8 NATURE 
[ NoveMBER 5, 1896 
the superficial friction of a gas and a liquid which 
did not evaporate, and of a vapour in contact with its 
own liquid. In one case there would be no exchange of 
molecules between the two bodies that were sliding past 
one another, while in the second case there would be an 
exchange. A study of the conduction of heat between 
a gas and a liquid might also help to elucidate the nature 
of the exchange which takes place between a liquid and 
its vapour. ‘ 
In the paper on the vapour pressure of mercury, there 
are some very rough approximations which are hardly 
sufficiently accurate for general application. One is as 
to the extent to which a saturated vapour obeys the laws 
of a perfect gas. Hertz assumes that this is more nearly 
true the lower the temperature. This is not generally so. 
For each kind of material there is a particular tempera- 
ture at which its saturated vapour most nearly obeys 
these laws, and below as well as above this temperature 
it departs from these laws. Again, there is a process, 
described at the bottom of p. 203 and top of 204, which 
cannot possibly be carried out. He says: “ Let a quantity 
of liquid at temperature T be brought to any other 
temperature. At this temperature it is converted into 
vapour without external work.” This 
impossible, and the equation he deduces from all this is 
not true, though it is sometimes a rough approximation | 
to the truth. 
There is a very interesting paper on the floating of 
bodies by thin sheets of rigid material like ice. Hertz 
shows that if the sheet be large enough it would be 
possible to cause a thin sheet of iron, which by itself 
would sink, to float by placing weights at its centre. The 
weights might so depress the centre and make the sheet 
so boat-shaped as to float both themselves and it. 
In 1883 Hertz published a deduction from first 
principles of Maxwell’s equations for the electromagnetic 
field in the symmetrical form, afterwards used by himself 
in his investigations on oscillatory discharge waves. He 
applies the very same arguments by which Helmholtz, 
Lord Kelvin, and others had argued from the work done 
by one electric current on another, that there must be a 
corresponding reaction of the second on the first current, 
and hence deduced electromagnetic induction. Hertz 
applies this argument to the case of a ring magnet | 
changing in strength and producing magnetic force on 
another ring magnet in its neighbourhood, and doing 
work there, and shows thereby that there should be a 
magnetic force due toa changing electric field exactly 
corresponding to the electric force due to a changing 
magnetic field. This, of course, is what Maxwell describes 
as the magnetic effect of the changing electric displace- 
ment, and its effects are expressed by the very same 
equations as Maxwell deduces. The argument is no 
more and no less conclusive than in the corresponding 
application of the principle of the conservation of energy 
to deduce ordinary electromagnetic induction. Hertz is 
careful to point this out, for he was early imbued by 
Helmholtz with the fact that the principle of the conserv- 
ation of energy is by itself utterly inadequate as a com 
plete explanation of physical phenomena. He specially 
mentions himself Helmholtz’s interest in this problem of 
the simplest basis for dynamics, and Hertz’s last great 
work was to place general dynamics on a sound 
basis. The simplest of all cases is the easiest in which 
to see how the principle of the conservation of energy 
fails to give a complete solution. A body moving without 
any action from other bodies describes a right line ata 
constant velocity. The principle of the conservation of 
energy requires the constant velocity. But, why the right 
line? Conservation of energy cannot solve even the 
simplest of all examples. It would be well if some 
modern chemists would mark, learn, and inwardly digest 
this. 
The part of Hertz’s work which is of greatest interest 
NO. 1410, VOL. 55] 
is absolutely | 
| as in a conducting sheet. 
just now is that in connection with kathode rays. He 
began with some very interesting observations on the 
aura accompanying spark discharges. It appeared to 
be projected from the positive electrode, and occasionally 
formed a vortex ring of incandescent gas, which lasted 
for an appreciable time between the electrodes of a jar 
discharging in air. Goldstein has noticed similar effects. 
and some recent experiments on the discharge of large 
Leyden batteries, in which some of the phenomena of 
globular lightning seem to have been reproduced, make 
it appear possible that this latter is a spherical vortex of 
incandescent air. 
Hertz’s study of kathode rays in 1883, set finally at rest 
two questions. In the first place he showed that the 
discharge in a gas may be as continuous as any other 
form of current. In no case are we absolutely certain 
that the current is absolutely continuous. On the large 
scale it certainly is ; but all we know of electrolysis seems 
to show that on a sufficiently small scale the current is. 
carried in detachments, and is consequently essentially 
discontinuous. This, however, was not the question at 
issue, and so far as a continuity of the same kind as that 
in any liquid electrolyte is concerned, Hertz showed that 
the discharge through a gas might be equally continuous. 
The second question he decided was as to the direction 
of flow of the average current in an exhausted space. He 
showed that the average flow at any point was nearly the 
| same as if the whole space were a conductor : that there 
was no connection between the kathode rays and the flow 
of the current. From experiments on kathode rays pro- 
jected down a tube, and quite away from both electrodes, 
he deduced that they produce no magnetic action outside 
the tube, although they are deflected by the magnet. His 
conclusion, that the kathode rays are not streams of 
electrified particles, was largely founded on this, and on 
another experiment on the action of electrostatic force on 
the particles. This experiment on the magnetic action 
of kathode rays is quite inconclusive, and it is very 
remarkable that Hertz should have attributed much 
importance to it. Whatever current was carried down 
his tube by the kathode ray must have come back the 
tube by the surrounding gas, and these two opposite 
currents should have produced no magnetic force outside 
the tube ; and this is exactly what Hertz observed. Ina 
similar way, what he observed in the case of a flat box 
was the average direction of the current, and he showed 
that this average direction was approximately the same 
This proved that if there were 
any concentration of the current along the direction of 
the kathode rays, this concentration was neutralised by a 
corresponding return current, so that the average current 
was as described. At the same time there does not seem 
much doubt but that the kathode rays only carry a very 
small part of the current. The third part of the paper is. 
concerned with the electrostatic effects due to kathode 
rays. The experiments do not seem to fully justify the 
conclusion drawn, that kathode rays cannot be charged 
molecules. Sufficient account does not seem to have been 
taken of the shielding action of the conducting gas 
surrounding the kathode ray, nor of the way in which the 
potential is distributed between two electrodes in a gas. 
Hertz describes an experiment with two plates inside the 
tube kept at a considerable difference of potential. He 
says: “ The phosphorescent image of the Ruhmkorff coil: 
discharge appeared somewhat distorted through deflec- 
tion in the neighbourhood of the negative strip ; but the 
part of the shadow in the middle between the two strips 
was not visibly displaced.” Now this is exactly what one 
might expect, because the fall of potential between two 
such strips is very small indeed, except close to the 
negative strip, and there the electric force dd deflect the 
rays. Hence the conclusion is just the reverse of the one 
Hertz gives. From the experiment it appears that kathode 
rays do behave like electrified particles. It is very 
‘ 
