass rod which fitted the 

 hole. The lead block had a circular cross-section of 10*7 cm. 

 diameter, and its greatest length was 16 cm. As the radium 

 was inserted to a depth of 7*7 cm. in the block, no rays could 

 emerge from it without passing through at least 5 cm. of 



* Communicated by the Author. 



