36 Mr. A. L. Fletcher on the Radioactivity of 



cyclones, i. e. during windy, wet weather, when the barometer 

 is low; and usually highest during anticyclones, i. e. during 

 dry weather with light variable winds and a high barometer. 



(4) In cases where (3) breaks down, a study o£ the tra- 

 jectories of the surface air-currents reveals that when air has 

 travelled over the sea to Cambridge or very rapidly over 

 land, the emanation-content is low, while if the air has spent 

 much time over land, the emanation-content is high. 



(5) The results of other experiments have been discussed. 

 Eve's results obtained by a similar method at Montreal differ 

 from the author's, but this is probably due to different geo- 

 graphical conditions. Experiments made by measuring the 

 active deposits on exposed wires seem to give misleading 

 and indefinite results. 



(6) The number of ions produced per c.c. per sec. in free 

 air at Cambridge due to the radium emanation present is 

 about 2*1 on the average, with a minimum value of *7 and a 

 maximum value of 7*0. 



In conclusion the author wishes to thank Prof. Sir J. J. 

 Thomson for permission to carry out these experiments in 

 the Cavendish Laboratory, and for his stimulating suggestions. 

 The author also wishes to express his thanks to Dr. W. N. 

 Shaw and his assistants at the Meteorological Office for help 

 in tracing the trajectories of the air. 



Cavendish Laboratory, Cambridge, 

 April 1910. 



II. On the Radioactivity of the Rocks of the Transandine 

 Tunnel. By Arnold L. Fletcher, B.A.I * 



rf^HE following determinations of the Radium content of a 

 JL series of volcanic rocks from the Andes of South 

 America were made as the result of a suggestion by Pro- 

 fessor Joly, that a systematic analysis of some of the larger 

 igneous masses was desirable. 



This is the more apparent in view of the various results 

 obtained upon igneous rocks from different localities^ and 

 the consequent necessity for a larger number of systematic 

 determinations, with a view to the establishment of some sort 

 of a mean in the case of such materials. 



The determinations were made upon a series of rock- 

 specimens taken at various points during the working of the 

 tunnel, and which were obtained by the kindness of Mr. E. 

 Manisty, M.Inst.CE. They were taken at an average depth 

 of about 1000 feet below surface-level. 



* . Communicated by the Author. 



