Notices respecting New Books. 659 
in cases where experiment indicates that true saturation is 
not obtained. For in order to settle the value of g and @ we 
require to know the form of the curve for a considerable 
range of voltages, and this is particularly difficult in cases of 
intense ionization. In other words, the failure to obtain 
saturation may be due to a small value of 0, in which case we 
are beyond the true saturation current, or it may be that 0 is 
negligible and that the voltage is insufficient for saturation. 
No doubt actual experiments will be complicated by 
inequality of the velocities of the positive and negative 
ions and of the rate of supply of these at positive and negative 
plates. This, however, only increases the difficulty I have 
mentioned. 
A comparison of the curves I have drawn with actual 
experiments where saturation is not obtained, shows clearly 
the uncertainty thus introduced in estimating for instance q. 
The point seems to me one which merits considerable 
attention. 
There are some other cases in which the equations can be 
integrated: for instance, when the ions are supplied entirely 
by the plates, and the positive and negative ions move with 
unequal velocities. 1 hope to consider some of these cases 
scon. 
Physical Laboratory, 
The University, Glasgow, 
22nd Aug. 1904. 
LXV. Notices respecting New Books. 
Radioactivity. By ¥rupK.Soppy, M.A. (The Electrician Printing 
and Publishing Co. Ltd.) 
THE account which Professor Soddy gives in this book of the deve- 
lopment of the science of Radioactivity up to the commence- 
ment of the present year is clear in style and full in treatment. 
Details of experimental work are given in many places; a feature 
which will be appreciated by the many who, fascinated by the 
interest of the subject, are endeavouring to pursue investigations 
for themselves. We feel indeed that we would have been thankful 
to: Professor Soddy had he given yet more practical details. All 
workers in this field should for instance be repeatedly warned 
against the risk of error arising from the subtle dispersal of emana- 
tion, or the obstinately persistent radioactivity of vessels or instru- 
ments once brought into contact with radium salts. 
Nothing can more strikingly mark the rapid progress of this 
branch of Science than to note how much has been done since the 
appearance of so recent a work. ‘Thus Paschen’s investigations 
into the nature of the y rays seem to suggest that in these rays 
we are dealing with something quite different from the X rays and, 
