THE CONDUCTIVITY OF GASES AND THE IONS
§ 1. THE CONDUCTIVITY OF GASES
If we were confined to the facts I have set forth above, we might
conclude that two classes of phenomena are to-day being interpreted
with increasing correctness in spite of the few difficulties which
have been pointed out. The hypothesis of the molecular constitution of
matter enables us to group together one of these classes, and the
hypothesis of the ether leads us to co-ordinate the other.
But these two classes of phenomena cannot be considered independent of
each other. Relations evidently exist between matter and the ether,
which manifest themselves in many cases accessible to experiment, and
the search for these relations appears to be the paramount problem the
physicist should set himself. The question has, for a long time, been
attacked on various sides, but the recent discoveries in the
conductivity of gases, of the radioactive substances, and of the
cathode and similar rays, have allowed us of late years to regard it
in a new light. Without wishing to set out here in detail facts which
for the most part are well known, we will endeavour to group the chief
of them round a few essential ideas, and will seek to state precisely
the data they afford us for the solution of this grave problem.
It was the study of the conductivity of gases which at the very first
furnished the most important information, and allowed us to penetrate
more deeply than had till then been possible into the inmost
constitution of matter, and thus to, as it were, catch in the act the
actions that matter can exercise on the ether, or, reciprocally, those
it may receive from it.
It might, perhaps, have been foreseen that such a study would prove
remarkably fruitful. The examination of the phenomena of electrolysis
had, in fact, led to results of the highest importance on the
constitution of liquids, and the gaseous media which presented
themselves as particularly simple in all their properties ought, it
would seem, to have supplied from the very first a field of
investigation easy to work and highly productive.
This, however, was not at all the case. Experimental complications
springing up at every step obscured the problem. One generally found
one's self in the presence of violent disruptive discharges with a
train of accessory phenomena, due, for instance, to the use of
metallic electrodes, and made evident by the complex appearance of
aigrettes and effluves; or else one had to deal with heated gases
difficult to handle, which were confined in receptacles whose walls
played a troublesome part and succeeded in veiling the simplicity of
the fundamental facts. Notwithstanding, therefore, the efforts of a
great number of seekers, no general idea disengaged itself out of a
mass of often contradictory information.
Many physicists, in France particularly, discarded the study of
questions which seemed so confused, and it must even be frankly
acknowledged that some among them had a really unfounded distrust of
certain results which should have been considered proved, but which
had the misfortune to be in contradiction with the theories in current
use. All the classic ideas relating to electrical phenomena led to the
consideration that there existed a perfect symmetry between the two
electricities, positive and negative. In the passing of electricity
through gases there is manifested, on the contrary, an evident
dissymmetry. The anode and the cathode are immediately distinguished
in a tube of rarefied gas by their peculiar appearance; and the
conductivity does not appear, under certain conditions, to be the same
for the two modes of electrification.
It is not devoid of interest to note that Erman, a German scholar,
once very celebrated and now generally forgotten, drew attention as
early as 1815 to the unipolar conductivity of a flame. His
contemporaries, as may be gathered from the perusal of the treatises
on physics of that period, attached great importance to this
discovery; but, as it was somewhat inconvenient and did not readily
fit in with ordinary studies, it was in due course neglected, then
considered as insufficiently established, and finally wholly
forgotten.
All these somewhat obscure facts, and some others--such as the
different action of ultra-violet radiations on positively and
negatively charged bodies--are now, on the contrary, about to be
co-ordinated, thanks to the modern ideas on the mechanism of conduction;
while these ideas will also allow us to interpret the most striking
dissymmetry of all, i.e. that revealed by electrolysis itself, a
dissymmetry which certainly can not be denied, but to which sufficient
attention has not been given.
It is to a German physicist, Giese, that we owe the first notions on
the mechanism of the conductivity of gases, as we now conceive it. In
two memoirs published in 1882 and 1889, he plainly arrives at the
conception that conduction in gases is not due to their molecules, but
to certain fragments of them or to ions. Giese was a forerunner, but
his ideas could not triumph so long as there were no means of
observing conduction in simple circumstances. But this means has now
been supplied in the discovery of the X rays. Suppose we pass through
some gas at ordinary pressure, such as hydrogen, a pencil of X rays.
