220 
NATURE 
We must now turn to the next part of this volume, 
in which the mathematical electrician, now established as 
a Professor at Glasgow, turns his attention to the 
practical and experimental work of his science. In such 
work the mathematician, if he succeeds at all, proves 
himself no mere mathematician, but a thoroughly furnished 
man of science. And first we have an account of that 
research into atmospheric electricity which created a de- 
mand for electrometers ; then a series of electrometers of 
gradually improving species; and lastly, an admirable 
report on electrometers and electrostatic measurements, 
in which the results of many years’ experience are given 
in a most instructive and scientific form. In this report 
the different instruments are not merely described, but 
classified, so that the student is furnished with the means 
of devising a new instrument to suit his own wants. He 
may also study, in the recorded history of electrometers, 
the principles of natural selection, the conditions of the 
permanence of species, the retention of rudimentary 
organs in manufactured articles, and the tendency to 
reversion to older types in the absence of scientific control. 
A good deal of Sir W. Thomson’s practical electrical 
work is not referred to in this volume. It isto be hoped 
that he will yet find time to give some account of his many 
admirable telegraphic contrivances in galvanometers, 
suspended coils, and recording instruments, and to 
complete this collection by his papers on electrolysis, 
measurement of resistance, electric qualities of metals, 
thermo-electricity, and electro-magnetism in general. 
The second division of the book contains the a of 
magnetism. 
The first paper, communicated to the Royal Society in 
1849 and 1850, is the best introduction to the theory of 
magnetism that we know of. The discussion of particular 
distributions of magnetisation is altogether original, and 
prepares the way for the theory of electro-magnets which 
follows. This paper on electro-magnets is interesting as 
having been in manuscript for twenty-three years, during 
which time a great deal has been done both at home and 
abroad on the same subject, but without in any degree 
trenching upon the ground occupied by Thomson in 
1847. Though in these papers we find several formidable 
equations bristling with old English capitals, the reader 
will do well to observe that the most important results 
are often obtained without the use of this mathematical 
apparatus, and are always expressed in plain scientific 
English. 
As regards the most interesting of all subjects, the 
history of the development of scientific ideas—we know 
of few statements so full of meaning as the note at p. 419 
relating to Ampéres’ theory of magnetism, as depending 
on electric currents, flowing in circuits within the mole. 
cules of the magnet ; he goes on to say :—. 
“ From twenty to five-and-twenty years ago, when the 
materials of the present compilation were worked out, I 
had no belief in the reality of this theory ; but I did not 
then know that motion is the very essence of what has 
been hitherto called matter. At the 1847 meeting of the 
British Association in Oxford, I learned from Joule the 
dynamical theory of heat, and was forced to abandon at 
once many, and gradually from year to year all other, 
statical preconceptions regarding the ultimate causes of 
apparently statical phenomena.” 
rotation of the plane of polarised light discovered by 
Faraday implies an actual rotatory motion of something, 
and that this motion is part of the phenomenon of mag- 
netism, he adds: — 
“ The explanation of all phenomena of electro-magnetic 
attraction or repulsion, and of ele¢tro-magnetic induction, — 
is to be looked for simply in the inertia and pressure of 
the matter of which the motions constitute heat. Whether 
this matter is or is not electricity, whether it is a con- 
tinuous fluid interpermeating the spaces between molecu- 
lar nuclei, or is itself molecularly grouped ; or whether 
all matter is continuous, and molecular heterogeneous- 
ness consists in finite vortical or other relative motions of 
contiguous parts of a body; it is impossible to decide, 
and perhaps i in vain to speculate, in the present state of 
science.” 
The date of these remarks is 1856. In 1861 and 1862 
appeared Maxwell’s “theory of molecular vortices applied 
to magnetism, electricity, &c.” which may be considered 
as a development of Thomson’s idea ina form which, 
though rough and clumsy compared with the realities of 
nature, may have served its turn as a provisional hypo- 
thesis. 
The concluding sections of the book before us are 
devoted to illustrations of magnetic force derived from 
the motion of a perfect fluid. They are not put forward 
as explanations of magnetic force, for in fact the forces 
are of the opposite kind to those of magnets, They — 
belong more properly to that remarkable extension of the 
science of hydrokinetics which was begun by Helm- 
holtz and so ably followed up by Thomson himself. 
The conception of a perfectly homogeneous, incom- 
pressible frictionless fluid is as essential a part of pure 
dynamics as that of a circle is of pure geometry. It is 
true that the motions of ordinary fluids are very imperfect 
illustrstions of those of the perfect fluid. But it is equally 
true that most of the objects which we are pleased to call 
circles are very imperfect representations of a true circle. 
Neither a perfect fluid nor a perfect circle can be formed 
from the materials which we deal with, for they are 
assemblages of molecules, and therefore not homoge- 
neous except when regarded roughly in large masses. 
The perfect circle is truly continuous and the perfect 
fluid is truly homogeneous. 
It follows, however, from the investigations of Helm- 
holtz and Thomson that if a motion of the kind called 
rotational is once set up in the fluid, the portion of the 
fluid to which this motion is communicated, retains for 
ever, during all its wanderings through the fluid mass, 
the character of the motion thus impressed on it. 
This vortex then, as Helmholtz calls it, be it large or 
small, possesses that character of permanence and indi- 
viduality which we attribute to a material molecule, while 
at the same time it is capable, while retaining its essential 
characteristics unchanged both in nature and value, of 
changing its form in an infinite variety of ways, and of 
executing the vibrations which excite those rays of the 
spectrum by which the species of the molecule may be 
discovered. It would puzzle one of the old-fashioned 
little round hard molecules to execute vibrations at all. 
There was no music in those spheres. 
But besides this application of hydrokinetics to this 
new conception of the old atom, there is a vast field of 
After a short, but sufficient, proof that the magnetic | high mathematical inquiry opened up by the papers of 
| Xan. 23, 1873 
