; 
Fan. 12, 1882] 
NATURE 
239 
sense, and (resultant) magnetic force used in the sense 
in which magnetic intensity occurs in the above note, 
and, what is worse, in place of electric intensity or 
electromotive intensity we find on, p. 239, “ electromotive 
force,” and on p. 244, “electromotive force at a point”’ 
used in its place. This result of the interruption of the 
issue of the new edition is unfortunate, for it renders the 
confusion of terms greater than ever, and affords a kind 
of cover to those who excuse or justify the inaccuracy of 
their own ideas by appealing to the terminological incon- 
sistencies of standard works. Yet we can scarcely blame 
the editor, for it is a very delicate matter to interfere with 
another’s work, even in points like this. 
The want of definiteness in electrical terminology 
makes itself felt in the definition of the electric strength 
of a dielectric ; thus we find, on p. 51— 
“The value of the electromotive force which can exist 
in a dielectric without a discharge taking place, is called 
the electric strength of the dielectric.” 
And again on p. 54 :— 
“The intensity of the electromotive force when this 
takes place is a measure of what we may call ‘the electric 
strength of the dielectric.’’ 
Assuming for a moment that any accurate definition of 
electric strength can be given, which at present, experi- 
mentally speaking, is open to doubt ; or, which comes to 
the same thing, taking, as Maxwell probably means us 
to do, an ideal case, the second of these definitions is 
right, if we understand by intensity of the electromotive 
force resultant electric intensity, or electromotive inten- 
sity, as previously defined; while the first, strictly read, 
ze. taking electromotive force to mean the electromotive 
force between two points, is clearly wrong ; for, if it were 
right, a spark would always pass between two conductors 
in the same medium when the difference of potential 
between them is the same, no matter what their form or 
surroundings, which is well known to be untrue. 
In this connection we may mention that the account 
formerly given of Thomson’s classical experiments on the 
electric strength of air no longer finds a place in Art. 57. 
No doubt the author had intended to describe them later 
on along with what has been done of late in the same 
direction ; but no mention of them occurs, except a cursory 
one in Art. 59; yet we are still referred in Art. 369 to Art. 
57 for the account which is no longer there. It is a pity 
that a footnote was not inserted referring the reader to 
the reprint of Thomson’s papers. 
In Chapter IJ. we may note, as new, an interesting 
account of Cavendish’s experiment, on which, mainly, we 
may now rest the evidence for the elementary law of 
electrostatic action ; a simplification of the treatment of 
the variation of the potential at charged surfaces ; farther 
direct application of the theory of lines of force in proving 
electrical theorems of considerable interest and gene- 
rality ; anda clearer explanation than was formerly given 
of the distinction between the real electrification accord- 
ing to the medium theory and the apparent electrification 
which may be used to represent it, if we abstract the 
specific inductive capacity of the dielectric, 
Chapter III. has been in great part rewritten, and seve- 
ral very interesting and practically useful calculations of 
the approximate values of coefficients of induction and 
potential have been added. In Chapter IV., which con- 
tains the general theory of electrical equilibrium, the 
changes have been greater still; the result has been, on 
the whole, we think, considerable simplification ; it would 
appear, however, from the way in which the chapter ends, 
that the author had contemplated some farther additions, 
Chapter IX. has also been greatly modified ; in particular 
the problem of two spheres has been worked out in great 
detail, and series given to a high degree of approximation 
for the coefficients of induction and potential. 
The rest of the work is practically a reprint from the 
former edition, and calls for little remark. We may, 
however, call the attention of our readers to Art. 261, the 
ideas and notation of which will, we believe, be found 
discordant with the best modern chemical views.’ The 
editor has very properly appended a note to Art. 357, call- 
ing attention to Mr. Lodge’s exposure of the fundamental 
defect of Mance’s method (in the Philosophical Magazine 
for 1877, not 1857, as the reference is printed). Mr, 
Lodge’s remarks are, so far as we know, the first published 
allusion to the matter, but the defect in question was well 
known to Prof. Clerk Maxwell, for it was discussed with 
him by the present writer some time before the above 
date. It used to be not uncommon to set over-confident 
students in the Cavendish Laboratory the problem of 
measuring the internal resistance of a battery, and then 
to explain to them the reason of the hopelessly indefinite 
character of the results obtained under certain circum- 
stances by Mance’s method. Notwithstanding Mr. Lodge’s 
remarks, one sees, even in the most recent text-books, 
this method confidently cited as apparently irreproach- 
able.2 So tenacious is scientific error! Another correc- 
tion we may mention—the interlacing circuits in Art. 421 
arenow so arranged that the integral //11 adsdo vanishes. 
Before taking leave of the work, we have to express the 
gratitude which the scientific public owes to its editor, Mr. 
Niven. It has been our duty to indicate some points in 
which there might have been improvement, and we have 
said little as to what has been actually done by Mr. 
Niven. It is but justice to him, therefore, to add that we 
went over the new edition, and compared it with our copy 
of the first edition, and we found that in almost every 
case the errors we had noted were corrected, while ex- 
planations had been inserted at many of the places where 
we had found them necessary. The labour involved in 
doing all this will be best understood by those who are 
perfectly familiar with the whole of Maxwell’s great work, 
and all such will know how to appreciate the conscientious 
labour which Mr. Niven has so unostentatiously bestowed 
on the editing of this edition. 
The regret one feels that Prof. Maxwell did not live to 
complete his work is much increased when we read the 
elementary treatise. In the earlier parts it is charac- 
terised by that originality, freshness, and exemplary 
clearness familiar to the readers of his Heat. Chapters 
I. and II. reproduce with more ample experimental detail ~ 
the admirable introduction to the larger work. Chapter 
III. gives the mathematical theory of electrical work and 
energy in a form accessible to students of moderate mathe- 
matical acquirements, although such need not expect to 
1 This applies also to the corresponding passage in the elementary work. 
2 It isunfortunate that Mr. Garnett should have transferred the description 
of Mance’s method to the Elementary Treatise, without correcting or alluding 
to the error it contains, 
