480 
7. The subject of Spherical Harmonics (Laplace’s co- 
efficients) is also treated at some length, and in a some- 
what novel way, which leads to one of the quaternion 
methods of attack though without actually employing that 
calculus itself. 
8. Further, there is given, for the first time, the complete 
solution of the problem of /zduction of Currents in a disc 
rotating in the magnetic field, taking account of the 
mutual action of the currents on one another—a condition 
which very materially increases the difficulty of the 
problem, The result is a very curious one; and it 
appears especially curious when we compare the very 
simple, almost homely, methods employed by Maxwell 
with the elaborate analysis by means of which Jochmann 
and others, some years ago, attacked the incomplete and 
(comparatively) easy statement of the problem where the 
mutual action of the currents is not taken account of, 
This piece of work is worthy of being placed beside that 
of Thomson, referred to under (6) above, as among the 
very best things ever done in Mathematical Physics. 
9. Theratio of the electro-magnetic to the electrostatic 
unit of electricity is a Velocity whose absolute value is 
independent of the magnitude of the fundamental units 
employed. This has been shown by Maxwell to be the 
velocity with which waves of transverse vibration will be 
propagated in the medium whose stresses &c. account on 
his theory for the apparent action at a distance. Neither 
the velocity of light in free space, nor the ratio of the 
electric units, is certainly known as yet within five or six 
per cent., but it is assuredly a most striking fact that (in 
millions of metres per second) three of the best determi- 
nations of the former of these quantities give 
314, 308, 298, 
while apparently equally good determinations of the latter 
give 
311, 288, 282, 
Such approximation is evidently much more than a 
mere fortuitous coincidence, it shows that a great step 
has been taken in the grand question of the connection 
between radiation and electrical phenomena. 
Having said so much in hearty admiration of this noble 
work, a work which will do more to raise our country in 
the eyes of really competent judges than cartloads of 
more pretentious publications, it is only natural to seek 
some of its defects. There are spots on every sun; and 
they are, as phenomena, sometimes more instructive and 
therefore more worthy of observation than the sun itself. 
But, as they are not visible save to those whose eyes can 
bear the full glare of the glowing orb, and who therefore 
do not require our aid, itis unnecessary to point them out. 
Such a proceeding would be mere pandering to that 
miserable form of envy which leads inferior minds to 
gloat over the defects of their superiors. 
The concluding section of the work is particularly well 
fitted to terminate our article. 
“We have seen that the mathematical expressions for 
electrodynamic action led, in the mind ot Gauss, to the 
conviction that a theory of the propagation of electric 
action in time would be found to be the very keystone of 
electrodynamics. Now we are unable to conceive of 
propagation in time, except either as the flight of a mate- 
rial substance through space, or as the propagation of a 
condition of motion or stress in a medium already existing 
in space. Inthe theory of Neumann, the mathematical 
NATURE 
i yee 
conception called Potential, which we are unable to con- 
ceive as a material substance, is supposed to be projected 
from one particle to another, in a manner which is quite 
independent of a medium, and which, as Neumann has 
himself po:nted out, is extremely different from that of 
the propagation of light. In the theories of Riemann 
and Betti it would appear that the action is supposed to 
be propagated in a manner somewhat more similar to 
that of light. 
“ But in all of these theories the question naturally 
occurs :—If something is transmitted from one particle to 
another at a distance, what is its condition after it has 
left the one particle and before it has reached the other ? 
If this something is the potential energy of the two par- 
ticles, as in Neumann’s theory, how are we to conceive 
this energy as existing in a point of space, coinciding 
neither with the one particle nor with the other? In fact, 
whenever energy is transmitted from one body to another 
in time, there must be a medium or substance in which 
the energy exists after it leaves one body and before it 
reaches the other, for energy, as Torricelli* remarked, 
‘is a quintessence of so subtle anature that it cannot be 
contained in any vessel except the inmost substance of 
material things.’ 
conception of a medium in which the propagation takes 
place, and if we admit this medium as an hypothesis, I 
think it ought to occupy a prominent place in our investi- 
gations, and that we ought to endeavour to construct a 
mental representation of all the details of action, and 
this has been my constant aim in this treatise.” 
OUR BOOK SHELF 
Medizinische Fahrbiither Herausgegeben, Von der K. K. 
Gesellschaft der Aerzte redigert von S. Stricker. Jahr- 
gang 1872. Hefte i. ii. iii. iv., pp. 5133; mit xil. 
Tafein. 
THE second volume of Stricker’s Medizinische Fahr- 
biicher, now before us, maintains the promise of the first. 
It is made up of a number of separate essays on various 
subjects, physiological, pathological, and medical; the 
physiological essays predominating over the others. 
Amongst these the following deserve notice, 
1. Researches on the heart and blood-vessels, by Dr. 
Sigismund Mayer. In this paper Dr. Mayer shows that 
the extraordinary increase of pressure of the blood 
against the walls of the blood-vessels which occurs as a 
result of the administration of strychnia is due to intense 
excitation of the vaso-motor centre in the brain, which 
leads to contraction of the small arteries, 
2. A paper by Ewald Hering on the influence of the 
respiration upon the circulation. In this he shows that 
moderate expansion of the lungs by insufflation causes 
diminished blood pressure in the arteries and increased 
rapidity of the heart’s action. This latter effect, how- 
ever, is not due to the increased pressure exerted 
upon the external surface of the heart, nor to alterations 
in the conditions of resistance in the circulation ; nor to 
differences in the exchange of gases ; nor to any disloca- 
tion of the heart’s position, but is effected reflectorially 
through the pneumogastrics in such a manner that the 
activity of the cerebral centre of the inhibitory nerves is 
lowered. 
3. MM. Oser and Schlesinger give the details, in an 
elaborate essay, of many experiments they have made to 
determine the causation of the movements of the uterus, 
and sate that they have arrived at the conclusion that 
these movements can be induced. by suspeysion of the 
respiration ; by rapid loss of blood ; or by arrest of the 
supply of arterial blood to the brain, 
4. M. M. Rosenthal contributes an interesting essay on 
the death of the muscles of the body, and on apparent 
death, He made many experiments on about twenty sub- 
* Lesionit Accademiche (Firenze, 1715), p. 25. 
[April 24, 1873 
Hence all these theories lead to the | 
se 
-i ) a ool 
