propulsion of the ball was given by the arm of the man who pumped in
the air. In ordinary firearms, the condensed mass of air which propels
the bullet is obtained in a totally different manner, namely, by the
combustion of the powder. Gunpowder is transformed by combustion for
the most part into gaseous products, which endeavor to occupy a much
larger space than that previously taken by the volume of the powder.
Thus, you see, that, by the use of gunpowder, the work which the human
arm must accomplish in the case of the air-gun is spared.
In the mightiest of our machines, the steam engine, it is a strongly
compressed aeriform body, water, vapour, which, by its effort to
expand, sets the machine in motion. Here, also, we do not condense the
steam by means of an external mechanical force, but by communicating
heat to a mass of water in a closed boiler, we change this water
into steam, which, in consequence of the limits of the space, is
developed under strong pressure. In this case, therefore, it is the
heat communicated which generates the mechanical force. The heat thus
necessary for the machine we might obtain in many ways; the ordinary
method is to procure it from the combustion of coal.
Combustion is a chemical process. A particular constituent of our
atmosphere, oxygen, possesses a strong force of attraction, or, as
it is named in chemistry, a strong affinity for the constituents of
the combustible body, which affinity, however, in most cases, can
only exert itself at high temperatures. As soon as a portion of the
combustible body, for example, the coal, is sufficiently heated,
the carbon unites itself with great violence to the oxygen of the
atmosphere and forms a peculiar gas, carbonic acid, the same which we
see foaming from beer and champagne. By this combination, light and
heat are generated; heat is generally developed by any combination
of two bodies of strong affinity for each other; and when the heat
is intense enough, light appears. Hence, in the steam engine, it is
chemical processes and chemical forces which produce the astonishing
work of these machines. In like manner the combustion of gunpowder is a
chemical process which, in the barrel of the gun, communicates living
force to the bullet.
While now the steam engine develops for us mechanical work out of
heat, we can conversely generate heat by mechanical forces. A skilful
blacksmith can render an iron wedge red hot by hammering. The axes of
our carriages must be protected, by careful greasing, from ignition
through friction. Even lately this property has been applied on a large
scale. In some factories, where a surplus of water power is at hand,
this surplus is applied to cause a strong iron plate to rotate swiftly
upon another, so that they become strongly heated by friction. The heat
so obtained warms the room, and thus a stove without fuel is provided.
Now, could not the heat generated by the plates be applied to a small
steam engine, which in its turn should be able to keep the rubbing
plates in motion? The perpetual motion would thus be at length found.
This question might be asked, and could not be decided by the older
mathematico-mechanical investigations. I will remark, beforehand, that
the general law which I will lay before you answers the question in the
negative.
By a similar plan, however, a speculative American set some time ago
the industrial world of Europe in excitement. The magneto-electric
machines often made use of in the case of rheumatic disorders are well
known to the public. By imparting a swift rotation to the magnet of
such a machine, we obtain powerful currents of electricity. If those
be conducted through water, the latter will be reduced into its two
components, oxygen and hydrogen. By the combustion of hydrogen, water
is again generated. If this combustion takes place, not in atmospheric
air, of which oxygen only constitutes a fifth part, but in pure oxygen,
and if a bit of chalk be placed in the flame, the chalk will be raised
to a white heat, and give us the sun-like Drummond’s light. At the same
time, the flame develops a considerable quantity of heat. Our American
proposed to utilize in this way the gases obtained from electrolytic
decomposition, and asserted that by the combustion a sufficient amount
of heat was generated to keep a small steam engine in action, which
again drove his magneto-electric machine, decomposed the water, and
thus continually prepared its own fuel. This would certainly have been
the most splendid of all discoveries; a perpetual motion which, besides
the force which kept it going, generated light like the sun, and
warmed all around it. The matter was by no means badly cogitated. Each
practical step in the affair was known to be possible; but those who at
that time were acquainted with the physical investigations which bear
upon this subject could have affirmed, on first hearing the report,
that the matter was to be numbered among the numerous stories of the
fable-rich America; and indeed a fable it remained.