The gas, which till then has behaved as a perfect insulator,[29]
suddenly acquires a remarkable conductivity. If into this hydrogen two
metallic electrodes in communication with the two poles of a battery
are introduced, a current is set up in very special conditions which
remind us, when they are checked by experiments, of the mechanism
which allows the passage of electricity in electrolysis, and which is
so well represented to us when we picture to ourselves this passage as
due to the migration towards the electrodes, under the action of the
field, of the two sets of ions produced by the spontaneous division of
the molecule within the solution.
[Footnote 29: At least, so long as it is not introduced between the
two coatings of a condenser having a difference of potential
sufficient to overcome what M. Bouty calls its dielectric cohesion. We
leave on one side this phenomenon, regarding which M. Bouty has
arrived at extremely important results by a very remarkable series of
experiments; but this question rightly belongs to a special study of
electrical phenomena which is not yet written.]
Let us therefore recognise with J.J. Thomson and the many physicists
who, in his wake, have taken up and developed the idea of Giese, that,
under the influence of the X rays, for reasons which will have to be
determined later, certain gaseous molecules have become divided into
two portions, the one positively and the other negatively electrified,
which we will call, by analogy with the kindred phenomenon in
electrolysis, by the name of ions. If the gas be then placed in an
electric field, produced, for instance, by two metallic plates
connected with the two poles of a battery respectively, the positive
ions will travel towards the plate connected with the negative pole,
and the negative ions in the contrary direction. There is thus
produced a current due to the transport to the electrodes of the
charges which existed on the ions.
If the gas thus ionised be left to itself, in the absence of any
electric field, the ions, yielding to their mutual attraction, must
finally meet, combine, and reconstitute a neutral molecule, thus
returning to their initial condition. The gas in a short while loses
the conductivity which it had acquired; or this is, at least, the
phenomenon at ordinary temperatures. But if the temperature is raised,
the relative speeds of the ions at the moment of impact may be great
enough to render it impossible for the recombination to be produced in
its entirety, and part of the conductivity will remain.
Every element of volume rendered a conductor therefore furnishes, in
an electric field, equal quantities of positive and negative
electricity. If we admit, as mentioned above, that these liberated
quantities are borne by ions each bearing an equal charge, the number
of these ions will be proportional to the quantity of electricity, and
instead of speaking of a quantity of electricity, we could use the
equivalent term of number of ions. For the excitement produced by a
given pencil of X rays, the number of ions liberated will be fixed.
Thus, from a given volume of gas there can only be extracted an
equally determinate quantity of electricity.
The conductivity produced is not governed by Ohm's law. The intensity
is not proportional to the electromotive force, and it increases at
first as the electromotive force augments; but it approaches
asymptotically to a maximum value which corresponds to the number of
ions liberated, and can therefore serve as a measure of the power of
the excitement. It is this current which is termed the _current of
saturation_.
M. Righi has ably demonstrated that ionised gas does not obey the law
of Ohm by an experiment very paradoxical in appearance. He found that,
the greater the distance of the two electrode plates from each, the
greater may be, within certain limits, the intensity of the current.
The fact is very clearly interpreted by the theory of ionisation,
since the greater the length of the gaseous column the greater must be
the number of ions liberated.
One of the most striking characteristics of ionised gases is that of
discharging electrified conductors. This phenomenon is not produced by
the departure of the charge that these conductors may possess, but by
the advent of opposite charges brought to them by ions which obey the
electrostatic attraction and abandon their own electrification when
they come in contact with these conductors.
This mode of regarding the phenomena is extremely convenient and
eminently suggestive. It may, no doubt, be thought that the image of
the ions is not identical with objective reality, but we are compelled
to acknowledge that it represents with absolute faithfulness all the
details of the phenomena.
Other facts, moreover, will give to this hypothesis a still greater
value; we shall even be able, so to speak, to grasp these ions
individually, to count them, and to measure their charge.
§ 2. THE CONDENSATION OF WATER-VAPOUR BY IONS
If the pressure of a vapour--that of water, for instance--in the
atmosphere reaches the value of the maximum pressure corresponding to
the temperature of the experiment, the elementary theory teaches us
that the slightest decrease in temperature will induce a condensation;
that small drops will form, and the mist will turn into rain.
