Fesruary 25, 1897 | 
NATURE 389 
vegetable oils, and mineral oils are dealt with, the pro- 
cesses of manufacture and purification, and the various 
kinds of lamps being described. A brief comparative 
statement of the prices and efficiency of different systems 
of lighting concludes the book. 
The cellular substance of plants now used in the 
manufacture of paper, are known collectively as succéd- 
anés des chiffons. M. Urbain describes the different 
kinds of straw, Sparta grass, and wood used for this 
purpose ; the physical and chemical constitution, so far 
as it is known, of cellulose ; the manufacture of pulp 
from different cellular substances ; and the methods of 
bleaching the paper. His book should be of use in 
showing how the structural elements of plants are now 
utilised in paper manufacture. 
Alterations of Personality. By Alfred Binet. Translated 
by Helen Green Baldwin ; with notes and a preface by 
Prof. J. Mark Baldwin. Pp. xii + 356. (London: 
Chapman and Hall, Ltd., 1896.) 
M. BINET’s volume originally appeared in the “ Biblio- 
théque Scientifique Internationale,” and was reviewed in 
NATURE in July 1892 (vol. xlvi. p. 219). The subject with 
which it deals is beset with peculiar difficulties, and great 
caution is necessary before coming to any definite con- 
clusions concerning the psychological phenomena in- 
volved ; for though many observers have recorded strange 
alterations and modifications of personality, the cause of 
this spontaneous somnambulism is much disputed. M. 
Binet holds “that in a great many cases, and in very 
diverse conditions, the normal unity of consciousness is 
broken up, and several distinct consciousnesses are 
formed, each of which may have its own system of per- 
ceptions, its own memory, and even its own moral 
character.” His book contains a detailed account of the 
results of researches by various psychologists on these 
alterations of personality. It is an authoritative state- 
ment of facts, and the translation, with Prof. Baldwin’s 
notes, will be read with interest by the more intelligent 
section of the general public, as well as by the student 
of psychology. 
The Hemiptera-Homoptera of the British Islands. By 
James Edwards, F.E.S. Pp. vi + 271. (London: 
L. Reeve and Co., 1896.) 
STUDENTS of the insects of the Homopterous sub-order 
of the Hemiptera will find this volume very serviceable 
in the determination of their captures. The work is a 
descriptive catalogue of the families, genera, and species 
of the Cicadina and Psyllina indigenous to Great Britain 
and Ireland, with notes as to localities, habitats, &c. 
Particular attention is given to the consideration of 
characters which are of the greatest service in deter- 
mining the several species and larger divisions of the 
insects described. 
Analytical Keys to the Genera and Species 07 North 
American Mosses. By C. R. Barnes. Revised and 
extended by F. D. Heald. (Madison, Wis. : published 
by the University, 1897.) 
ALTHOUGH a revision and extension of previous works 
by the same author, this is an important and valuable 
addition to the literature of bryology. It consists in the 
first place of a key to all the genera of Musci, including 
Sphagnacez, found in North America, and secondly of 
a similar key to all the species in each genus. Some 
idea of the labour involved will be gathered when it is 
stated that the genera number over 140; and that in 
some of the genera—e.g. Sphagnum, Orthotrichum, 
BLryum, Hypnum—there are from 50 to over go species. 
In a copious appendix is given a diagnosis of all the 
new species described between 1884 and 1896, 
NO. 1426, VOL. 55 | 
LETTERS TO THE EDITOR. 
(Lhe Editor does not hold himself responsible for opinions ex- 
pressed by hts correspondents. Netther can he undertake 
to return, or to correspond with the writers of, rejected 
manuscripts tntended for this or any other part of NATURE. 
No notice ts taken of anonymous communications. | 
Dynamical Units, 
MAny of the writers of letters on this subject seem to have 
forgotten that the question Prof. Perry raised was as to the best 
system of units to use with a class of exgineering students. 
This question is very seriously complicated by the fact that all 
their books, and almost all their teachers, use a system of units 
which is of that of the poundal, but is essentially the one that 
Prof. Perry advocates. This is a very serious fact that every 
teacher of engineering students must take account of, and the 
question is, ‘‘What system shall the teacher use with engineer- 
ing classes?” I entirely agree with Prof. Perry in thinking that 
it is much better for the teacher to accommodate himself to the 
requirements of his class than for him to force his class to use 
one system when working for him, and another outside his class- 
rooms. This latter plan tends to perpetuate the prevalent 
notion that science has nothing to do with practice. 
As regards the question of why students find dynamics and 
the notion of mass in particular so difficult, I do not believe 
that this is due to any difficulties about various systems of units. 
In matters upon which their ideas are clear and distinct, such as 
length and time, the existence of different units, feet, yards, 
miles, &c., minutes, days, &c., presents very little difficulty. 
To British students these varieties of units in which to measure 
the same quantity are so familiar, that they naturally look upon 
varieties of units with contempt. It is only when the thing 
measured is not clearly and distinctly conceived, that confusion 
and all sorts of difficulties arise. Hence the importance of 
getting students actually to come into contact with the things 
themselves. Untila student has some ideas of density, accelera- 
tion, &c., as things to be measured, he will be quite certain to 
misapply the rules he has learnt for dealing with the black 
marks he makes on a piece of paper, and which he calls by their 
names. Now of all these dynamical quantities, of which 
students are generally expected to form clear and distinct ideas 
without any actual experience of the things themselves, the most 
abstruse, and the one about which the most metaphysical state- 
ments are made, is ‘‘ quantity of matter”’ or ‘‘ mass.” A priori, 
there is no way by which we can determine whether a quantity of 
gold is equal to a quantity of iron. In ordinary practice there are 
two kinds of equality which are commonly used : volume and 
weight. IfI tell any ordinary man to mix equal quantities of whisky 
and water, he will mix equal volumes. Itis quite as common to 
mean equal volumes as equal weights by equal quantities in 
common language. When a student is told, as an explanation 
of the word ‘“‘ mass,” that it means ‘‘ quantity of matter,” there 
is an appeal made from the obscure to the more obscure. It is a 
case of huggermugger. The student thinks the teacher must 
have seme clear and distinct idea of what he means by “‘ quantity 
of matter,” and is ashamed to say that to him it is no explana- 
tion of mass to call it ‘‘ quantity of matter.” Thus begins the 
demoralisation of the student. He is demoralised by having to 
swallow undigested a term of which neither he nor his teacher 
has a clear and distinct idea, and he naturally concludes that 
the whole subject is one that ‘‘no fellow can understand.” _ 
If teachers and books would give up this metaphysical notion 
of ‘‘quantity of matter,” and would deign to confine their 
attentions to actually measurable quantities, like volume, weight, 
and inertia, the student could be given, by making experiments, 
clear and distinct ideas of these properties of matter as actual 
quantities to be measured. Once he had these clear and distinct 
ideas, a variety of units for measuring each of them would not 
present any serious difficulty. That the inertia of matter is pro- 
portional to its weight, that the inertia of a body is the same here 
as inthe moon and Jupiter—these are most important physical 
facts to be proved by experiment, because we have no other way of 
ascertaining them. It is often asszed that the inertia of a hot 
body is the same as of the same body when cold; but I do not 
know of any accurate: experiments having ever been made to 
prove it, and I am quite certain that a great deal too little is 
known of the structure of matter, and of its relations with the 
ether, to be able to prove @ frzor7 that the inertias are the same. 
The suggestion would probably involve the further suggestion 
