those whose powers of generalisation do not carry them 
far enough to put them above regarding the small details 
of experiment as the ultimate end of science. For both 
classes there are lessons in Maxwell’s pages. 
Another most important feature of the work under 
review is that it is in the strictest sense a Treatise on 
Electrical Measurement. It looks at electrical actions 
almost exclusively as measurable; it does not profess to 
be a complete experimental treatise of all electrical 
phenomena qualitatively or quantitatively observed. In 
this respect it is a continuation of the labours of its 
author in conjunction with the rest of the distinguished 
band of electricians who formed the Committee of the 
British Association on Electrical Measurements. Max- 
well’s work is in many parts, particularly in the second 
volume, a development of the methods employed by this 
Committee. Many of the electrical measurements there 
described had, when the volumes were first published, 
been actually carried out only by the author himself or by 
very few beside ; but now the great majority of them have 
become the commonplaces of a physical laboratory. 
The effect on practice of the work thus consummated 
in Maxwell’s Treatise has been, both directly and indi- 
rectly, enormous. The extension of technical applications 
has been immensely facilitated by the introduction of 
definite units. Instead of the old vague, unscientific, and 
still more, unbusinesslike statements of quantity and in- 
tensity, we have the precise ideas of electromotive force, 
resistance, current, and so on, measured in their respective 
units, the volt, the ohm, the ampére, &c.; and now elec- 
trical commodities can be bought and sold by rule and 
measure, as heretofore cloth, coals, or horse-power. And 
yet we have noticed a tendency now and then in technical 
journals, on the part of men of practice, evidently ignorant 
of the history of the science they apply, to depreciate 
unduly the services of their theoretical brethren. One 
would have thought that in electrical science, beyond all 
others, where the mutual obligation is so great and so 
equally balanced, the folly of either the man of theory or 
the man of practice attempting to minimise the services 
of his fellow worker would have been evident. 
The seal has been set to the work of the B.A. Com- 
mittee by the Congress of Electricians which met last 
autumn in Paris, by the adoption of the B.A. units as 
the basis of an international system. To this result 
Maxwell's treatise has powerfully contributed, but it 
would be little in the spirit of its author to boast of this 
as a national, much less as his personal triumph; it is 
more fitting to remind the readers of NATURE that in the 
work thus consummated the English electricians were 
the followers of Gauss and Weber, and, more remotely, 
the disciples of Coulomb, Poisson and Ampére, so that 
they have simply acted up to the motto of all true scien- 
tific men, Aapuradia €yovres Siaddcovew adAndots ; they have 
but passed the torch from hand to hand. 
The electromagnetic theory of light formed a fitting 
crown to the first edition of Maxwell’s Electricity. It 
was left by its author in the form of a general sketch, 
carried just so far as was necessary for comparison with 
experiment. Concerning the progress of this theory 
during the last eight years much might be said, and it is 
greatly to be regretted that we have not before .us what 
Maxwell himself would undoubtedly have said, had he 
lived to superintend the publication of the second volume 
of his work. It has formed the basis, as every good 
physical theory should, for a large number of further re- 
searches, both theoretical and experimental. We need 
only mention the work of Helmholtz, Boltzmann, Rayleigh, 
Silow, Hopkinson, Fitzgerald, Glazebrook, J. J. Thomson, 
and many others. The theory has not proved, and its 
author certainly never expected it to prove, a framework 
ready made with appropriated pigeon-holes, into which 
would naturally fall every electrical fact to be discovered 
in all time coming ; but it has proved itself, so far, the 
best theory with anything like a physical basis that has 
been proposed to explain the facts with which it deals. 
The more it has been worked out, the more it has been 
found to explain in a natural way the known phenomena 
of electricity and light; and it does not appear to have 
been shown as yet that there is any observed fact that 
may not ultimately be reconciled with it, either by farther 
development of the theory, or by deeper probing of the 
experimental results. This is really all that could be 
expected when we reflect that, much as we know about 
electricity, there is an infinity yet unknown, 
We have now to allude briefly to the changes that have 
been made in the second edition. 
In the introductory chapter we are glad to see that few 
changes have been made; we need, therefore, only 
recommend our readers to peruse it again, as perhaps the 
most admirable thing of the kind that has been written in 
any language; we direct their attention more particularly 
to the distinctions drawn between electricity, force, and 
energy, of which some of our scientific men seem strangely 
oblivious, and to the admirable remarks on the two fluid 
theory. We must at the same time warn the student as 
to a radical change that has been introduced into the 
terminology of the subject. He is aware that at every 
point of the electric field there is conceived a directed 
quantity, which in the former edition of this work was 
called resultant electric force, or the electromotive force 
at the point, according as it was regarded from the pon- 
deromotive or electromotive point of view, and he is also 
aware that the electromotive force at a Point was a very 
different thing from the electromotive force defween two 
points, the latter being in point of fact of different dimen - 
sions. It was always difficult, even for those who clearly 
understood the distinction, to avoid occasionally using 
the one term where the other was appropriate. Most 
probably from a feeling of this difficulty, our author 
has substituted for the two first of these terms 
resultant electric intensity and electromotive intensity 
respectively, wisely leaving the old established terms 
electromotive force between two points with its ori- 
ginal meaning, although in point of fact it involves 
an abuse of the word Force. We could have wished the 
danger of confusion still more effectively barred by drop- 
ping the word “ electromotive” in the first case altogether; 
but a more serious objection to this change is, that the 
author evidently intended, from his foot-note on p. 72— 
“The electric and magnetic intensity correspond in 
electricity and magnetism to the intensity of gravity, com- 
monly called g in the theory of heavy bodies,” 
to have made a corresponding change of terminology in 
the case of magnetism; whereas on turning to the second 
volume we find intensity of magnetisation used in its old 
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