Vill PREFACE 
radio-active processes, and also allows us to determine with con- 
siderable certainty the order of magnitude of the different 
quantities involved. 
For these reasons, it has been thought advisable to give a brief 
account of the electric properties of gases, to the extent that is 
necessary for the interpretation of the results of measurements 
in radio-activity by the electric method. The chapter on the 
ionization theory of gases was written before the publication 
of J. J. Thomson’s recent book on “Conduction of Electricity 
through Gases,” in which the whole subject is treated in a 
complete and connected manner. 
A short chapter has been added, in which an account is given 
of the methods of measurement which, in the experience of the 
writer and others, are most suitable for accurate work in radio- 
activity. It is hoped that such an account may be of some service 
to those who may wish to obtain a practical acquaintance with the 
methods employed in radio-active measurements. 
My thanks are due to Mr W. C. Dampier Whetham, F-.R.S., 
one of the editors of the Cambridge Physical Series, for many 
valuable suggestions, and for the great care and trouble he has 
taken in revising the proof sheets. I am also much indebted to 
my wife and Miss H. Brooks for their kind assistance in correcting 
the proofs, and to Mr R. K. M°Clung for revising the index. 

oa 
MacponaLp Puysics BUILDINGS, 
MONTREAL, 
February, 1904. 
