448 
NATURE 
[June 24, 1915 

gamete-formation again. It is a mistake to 
suppose that nutrition determines sexuality, but it 
sometimes determines the opportunity for the ex- 
pression of sexuality. Similarly, the nutritive 
supply is not necessarily related to differentiation 
of sex or size. The male gametophyte of Equise- 
tum is small because it is a male, and not male 
because it is small. 
The general theory suggested in this interesting 
essay is that decline of vegetative vigour favours 
the production of gametes with characteristic 
chemical substances. The zygotes that result 
from the pairing of gametes tend to lie dormant 
until environmental conditions improve—a useful 
and probably primary protective adaptation. The 
peculiarity of gametes is not to be found in their 
motility, or in their minuteness, or even in pairing 
(as is shown by nuclear fusions in endosperm- 
formation) ; what, then, is their essential feature ? 
In the reduction division their nuclei become 
peculiar, so that a new individual can only be 
produced after the nuclei have fused. And the 
advantage of this is probably in securing individu- 
ality or variation. Thus sexual reproduction 
makes, on the one hand, for protection, and, on 
the other, for variability. 

CASE-HARDENING. 
(1) La Cémentation de L’Acier. 
'  Giolitti. 
By) Prot. is 
Traduction frangaise revue par M. A. 
Portevin. Pp. 548. (Paris: A. Hermann et 
Fils, 1914.) Price 16 francs. 
(2) The Case-hardening of Steel. By H. Brearley. 
Pp. xv+164. (London: Iliffe and Sons, Ltd., 
1914.) Price 7s. 6d. . 
(1) ROF. GIOLITTI is probably the 
greatest living authority on _ the 
cementation of steel, and in the above-mentioned 
publication will be found by far the most compre- 
hensive and lucid presentation of this subject that 
has ever appeared. During recent years many 
important original researches in this domain have 
been published by him and his co-workers at 
Turin, and no one is better qualified than he to 
summarise present-day knowledge with regard to 
it. M. Portevin, in rendering as he has done an 
admirable French translation of the Italian text, 
has considerably enlarged the circle of students 
to whom the book will be available. 
As Prof. Giolitti remarks in his preface, there 
is probably no branch of present-day steel tech- 
nology in which empiricism is so supreme as 
cementation or case-hardening. Such a condition 
of things, justifiable no doubt at one time, can 
no longer be defended. Scientific knowledge is 
now available which permits this highly important 
NO. 2382, VOL. 95] 

process to be carried out under simple and easily 
controlled conditions with inexpensive materials, in 
such a way as to secure definite results with re- 
markable certitude. In spite of this, many works 
are content to go on buying at fancy prices mix- 
tures of a very ordinary character, the nature of 
which is entirely disguised by the trade names 
under which they are sold. 
Part i. of the book deals with cementation pro- 
cesses from a chemical point of view, and consists 
of five chapters, which trace the sequence of re- 
searches that laid the foundation of scientific case- 
hardening, and gradually lead up to the final 
chapter on present-day knowledge of the subject. 
The author has been exceedingly careful in 
mastering and summarising the literature avail- 
able, and in spite of his own large share of the 
experimental field he seems to have missed little 
or nothing that anybody else has done. It seems 
now to be well established that while pure carbon 
can and does under suitable conditions of heating 
carburise solid iron in the complete absence of 
any gas, yet such carburisation proceeds so slowly 
as to be useless from the technical point of view. 
All industrial case-hardening processes require the 
presence of gas, and to the question “ What are the 
respective parts played by the carbon and the gas 
in such processes?” it is impossible to return an 
answer that will hold for all cases. It is neces- 
sary to examine for every type of cementing 
material the specific action of the gas which may 
be formed, and then to study how this action is 
modified by the presence either of free carbon 
pre-existing in the cement or of carbon formed 
during cementation. 
Part ii. deals with the technical applications of 
cementation, and of particular interest are the 
chapters relating to liquid and gaseous cementing 
agents. It is unfortunate that the table of con- 
tents is very meagre, and that the book is without 
an index. It is to be hoped that the latter defect 
will be remedied when a second edition appears, 
for it detracts considerably from the usefulness 
of the book in its present form. 
(2) Mr. Brearley’s book has been written mainly 
for those who are engaged in the commercial 
production of case-hardened objects. Nothing 
could indicate its point of view better than the 
following quotation from his preface: “An ex- 
planation based on the mechanical structure of an 
object is intelligible, because most minds can 
appreciate the elements of design and pass judg- 
ment on the composite properties of materials. All 
kinds of steel have a mechanical structure which, 
when suitably magnified, is as obvious as that of 
reinforced concrete. It is in terms of such struc- 
tures that the properties of case-hardened: steels 