In reality, matters do not occur in so simple a manner. A more or
less considerable delay may take place, and the vapour will remain
supersaturated. We easily discover that this phenomenon is due
to the intervention of capillary action. On a drop of liquid a
surface-tension takes effect which gives rise to a pressure which
becomes greater the smaller the diameter of the drop.
Pressure facilitates evaporation, and on more closely examining this
reaction we arrive at the conclusion that vapour can never
spontaneously condense itself when liquid drops already formed are not
present, unless forces of another nature intervene to diminish the
effect of the capillary forces. In the most frequent cases, these
forces come from the dust which is always in suspension in the air, or
which exists in any recipient. Grains of dust act by reason of their
hygrometrical power, and form germs round which drops presently form.
It is possible to make use, as did M. Coulier as early as 1875, of
this phenomenon to carry off the germs of condensation, by producing
by expansion in a bottle containing a little water a preliminary mist
which purifies the air. In subsequent experiments it will be found
almost impossible to produce further condensation of vapour.
But these forces may also be of electrical origin. Von Helmholtz long
since showed that electricity exercises an influence on the
condensation of the vapour of water, and Mr C.T.R. Wilson, with this
view, has made truly quantitative experiments. It was rapidly
discovered after the apparition of the X rays that gases that have
become conductors, that is, ionised gases, also facilitate the
condensation of supersaturated water vapour.
We are thus led by a new road to the belief that electrified centres
exist in gases, and that each centre draws to itself the neighbouring
molecules of water, as an electrified rod of resin does the light
bodies around it. There is produced in this manner round each ion an
assemblage of molecules of water which constitute a germ capable of
causing the formation of a drop of water out of the condensation of
excess vapour in the ambient air. As might be expected, the drops are
electrified, and take to themselves the charge of the centres round
which they are formed; moreover, as many drops are created as there
are ions. Thereafter we have only to count these drops to ascertain
the number of ions which existed in the gaseous mass.
To effect this counting, several methods have been used, differing in
principle but leading to similar results. It is possible, as Mr C.T.R.
Wilson and Professor J.J. Thomson have done, to estimate, on the one
hand, the weight of the mist which is produced in determined
conditions, and on the other, the average weight of the drops,
according to the formula formerly given by Sir G. Stokes, by deducting
their diameter from the speed with which this mist falls; or we can,
with Professor Lemme, determine the average radius of the drops by an
optical process, viz. by measuring the diameter of the first
diffraction ring produced when looking through the mist at a point of
light.
We thus get to a very high number. There are, for instance, some
twenty million ions per centimetre cube when the rays have produced
their maximum effect, but high as this figure is, it is still very
small compared with the total number of molecules. All conclusions
drawn from kinetic theory lead us to think that in the same space
there must exist, by the side of a molecule divided into two ions, a
thousand millions remaining in a neutral state and intact.
Mr C.T.R. Wilson has remarked that the positive and negative ions do
not produce condensation with the same facility. The ions of a
contrary sign may be almost completely separated by placing the
ionised gas in a suitably disposed field. In the neighbourhood of a
negative disk there remain hardly any but positive ions, and against a
positive disk none but negative; and in effecting a separation of this
kind, it will be noticed that condensation by negative ions is easier
than by the positive.
It is, consequently, possible to cause condensation on negative
centres only, and to study separately the phenomena produced by the
two kinds of ions. It can thus be verified that they really bear
charges equal in absolute value, and these charges can even be
estimated, since we already know the number of drops. This estimate
can be made, for example, by comparing the speed of the fall of a mist
in fields of different values, or, as did J.J. Thomson, by measuring
the total quantity of electricity liberated throughout the gas.
At the degree of approximation which such experiments imply, we find
that the charge of a drop, and consequently the charge borne by an
ion, is sensibly 3.4 x 10^{-10} electrostatic or 1.1 x 10^{-20}
electromagnetic units. This charge is very near that which the study
of the phenomena of ordinary electrolysis leads us to attribute to a
univalent atom produced by electrolytic dissociation.
Such a coincidence is evidently very striking; but it will not be the
only one, for whatever phenomenon be studied it will always appear
that the smallest charge we can conceive as isolated is that
mentioned. We are, in fact, in presence of a natural unit, or, if you
will, of an atom of electricity.
We must, however, guard against the belief that the gaseous ion is
identical with the electrolytic ion. Sensible differences between
those are immediately apparent, and still greater ones will be
discovered on closer examination.