It is not necessary to multiply examples further. You will infer from
those given, in what immediate connection heat, electricity, magnetism,
light, and chemical affinity, stand with mechanical forces.
Starting from each of these different manifestations of natural forces
we can set every other in motion, for the most part not in one way
merely, but in many ways. It is here as with the weaver’s web--
Where a step stirs a thousand threads
The shuttles shoot from side to side,
The fibres flow unseen,
And one shock strikes a thousand combinations.
Now it is clear that if by any means we could succeed, as the above
American professed to have done, by mechanical forces, to excite
chemical, electrical, or other natural processes, which, by any circuit
whatever, and without altering permanently the active masses in the
machine, could produce mechanical force in greater quantity than that
at first applied, a portion of the work thus gained might be made use
of to keep the machine in motion, while the rest of the work might be
applied to any other purpose whatever. The problem was, to find in
the complicated net of reciprocal actions, a track through chemical,
electrical, magnetical, and thermic processes, back to mechanical
actions, which might be followed with a final gain of mechanical work;
thus would the perpetual motion be found.
But, warned by the futility of former experiments, the public had
become wiser. On the whole, people did not seek much after combinations
which promised to furnish a perpetual motion, but the question was
inverted. It was no more asked, how can I make use of the known and
unknown relations of natural forces so as to construct a perpetual
motion? but it was asked, if a perpetual motion be impossible, what are
the relations which must subsist between natural forces? Everything
was gained by this inversion of the question. The relations of natural
forces rendered necessary by the above assumption, might be easily
and completely stated. It was found that all known relations of force
harmonize with the consequences of that assumption, and a series of
unknown relations were discovered at the same time, the correctness of
which remained to be proved. If a single one of them could be proved
false, then a perpetual motion would be possible.
The first who endeavoured to travel this way was a Frenchman, named
Carnot, in the year 1824. In spite of a too limited conception of
his subject, and an incorrect view as to the nature of heat, which
led him to some erroneous conclusions, his experiment was not quite
unsuccessful. He discovered a law which now bears his name, and to
which I will return further on.
His labors remained for a long time without notice, and it was not
till eighteen years afterwards, that is, in 1842, that different
investigators in different countries, and independent of Carnot, laid
hold of the same thought.
The first who saw truly the general law here referred to, and expressed
it correctly, was a German physician, J. R. Mayer, of Heilbronn,
in the year 1842. A little later, in 1843, a Dane, named Colding,
presented a memoir to the Academy of Copenhagen, in which the same law
found utterance, and some experiments were described for its further
corroboration. In England, Joule began about the same time to make
experiments having reference to the same subject. We often find, in the
case of questions to the solution of which the development of science
points, that several heads, quite independent of each other, generate
exactly the same series of reflections.
I myself, without being acquainted with either Mayer or Colding, and
having first made the acquaintance of Joule’s experiments at the end of
my investigation, followed the same path. I endeavoured to ascertain
all the relations between the different natural processes, which
followed from our regarding them from the above point of view. My
inquiry was made public in 1847, in a small pamphlet bearing the title,
“On the Conservation of Force.”
Since that time the interest of the scientific public for this subject
has gradually augmented. A great number of the essential consequences
of the above manner of viewing the subject, the proof of which was
wanting when the first theoretic notions were published, have since
been confirmed by experiment, particularly by those of Joule; and
during the last year the most eminent physicist of France, Regnault,
has adopted the new mode regarding the question, and by fresh
investigations on the specific heat of gases has contributed much to
its support. For some important consequences the experimental proof
is still wanting, but the number of confirmations is so predominant,
that I have not deemed it too early to bring the subject before even a
non-scientific audience.
How the question has been decided you may already infer from what has
been stated. In the series of natural processes there is no circuit
to be found, by which mechanical force can be gained without a
corresponding consumption. The perpetual motion remains impossible. Our
reflections, however, gain thereby a higher interest.
