NoveMBER 5, 1896] 
NATURE - 
in addition to the magnetic inertia which accompanied 
their motion. To test this he tried two different forms 
of experiment, and obtained results which showed that if 
there were inertia of this kind, it must be small compared 
with that of the magnetic kind. The first method con- 
sisted essentially in a careful comparison of the extra 
current in a conductor with its calculated value; the 
second consisted in observing whether anything like the 
action by which the trade winds are deflected from a due 
northerly and southerly flow by the rotation of the earth, 
could be observed in a rotating conductor when traversed 
by an electric current. That there is some directed 
inertia in the conductor when traversed by an electric 
current is very probable, and in some cases we can be 
sure it exists. Hertz himself remarks that the inertia of 
the motion of the ions in electrolysis is considerably 
greater than what he was looking for in a metallic con- 
ductor. He could not make sufficiently delicate experi- 
ments with his apparatus to detect it, however, when 
using the small densities of current that were available 
in liquids; but the question is of great interest, and 
deserves further investigation. There can be no doubt 
that in gaseous discharges, kathode rays, as well as 
in electrolysis of liquids, there is a directed motion of 
matter accompanying the electric current which would 
be of the nature of the inertia Hertz was looking for, but 
failed to find. There seems much reason for thinking 
that in metallic conductors some similar actions are also 
taking place. Besides all this, there is the question as to 
how far the theory that all electricity is molecular and 
consists of electrons, involves the supposition of an 
inertia of this kind. Is the inertia of an electron com- 
pletely specified by the magnetic force accompanying 
it? Does it occupy no space itself, and is its external 
field its all? We are hardly in a position to answer such 
questions. We might, however, be able to answer the 
former question as to the inertia of the directed matter 
movements accompanying the current, and as to another 
interesting question of a similar kind, namely, as to how 
far we can legitimately assume the current inside a con- 
ductor to be absolutely homogeneous. The self-induc- 
tion of a single wire of a square m.m. in section is not 
exactly the same as that of, say, a hundred wires each of 
the thousandth of a square m.m. in section, and dis- 
tributed over the square m.m. Subdividing the current 
would increase its self-induction. Outside the wire the 
distribution of magnetic force would be practically the 
same as before, but inside we would have it concentrated 
into a hundred small wires instead of being uniformly 
distributed, and the effect of this would be to slightly 
increase the self-induction, and the more so the smaller 
the section of each wire into which the square m.m. were 
subdivided. Hence we conclude that if the current in a 
real wire be from molecule to molecule, and so be cen- 
centrated on certain lines, its inertia should be somewhat 
greater than that calculated from the hypothesis that it is 
uniformly distributed over the section of the conductor. 
The difference between these two views is most clear 
when we consider the case of a Leyden discharging by 
its insulating medium becoming a conductor. If the 
Leyden be completely closed, and the medium become a 
conductor in such a way that the strain in each cubic 
cm. is there destroyed by conductivity, there will be no 
magnetic force anywhere accompanying this discharge of 
the Leyden, and consequently no magnetic inertia, if the 
conduction be perfectly homogeneous. Now it seems 
almost impossible that any directed change can take 
place without some accompanying inertia, and we may 
consequently conclude that either (a) an electric current 
has inertia such as Hertz was looking for, or (4) electric 
conduction currents are essentially heterogeneous, or (c) 
electric conduction is essentially accompanied by material 
inertia, or (@) two or all three of these are true. That (c) 
certainly exists in this case is incontestable in view of the 
NO. I410, VOL. 55] : 
known directed strains that Kerr and Duter have proved 
to exist in matter subject to electric stress. What is the 
complete answer, is the important question. It is still 
unsolved. It lies at the foundation of every theory of 
electric conduction. Hertz attacked it. It is still wait- 
ing solution. 
The papers on the contact of elastic spheres and on 
hardness are most valuable contributions to the subject. 
They place the question of hardness on a scientjfic basis, 
and lay the foundation for a quantitative study of this most 
variable property of matter. There is no quality in which 
different‘materials differ more than in hardness. Electric 
conductivity is perhaps as various as hardness, compres- 
sibility, and viscosity, but hardly any other quality of 
matter is at all comparable with these in variety of range. 
Of these hardness is one of the most important and least 
known, and since Hertz’s work on it can be scientifically 
studied. Innumerable subsidiary questions arise in con- 
nection with it. Why are some bodies so easily polished ? 
Is the polishing of marble connected with the ease with 
which crystals of calespar can be twinned by pushing 
over one corner? What is the essential difference 
between polishing and grinding ? What is the effect of 
impurities on hardness? Is it comparable to their effect 
on electric conductivity? What is the cause of this 
effect ? 
In considering the cracking of a material like glass, 
Hertz seems to think that its cracking will depend only 
on the tension ; that it will crack where the tension ex- 
ceeds a certain limit. He does not seem to consider 
whether it might not crack by shearing with hardly any 
tension. It is doubtful whether a material in which 
there were sufficient general compression to prevent any 
tensions at all, would crack. Rocks seem capable of 
being bent about and distorted to almost any extent 
without cracking, and this might very well be expected 
if they were at a sufficient depth under other rocks to 
prevent their parts being under tension. It is an interest- 
ing questien whether a piece of glass could be bent with- 
out breaking if it were strained at the bottom of a 
sufficiently deep ocean. On the other hand, there seems 
very little doubt that the parts of a body might slide 
past one another under the action of a shear, and would 
certainly crack unless there were a sufficiently great 
compressive stress to prevent the crack, and that con- 
sequently a body might crack, even though the tensions 
were not by themselves sufficiently great to cause separa- 
tion, and might crack where the shear was greatest, and 
not where the tensions were greatest. 
Then follow some papers on hygrometry and evapora- 
tion. A very interesting point is raised in this latter 
connection. Can a liquid evaporate at an unlimited rate 
if the vapour produced is removed as rapidly as it is 
evolved? From two points of view Hertz shows that 
there is a limit, and by his experiments went far to show 
that there was no other cause limiting the rate of evapora- 
tion. The first point of view was that a limit is imposed 
by the difficulty of supplying heat sufficiently rapidly to 
keep the surface temperature constant. He does not 
seem in his experiments to have attempted to supply 
this by radiation, but was content to allow the liquid to 
supply itself by conduction and convection from below. 
The second point of view was that the molecules could 
not leave the surface faster than they would be moving 
in the vapour that was formed. Hertz’s investigation of 
this case only assumes an average velocity for the mole- 
cules ; he does not consider the distribution of velocity 
among the molecules, nor whether they escape equally 
easily in all directions. The experimental investigation 
of the conditions of evaporation is extremely difficult ; and 
until some more satisfactory method of studying these 
conditions be invented, the rough approximation seems 
to be sufficient to explain the observations. It might be 
interesting to see whether there was any difference between 