As M. Perrin has shown, the ionisation produced by the X-rays in no
way depends on the chemical composition of the gas; and whether we
take a volume of gaseous hydrochloric acid or a mixture of hydrogen
and chlorine in the same condition, all the results will be identical:
and chemical affinities play no part here.
We can also obtain other information regarding ions: we can ascertain,
for instance, their velocities, and also get an idea of their order of
magnitude.
By treating the speeds possessed by the liberated charges as
components of the known speed of a gaseous current, Mr Zeleny measures
the mobilities, that is to say, the speeds acquired by the positive
and negative charges in a field equal to the electrostatic unit. He
has thus found that these mobilities are different, and that they
vary, for example, between 400 and 200 centimetres per second for the
two charges in dry gases, the positive being less mobile than the
negative ions, which suggests the idea that they are of greater
mass.[30]
[Footnote 30: A full account of these experiments, which were executed
at the Cavendish Laboratory, is to be found in _Philosophical
Transactions_, A., vol. cxcv. (1901), pp. 193 et seq.--ED.]
M. Langevin, who has made himself the eloquent apostle of the new
doctrines in France, and has done much to make them understood and
admitted, has personally undertaken experiments analogous to those of
M. Zeleny, but much more complete. He has studied in a very ingenious
manner, not only the mobilities, but also the law of recombination
which regulates the spontaneous return of the gas to its normal state.
He has determined experimentally the relation of the number of
recombinations to the number of collisions between two ions of
contrary sign, by studying the variation produced by a change in the
value of the field, in the quantity of electricity which can be
collected in the gas separating two parallel metallic plates, after
the passage through it for a very short time of the Röntgen rays
emitted during one discharge of a Crookes tube. If the image of the
ions is indeed conformable to reality, this relation must evidently
always be smaller than unity, and must tend towards this value when
the mobility of the ions diminishes, that is to say, when the pressure
of the gas increases. The results obtained are in perfect accord with
this anticipation.
On the other hand, M. Langevin has succeeded, by following the
displacement of the ions between the parallel plates after the
ionisation produced by the radiation, in determining the absolute
values of the mobilities with great precision, and has thus clearly
placed in evidence the irregularity of the mobilities of the positive
and negative ions respectively. Their mass can be calculated when we
know, through experiments of this kind, the speed of the ions in a
given field, and on the other hand--as we can now estimate their
electric charge--the force which moves them. They evidently progress
more slowly the larger they are; and in the viscous medium constituted
by the gas, the displacement is effected at a speed sensibly
proportional to the motive power.
At the ordinary temperature these masses are relatively considerable,
and are greater for the positive than for the negative ions, that is
to say, they are about the order of some ten molecules. The ions,
therefore, seem to be formed by an agglomeration of neutral molecules
maintained round an electrified centre by electrostatic attraction. If
the temperature rises, the thermal agitation will become great enough
to prevent the molecules from remaining linked to the centre. By
measurements effected on the gases of flames, we arrive at very
different values of the masses from those found for ordinary ions, and
above all, very different ones for ions of contrary sign. The negative
ions have much more considerable velocities than the positive ones.
The latter also seem to be of the same size as atoms; and the
first-named must, consequently, be considered as very much smaller,
and probably about a thousand times less.
Thus, for the first time in science, the idea appears that the atom is
not the smallest fraction of matter to be considered. Fragments a
thousand times smaller may exist which possess, however, a negative
charge. These are the electrons, which other considerations will again
bring to our notice.
§ 3. HOW IONS ARE PRODUCED
It is very seldom that a gaseous mass does not contain a few ions.
They may have been formed from many causes, for although to give
precision to our studies, and to deal with a well ascertained case, I
mentioned only ionisation by the X rays in the first instance, I ought
not to give the impression that the phenomenon is confined to these
rays. It is, on the contrary, very general, and ionisation is just as
well produced by the cathode rays, by the radiations emitted by
radio-active bodies, by the ultra-violet rays, by heating to a high
temperature, by certain chemical actions, and finally by the impact of
the ions already existing in neutral molecules.
Of late years these new questions have been the object of a multitude
of researches, and if it has not always been possible to avoid some
confusion, yet certain general conclusions may be drawn. The
ionisation by flames, in particular, is fairly well known. For it to
be produced spontaneously, it would appear that there must exist
simultaneously a rather high temperature and a chemical action in the
gas. According to M. Moreau, the ionisation is very marked when the
flame contains the vapour of the salt of an alkali or of an alkaline
earth, but much less so when it contains that of other salts.
