tas 26, Me 1923] 
. 
7 
5 

NATURE 
697 

“condition of deflocculation (suspension or peptisation 
are other terms used) flotation of such particles will be 
rendered impossible. A main object of flotation is, 
therefore, to flocculate the valuable mineral in a pulp, 
and to deflocculate the gangue or unwanted mineral. 
It may be pointed out that Brownian motion (p. 510, 
para. 6) is not due to the “inherent kinetic energy 
of extremely fine particles” ; these indeed are passive 
agents, their motion being imparted by the kinetic 
energy of the water molecules which continuously 
bombard them—as shown in Perrin’s classic experi- 
ments. Some other statements will provoke critical 
comment and seem to require qualifications—such, 
_ for example, as that on p. 506, where it is stated that 
the necessary filming of a mineral with oil cannot be 
achieved if the oil be completely emulsified. In the 
form stated it is not a fact, and theoretically it seems 
to overlook the phenomena of adsorption. 
It should be mentioned that the book makes no 
serious attempt to deal with many of the problems 
which invariably confront the designers of mills, such 
as grades for launders and pipes, wet and dry pulp 
elevators and pumps, ore-bin construction, auto- 
matic feeders, etc. This, however, cannot be regarded 
_ as a shortcoming, and is perhaps to be commended, 
for the student should not be encouraged to imagine 
that he is fully competent to design a plant. It should 
 besufficient for him to obtain a thorough understanding 
of all ore-dressing appliances and methods in general 
use, and it is difficult to conceive how he could obtain 
so much sound knowledge so readily, and conveniently 
prepared for him, as he can in this book. Prof. Truscott 
apparently refrained from showing partiality towards 
any particular machines or methods, and has been 
cautious in his references to their merits or demerits. 
Practically everything said may be accepted as trust- 
worthy, though a few statements noticed in respect, 
of metallurgical matters are not strictly correct ; for 
-example, that much high-grade lead ore is smelted in 
reverberating furnaces, that zinc necessarily renders 
slags pasty, and that lead is highly objectionable in 
the retorting of zinc. These slight inaccuracies do 
not, however, affect the arguments they are used to 
illustrate. Soipese 

New Works on Relativity. 
The Mathematical Theory of Relativity. By Prof. A. S. 
Eddington. Pp. ix+247. (Cambridge: At the 
University Press, 1923.) 20s. net. 
The Principle of Relativity with Applications to Physical 
Science. By Prof. A. N. Whitehead. Pp. xii+1go. 
(Cambridge: At the University Press, 1922.) 
Tos. 6d. net. 
NO. 2795, VOL. 111] 
The Meaning of Relativity: Four Lectures delivered at 
' Princeton University, May 1921. By Albert Ein- 
stein. Translated by Prof. Edwin Plimpton Adams. 
* Pp. v+123. (London: Methuen and Co., Ltd., 
BTo22.). 5s.met. 
Modern Electrical Theory: Supplementary Chapters. 
~ By Dr. Norman R. Campbell. Chapter XVI. : 
Relativity. (Cambridge Physical Series.) Pp. 
vilit+116. (Cambridge: At the University Press, 
1923.) 7s. 6d. net. 
La Théorie de la relativité d’Einstein et ses bases 
. physiques: exposé élémentaire. Par Max Born. 
~’ Traduit de l’allemand d’aprés la seconde édition par 
Dr. F.-A. Finkelstein et J.-G. Verdier. Pp. xi+339. 
“(Paris : Gauthier-Villars et Cie, 1923.) 25 francs. 
The General Principle of Relativity in its Philosophical 
" and Historical Aspect. By Prof. H. Wildon Carr. 
“ Second edition, revised and enlarged. Pp. vili+ 200. 
(London: Macmillan and Co., Ltd., 1922.) 7s. 6d. 
» net. 
The Theory of General Relativity and Gravitation : 
Based on a Course of Lectures delivered at the Confer- 
~~ ence on Recent Advances in Physics held at the Uni- 
~ versity of Toronto,in January 7927. By Dr. Ludwik 
‘Silberstein. Pp. vi+1q1. (Toronto: University of 
~ Toronto Press, 1922.) 2.50 dollars. 
The Mathematical Theory of Relativity. By Prof. A. 
‘Kopff. Translated by Prof. H. Levy. Pp. viii+214. 
*'(London: Methuen and Co., Ltd., 1923.) 8s. 6d. net. 
Vector Analysis and the Theory of Relativity. By Prof. 
~ Francis D, Murnaghan, Pp. x+125. (Baltimore : 
The Johns Hopkins Press, 1922.) n.p. 
I’Evidence de la théorie d’Einstein. Par Prof. Paul 
Drumaux. Pp. 72. (Paris: J. Hermann, 1923.) 
~ 6 francs. 
6a all the books on the Principle of Relativity 
which it has been our good and ill fortune to 
peruse during the last three years, there are none 
which have given such food for thought as those of 
Profs. Eddington and Whitehead. Other books, and 
their name is legion, fall into several well-defined 
classes. Among those before us are two serious 
and well-executed books addressed to students of ex- 
perimental physics by Dr. Norman Campbell and Dr. 
Max Born. We are glad to note a cessation of the 
flood of popular accounts in which, mainly without 
success, more and less well-equipped writers have felt 
called upon to try their skill at hitting off the average 
man’s understanding. Then there are the books in 
which metaphysicians have told us the effect which 
heir reading around the subject, largely in semi- 
popular treatises, has had upon their thinking in regard 
to theology, sociology, and things in general. To this. 
| very important branch of the literature Prof. Wildon 
xX 1 