We have thus far regarded the development of force by natural
processes, only in its relation to its usefulness to man, as mechanical
force. You now see that we have arrived at a general law, which holds
good wholly independent of the application which man makes of natural
forces; we must therefore make the expression of our new law correspond
to this more general significance. It is in the first place clear, that
the work which, by any natural process whatever, is performed under
favourable conditions by a machine, and which may be measured in the
way already indicated, may be used as a measure of force common to
all. Further, the important question arises, “If the quantity of force
cannot be augmented except by corresponding consumption, can it be
diminished or lost?” For the purpose of our machines it certainly can,
if we neglect the opportunity to convert natural processes to use, but
as investigation has proved, not for a nature as a whole.
In the collision and friction of bodies against each other, the
mechanics of former years assumed simply that living force was lost.
But I have already stated that each collision and each act of friction
generates heat; and, moreover, Joule has established by experiment
the important law that for every foot-pound of force which is lost a
definite quantity of heat is always generated, and that when work is
performed by the consumption of heat, for each foot-pound thus gained
a definite quantity of heat disappears. The quantity of heat necessary
to raise the temperature of a pound of water a degree of the centigrade
thermometer, corresponds to a mechanical force by which a pound weight
would be raised to the height of 1350 feet; we name this quantity the
mechanical equivalent of heat. I may mention here that these facts
conduct of necessity to the conclusion, that the heat is not, as was
formerly imagined, a fine imponderable substance, but that, like
light, it is a peculiar shivering motion of the ultimate particles of
bodies. In collision and friction, according to this manner of viewing
the subject, the motion of the mass of a body which is apparently lost
is converted into a motion of the ultimate particles of the body; and
conversely, when mechanical force is generated by heat, the motion of
the ultimate particles is converted into a motion of the mass.
Chemical combinations generate heat, and the quantity of this heat is
totally independent of the time and steps through which the combination
has been effected, provided that other actions are not at the same
time brought into play. If, however, mechanical work is at the same
time accomplished, as in the case of the steam engine, we obtain as
much less heat as is equivalent to this work. The quantity of work
produced by chemical force is in general very great. A pound of the
purest coal gives when burnt, sufficient heat to raise the temperature
of 8086 pounds of water one degree of the centigrade thermometer; from
this we can calculate that the magnitude of the chemical force of
attraction between the particles of a pound of coal and the quantity
of oxygen that corresponds to it is capable of lifting a weight of one
hundred pounds to a height of twenty miles. Unfortunately, in our steam
engines, we have hitherto been able to gain only the smallest portion
of this work; the greater part is lost in the shape of heat. The best
expansive engines give back as mechanical work only eighteen per cent.
of the heat generated by the fuel.
From a similar investigation of all the other known physical and
chemical processes, we arrive at the conclusion that Nature as a whole
possesses a store of force which cannot in any way be either increased
or diminished. And that, therefore, the quantity of force in Nature is
just as eternal and unalterable as the quantity of matter. Expressed
in this form, I have named the general law “The Principle of the
Conservation of Force.”
We cannot create mechanical force, but we may help ourselves from the
general store-house of Nature. The brook and the wind, which drive our
mills, the forest and the coal-bed, which supply our steam engines and
warm our rooms, are to us the bearers of a small portion of the great
natural supply which we draw upon for our purposes, and the actions of
which we can apply as we think fit. The possessor of a mill claims the
gravity of the descending rivulet, or the living force of the moving
wind, as his possession. These portions of the store of Nature are what
give his property its chief value.
Further, from the fact that no portion of force can be absolutely lost,
it does not follow that a portion may not be inapplicable to human
purposes. In this respect the inferences drawn by William Thomson from
the law of Carnot are of importance. This law, which was discovered
by Carnot during his endeavours to ascertain the relations between
heat and mechanical force, which, however, by no means belongs to the
necessary consequences of the conservation of force, and which Clausius
was the first to modify in such a manner that it no longer contradicted
the above general law, expresses a certain relation between the
compressibility, the capacity for heat, and the expansion by heat of
all bodies. It is not yet considered as actually proved, but some
remarkable deductions having been drawn from it, and afterwards proved
to be facts by experiment, it has attained thereby a great degree
of probability. Besides the mathematical form in which the law was
first expressed by Carnot, we can give it the following more general
expression:--“Only, when heat passes from a warmer to a colder body,
and even then only partially, can it be converted into mechanical work.”