Arrhenius, Mr C.T.R. Wilson, and M. Moreau, have studied all the
circumstances of the phenomenon; and it seems indeed that there is a
somewhat close analogy between what first occurs in the saline vapours
and that which is noted in liquid electrolytes. There should be
produced, as soon as a certain temperature is reached, a dissociation
of the saline molecule; and, as M. Moreau has shown in a series of
very well conducted researches, the ions formed at about 100°C. seem
constituted by an electrified centre of the size of a gas molecule,
surrounded by some ten layers of other molecules. We are thus dealing
with rather large ions, but according to Mr Wilson, this condensation
phenomenon does not affect the number of ions produced by
dissociation. In proportion as the temperature rises, the molecules
condensed round the nucleus disappear, and, as in all other
circumstances, the negative ion tends to become an electron, while the
positive ion continues the size of an atom.
In other cases, ions are found still larger than those of saline
vapours, as, for example, those produced by phosphorus. It has long
been known that air in the neighbourhood of phosphorus becomes a
conductor, and the fact, pointed out as far back as 1885 by Matteucci,
has been well studied by various experimenters, by MM. Elster and
Geitel in 1890, for instance. On the other hand, in 1893 Mr Barus
established that the approach of a stick of phosphorus brings about
the condensation of water vapour, and we really have before us,
therefore, in this instance, an ionisation. M. Bloch has succeeded in
disentangling the phenomena, which are here very complex, and in
showing that the ions produced are of considerable dimensions; for
their speed in the same conditions is on the average a thousand times
less than that of ions due to the X rays. M. Bloch has established
also that the conductivity of recently-prepared gases, already studied
by several authors, was analogous to that which is produced by
phosphorus, and that it is intimately connected with the presence of
the very tenuous solid or liquid dust which these gases carry with
them, while the ions are of the same order of magnitude. These large
ions exist, moreover, in small quantities in the atmosphere; and M.
Langevin lately succeeded in revealing their presence.
It may happen, and this not without singularly complicating matters,
that the ions which were in the midst of material molecules produce,
as the result of collisions, new divisions in these last. Other ions
are thus born, and this production is in part compensated for by
recombinations between ions of opposite signs. The impacts will be
more active in the event of the gas being placed in a field of force
and of the pressure being slight, the speed attained being then
greater and allowing the active force to reach a high value. The
energy necessary for the production of an ion is, in fact, according
to Professor Rutherford and Professor Stark, something considerable,
and it much exceeds the analogous force in electrolytic decomposition.
It is therefore in tubes of rarefied gas that this ionisation by
impact will be particularly felt. This gives us the reason for the
aspect presented by Geissler tubes. Generally, in the case of
discharges, new ions produced by the molecules struck come to add
themselves to the electrons produced, as will be seen, by the cathode.
A full discussion has led to the interpretation of all the known
facts, and to our understanding, for instance, why there exist bright
or dark spaces in certain regions of the tube. M. Pellat, in
particular, has given some very fine examples of this concordance
between the theory and the facts he has skilfully observed.
In all the circumstances, then, in which ions appear, their formation
has doubtless been provoked by a mechanism analogous to that of the
shock. The X rays, if they are attributable to sudden variations in
the ether--that is to say, a variation of the two vectors of Hertz--
themselves produce within the atom a kind of electric impulse which
breaks it into two electrified fragments; _i.e._ the positive centre,
the size of the molecule itself, and the negative centre, constituted
by an electron a thousand times smaller. Round these two centres, at
the ordinary temperature, are agglomerated by attraction other
molecules, and in this manner the ions whose properties have just been
studied are formed.
§ 4. ELECTRONS IN METALS
The success of the ionic hypothesis as an interpretation of the
conductivity of electrolytes and gases has suggested the desire to try
if a similar hypothesis can represent the ordinary conductivity of
metals. We are thus led to conceptions which at first sight seem
audacious because they are contrary to our habits of mind. They must
not, however, be rejected on that account. Electrolytic dissociation
at first certainly appeared at least as strange; yet it has ended by
forcing itself upon us, and we could, at the present day, hardly
dispense with the image it presents to us.
