AucustT 3, 1899] NATURE 315 
theorems and formulz, dimensional relations and | without being stimulating, we may mention the very 
numbers belonging to physics are represented and put 
together in a systematic manner, and in a way which 
would do justice to two different intentions: to give on 
the one hand to the reader a general view of the whole, 
without disturbing him by methodical, historical and 
other details, and on the other hand to give without 
circumlocution, to any one who may consult the book, a 
definite answer to his questions. The great difficulty in 
principle of such a ‘Kanon’ of physics lies clearly in 
the fact that it is in exact science often, perhaps gen- 
erally, impossible to give a short answer to a short ques- 
tion. Ifa scientific man is asked, What is mass? What 
is elasticity? What is entropy? he takes a long breath 
and begins with a long introduction—and not without 
reason ; for the difficulty of a short and detached answer 
lies in the nature of the thing. But we must not consider 
the difficulty to be insurmountable. We must fix our 
mind on what is essential and characteristic, and give 
expression to it in our answer ; as for the rest, so far as 
is necessary, it may be added afterwards by supple- 
mentary remarks.” 
The volume which the author has produced to satisfy 
his requirements is worth reading because it is stimu- 
lating. Irritating would perhaps be the better word for 
the principal sensation felt in perusing it, because we 
constantly come across statements which do not seem to 
coincide with our own views, or with explanations which 
do not satisfy ; yet in spite of trying hard, it is difficult 
sometimes to point out what it is that does not satisfy, 
and even if one succeeds one feels that the thought one 
has been obliged to give to the matter has cleared and 
perhaps modified one’s own views. The book begins 
with a number of chapters on general principles, space, 
time, motion, force and mass and the properties of 
matter. The subject is treated in a concise, short and 
instructive manner, but the author does not always 
succeed in giving us, as promised in the above passage, 
a short answer to a short question. His definition of 
dispersion, ¢.g., takes up eight lines and wants reading 
eight times before it can be understood. After a short 
chapter in which the principal equations of the potential 
theory are put together and explained, the author enters 
into the two chief divisions of the book, ‘‘energy” and 
“entropy.” He takes a rather wide and unusual view 
ofthe latter word, including under it all transformations 
of energy. Without entering into the difficult question 
of classification, we may commend these two chapters, 
which most physicists will read with profit. But surely 
a better definition of electric current might be given 
than the one on p. 250 :— 
“When one observes, that the potential has different 
values at different points of a conductor, one expresses 
this fact also by saying: a certain quantity of electricity 
moves in the conductor, or an electric current flows in it.” 
The difference of potential at different points is by no 
means characteristic of an electric current, as for instance 
in air, where we are constantly dealing with such differ- 
ences. Dr. Auerbach, to make his explanation correct, 
must therefore lay stress on a sharp distinction between 
conductors and non-conductors ; but how would he de- 
fine a conductor except by arguing in a circle, and saying 
that a body is a conductor when a fall of potential 
causes an electric current. 
Among the points of the book which are irritating 
NO. 1553, VOL. 60] 
annoying method of numbering concurrently and inde- 
pendently his paragraphs, according as they contain 
matters of principle or laws and propositions. Thus 
§ 52, printed in fat type, follows § 158 printed in some- 
what leaner characters, and whenever a reference has to 
be looked up, one has to investigate the type carefully, 
and if it is, e.g, 91 fat, turn to p. 385 ; whileif it is 91 lean, 
find the required passage on p. 118. We hope that in 
future editions a different system will be adopted. 
Lecturers will find one use, perhaps not a very high 
one, for this book ; it will save them thought and labour, 
by helping them to arrange their course in a systematic 
and orderly fashion. ARTHUR SCHUSTER. 
OUR BOOK SHELF. 
Insects: their Structure and Life. A Primer of Ento- 
mology. By George H. Carpenter, B.Sc. (Lond.). Pp. 
xi + 404. (London: J. M. Dent and Co., 1899.) 
Mr. CARPENTER, Assistant Naturalist in the Science 
and Art Museum in Dublin, is favourably known to 
entomologists by numerous valuable papers on Lefz- 
doptera, Odonata, cave-insects, economic entomology, 
&c.; and we are very pleased to welcome a useful 
introductory manual of entomology from his pen. It 
is compiled from a variety of sources, special use having 
been made, in the chapters on the form and life-history 
of insects, of the well-known work on the cockroach by 
Profs. Miall and Denny. These chapters will be found 
very useful, especially as the names attached to the 
various parts of insects are clearly and carefully ex- 
plained. Classification and the principal orders and 
families of insects are then dealt with as fully as the 
space at the author’s disposal would allow ; and chapters 
on insects and their surroundings and on the pedigree 
of insects close the body of the book, which concludes 
with a short bibliography and a good index. Perhaps 
Chapter v., on insects and their surroundings, will be 
found most interesting to the general reader ; for it treats 
of such subjects as cave-insects, fresh-water insects, 
marine insects, geographical distribution, mimicry, &c. 
Mr. Carpenter usually expresses himself very cautiously, 
but when he says that the number of described species 
of insects amounts to a quarter of a million, and that 
there are probably two millions of species still unde- 
scribed, we are inclined to think that both his estimates 
are very much below the mark. The number of described 
species of insects cannot be less than 300,000 at present, 
and many entomologists think that the late Prof. Riley’s 
estimate of the number of existing species of insects as 
ten millions is by no means to be regarded as ex- 
travagant. Mr. Carpenter’s remarks on the various sub- 
jects connected with evolution are very well expressed 
and reasoned out. W. F. K. 
Year-book of the United States Department of Agri- 
culture, 1898. Pp. 768. (Washington: Government 
Printing Office, 1899.) 
THE volume before us, like so many of its predecessors 
which have been noticed in these columns, is full of con- 
tributions of interest and value to students of science, 
agriculturists and others. Although appealing primarily 
to residents in the States, many useful hints and sug- 
gestions may be gleaned from the year-book by its 
readers in this country. The report of the Secretary of 
the Department shows that the varied operations carried 
on have been prosecuted with vigour. The Department 
has at present four scientific explorers abroad getting 
seeds and plants—one in Russia, one in the countries 
