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 radio-active processes, and also allows us to determine with considerable 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^cClung for revising the index.

E. R.

, February, 1904.