The idea that the conductivity of metals is not essentially different
from that of electrolytic liquids or gases, in the sense that the
passage of the current is connected with the transport of small
electrified particles, is already of old date. It was enunciated by W.
Weber, and afterwards developed by Giese, but has only obtained its
true scope through the effect of recent discoveries. It was the
researches of Riecke, later, of Drude, and, above all, those of J.J.
Thomson, which have allowed it to assume an acceptable form. All these
attempts are connected however with the general theory of Lorentz,
which we will examine later.
It will be admitted that metallic atoms can, like the saline molecule
in a solution, partially dissociate themselves. Electrons, very much
smaller than atoms, can move through the structure, considerable to
them, which is constituted by the atom from which they have just been
detached. They may be compared to the molecules of a gas which is
enclosed in a porous body. In ordinary conditions, notwithstanding the
great speed with which they are animated, they are unable to travel
long distances, because they quickly find their road barred by a
material atom. They have to undergo innumerable impacts, which throw
them first in one direction and then in another. The passage of a
current is a sort of flow of these electrons in a determined
direction. This electric flow brings, however, no modification to the
material medium traversed, since every electron which disappears at
any point is replaced by another which appears at once, and in all
metals the electrons are identical.
This hypothesis leads us to anticipate certain facts which experience
confirms. Thus J.J. Thomson shows that if, in certain conditions, a
conductor is placed in a magnetic field, the ions have to describe an
epicycloid, and their journey is thus lengthened, while the electric
resistance must increase. If the field is in the direction of the
displacement, they describe helices round the lines of force and the
resistance is again augmented, but in different proportions. Various
experimenters have noted phenomena of this kind in different
substances.
For a long time it has been noticed that a relation exists between the
calorific and the electric conductivity; the relation of these two
conductivities is sensibly the same for all metals. The modern theory
tends to show simply that it must indeed be so. Calorific conductivity
is due, in fact, to an exchange of electrons between the hot and the
cold regions, the heated electrons having the greater velocity, and
consequently the more considerable energy. The calorific exchanges
then obey laws similar to those which govern electric exchanges; and
calculation even leads to the exact values which the measurements have
given.[31]
[Footnote 31: The whole of this argument is brilliantly set forth by
Professor Lorentz in a lecture delivered to the Electrotechnikerverein
at Berlin in December 1904, and reprinted, with additions, in the
_Archives Néerlandaises_ of 1906.--ED.]
In the same way Professor Hesehus has explained how contact
electrification is produced, by the tendency of bodies to equalise
their superficial properties by means of a transport of electrons, and
Mr Jeans has shown that we should discover the existence of the
well-known laws of distribution over conducting bodies in electrostatic
equilibrium. A metal can, in fact, be electrified, that is to say, may
possess an excess of positive or negative electrons which cannot
easily leave it in ordinary conditions. To cause them to do so would
need an appreciable amount of work, on account of the enormous
difference of the specific inductive capacities of the metal and of
the insulating medium in which it is plunged.
Electrons, however, which, on arriving at the surface of the metal,
possessed a kinetic energy superior to this work, might be shot forth
and would be disengaged as a vapour escapes from a liquid. Now, the
number of these rapid electrons, at first very slight, increases,
according to the kinetic theory, when the temperature rises, and
therefore we must reckon that a wire, on being heated, gives out
electrons, that is to say, loses negative electricity and sends into
the surrounding media electrified centres capable of producing the
phenomena of ionisation. Edison, in 1884, showed that from the
filament of an incandescent lamp there escaped negative electric
charges. Since then, Richardson and J.J. Thomson have examined
analogous phenomena. This emission is a very general phenomenon which,
no doubt, plays a considerable part in cosmic physics. Professor
Arrhenius explains, for instance, the polar auroras by the action of
similar corpuscules emitted by the sun.
In other phenomena we seem indeed to be confronted by an emission, not
of negative electrons, but of positive ions. Thus, when a wire is
heated, not _in vacuo_, but in a gas, this wire begins to electrify
neighbouring bodies positively. J.J. Thomson has measured the mass of
these positive ions and finds it considerable, i.e. about 150 times
that of an atom of hydrogen. Some are even larger, and constitute
almost a real grain of dust. We here doubtless meet with the phenomena
of disaggregation undergone by metals at a red heat.