284 Notices respecting New Books. 
Lithium is included upon the supposition that the lines 
are douhlets with an infinitesimal separation. 
For the metals of the alkalies the formula 
atomic weight=log-1(0°65284 + 0°53869 log a) 
gives for ay 
Lathram) Gos 4:5 instead of 7:03 
Sadrums Sees. 20°82 * 23°05 
Potassium ... 39°62 # 39°15 
Rubidium...... 85°02 _ sor4 
Caesvam) .2ht. 133°94 tr 
It seems that magnesium fits in better with calcium and 
strontium than it does with zinc and cadmium, and _ that 
lithium and sodium will not go: with potassium, rubidium, 
and cesium, but fit in better with copper and silver. 
XXX. Notices respecting New Books. 
Radio-activity. By E. Rurnerrorn, D.Sc., F.RS., FRCS. 
(Cambridge: at the University Press.) 
i: the course of the eight years which have elapsed since 
Becquerel’s discovery of the radio-activity of an uranium salt, 
quite a new aspect has been given to our ideas of atomic structure 
by investigations arising from that discovery and the development 
of methods of detecting and measuring minute electric currents. 
The familiar gold-leat electroscope—the valuable qualities of which 
we see in a new light—has come forward pre-eminent among 
instruments of measurement as providing means of exploration 
into matters which were till recently thought to be probably 
forever beyond our ken. 
' Professor Rutherford’s book on Radio-activity will be taken as 
supplementing the treatise of Professor J. J. Thomson on the 
general subject of the conduction of electricity through gases, and, 
itis hardly necessary to add, is a book of the highest interest and 
value. The book of course appears while yet the field is not nearly 
exhausted and while still a large band of enquirers are eagerly 
pushing forward our understanding of the many issues involved. 
It, of necessity, marks but a stage of enquiry, leaving many pressing 
questions unanswered. The reader, however, feels convinced of 
the solidity of foundation of the central idea pervading the work— 
the intimately atomic nature of radio-activity. The whole experi- 
mental evidence so tar obtained for this great advance is before 
him, marshalled and criticised by the man who has played so 
brilliant a part in establishing it. Along with this record of 
patient experimental work, to read J. J. Thomson’s ‘ Electricity 
