APRIL 5, 1912] 
properties of each of these are considered at 
some length and the evidence pointing to the 
identity of the alpha particles with charged 
helium atoms is clearly presented. 
The second chapter is devoted to the radio- 
active constants and periods of average life of 
the radio-elements. Here, as elsewhere in the 
pages which follow, Mr. Soddy has shown a 
preference for denoting the relative stabilities 
of the different radio-elements by their so- 
called “ average life” periods rather than by 
their “ half-value” periods adopted and used 
by nearly (if not quite) all the other writers 
on the subject. To the reviewer the “ aver- 
age life” of a radio-element is merely the 
reciprocal of the constant of radioactive 
change, an occasionally convenient mathe- 
matical expression having little or no objec- 
tive significance. Mr. Soddy defines it as 
“the sum of the separate periods of future 
existence of all the individual atoms divided 
by the number in existence at the starting 
point.” The “half-value” period, on the 
other hand, is the time required for exactly 
one half of any given quantity of a radioac- 
tive substance to disintegrate or be trans- 
formed into other types of matter. 
A very interesting and suggestive section 
on radioactive equilibrium completes the sec- 
ond chapter. 
The title of the third chapter is the “ Classi- 
fication and Nomenclature of the Radio-ele- 
ments.—Analogies Between the Three Dis- 
integration Series,” but the subjects covered 
are much more comprehensive than this head- 
ing would imply and include many topics of 
extreme interest and importance. In the 
opinion of the reviewer this chapter is the 
most valuable and illuminating portion of 
the entire book and the one to be most profit- 
ably extended and developed in future edi- 
tions. It contains the essence of a new branch 
of science, radiochemistry, and Mr. Soddy has 
succeeded in collecting together and present- 
ing in a relatively small space, and in a very 
impressive and convincing manner, much that 
las previously been accessible only in scat- 
tered scientific publications of his own and of 
other workers in this field of scientific in- 
SCIENCE 
543 
quiry. Thus, for example, the conditions de- 
termining the possibility of separating and 
isolating any given radio-element in a state of 
comparative purity are briefly but compre- 
hensively considered, the general methods of 
obtaining the so-called “active deposits ”— 
the solid radioactive products resulting from 
the disintegration of the gaseous emanations 
—are outlined, and the inferences to be drawn 
as to the true chemical nature of the different 
radio-elements from a knowledge of the be- 
havior of these when mixed with large pro- 
portions of their apparent chemical analogues 
are discussed in some detail. 
This general introductory matter occupies 
in all only thirty pages and one can not avoid 
a feeling of regret that it was not found prac- 
tical to extend it still further. It is followed 
by a systematic presentation of the more im- 
portant physical and chemical properties of 
the thirty-odd radio-elements thus far identi- 
fied arranged in an orderly fashion under the 
separate headings, “Uranium,” “ Uranium 
X,” “Ionium,” “Radium,” ete. 
Mr. Soddy has provided us with a very 
valuable book of its kind, unique in respect to 
the field which it covers. It need scarcely be 
added that no one who desires to work intelli- 
gently in this modern branch of chemistry can 
well afford to be without it. 
B. B. Bottwoop 
Stability in Aviation: an Introduction to 
Dynamical Stability as Applied to the Mo- 
tion of Aeroplanes. By Professor G. H. 
Bryan, Se.D., F.R.S. Macmillan and Co. 
1911. Pp. xi+192. Price, $2.00. 
Any one seriously at work on the theory or 
the art of aviation would profit by reading 
what he can of this book, though precious few 
will have both the time and the ability wholly 
to master it and probably none, not even the 
author of it himself, fully to solve all the diffi- 
cult problems it sets. 
The first chapter gives, in 18 pages of con- 
cise non-mathematical language, a summary 
of existing knowledge on aeroplane stability, 
and incidentally forms a good mental appetizer 
for the very solid intellectual courses that fol- 