The heat of a body which we cannot cool further, cannot be changed
into another form of force; into the electric or chemical force, for
example. Thus, in our steam engines, we convert a portion of the heat
of the glowing coal into work, by permitting it to pass to the less
warm water of the boiler. If, however, all the bodies in nature had
the same temperature, it would be impossible to convert any portion of
their heat into mechanical work. According to this, we can divide the
total force store of the universe into two parts, one of which is heat,
and must continue to be such; the other, to which a portion of the heat
of the warmer bodies, and the total supply of chemical, mechanical,
electrical, and magnetical forces belong, is capable of the most varied
changes of form, and constitutes the whole wealth of change which takes
place in nature.
But the heat of the warmer bodies strives perpetually to pass to
bodies less warm by radiation and conduction, and thus to establish
an equilibrium of temperature. At each motion of a terrestrial body,
a portion of mechanical force passes by friction or collision into
heat, of which only a part can be converted back again into mechanical
force. This is also generally the case in every electrical and chemical
process. From this, it follows that the first portion of the store of
force, the unchangeable heat, is augmented by every natural process,
while the second portion, mechanical, electrical, and chemical force,
must be diminished; so that if the universe be delivered over to
the undisturbed action of its physical processes, all force will
finally pass into the form of heat, and all heat come into a state of
equilibrium. Then all possibility of a further change would be at an
end, and the complete cessation of all natural processes must set in.
The life of men, animals, and plants, could not of course continue if
the sun had lost its high temperature, and with it his light,--if all
the components of the earth’s surface had closed those combinations
which their affinities demand. In short, the universe from that time
forward would be condemned to a state of eternal rest.
These consequences of the law of Carnot are, of course, only valid,
provided that the law, when sufficiently tested, proves to be
universally correct. In the mean time there is little prospect of the
law being proved incorrect. At all events we must admire the sagacity
of Thomson, who, in the letters of a long known little mathematical
formula, which only speaks of the heat, volume, and pressure of bodies,
was able to discern consequences which threatened the universe, though
certainly after an infinite period of time, with eternal death.
I have already given you notice that our path lay through a thorny and
unrefreshing field of mathematico-mechanical developments. We have
now left this portion of our road behind us. The general principle
which I have sought to lay before you has conducted us to a point from
which our view is a wide one, and aided by this principle, we can now
at pleasure regard this or the other side of the surrounding world,
according as our interest in the matter leads us. A glance into the
narrow laboratory of the physicist, with its small appliances and
complicated abstractions, will not be so attractive as a glance at the
wide heaven above us, the clouds, the rivers, the woods, and the living
beings around us. While regarding the laws which have been deduced
from the physical processes of terrestrial bodies, as applicable also
to the heavenly bodies, let me remind you that the same force which,
acting at the earth’s surface, we call gravity (_Schwere_), acts
as gravitation in the celestial spaces, and also manifests its power in
the motion of the immeasurably distant double stars which are governed
by exactly the same laws as those subsisting between the earth and
moon; that, therefore, the light and heat of terrestrial bodies do not
in any way differ essentially from those of the sun, or of the most
distant fixed star; that the meteoric stones which sometimes fall from
external space upon the earth are composed of exactly the same simple
chemical substances as those with which we are acquainted. We need,
therefore, feel no scruple in granting that general laws to which all
terrestrial natural processes are subject, are also valid for other
bodies than the earth. We will, therefore, make use of our law to
glance over the household of the universe with respect to the store of
force, capable of action, which it possesses.
A number of singular peculiarities in the structure of our planetary
system indicate that it was once a connected mass with a uniform
motion of rotation. Without such an assumption, it is impossible to
explain why all the planets move in the same direction round the sun,
why they all rotate in the same direction round their axes, why the
planes of their orbits, and those of their satellites and rings all
nearly coincide, why all their orbits differ but little from circles;
and much besides. From these remaining indications of a former state,
astronomers have shaped an hypothesis regarding the formation of our
planetary system, which, although from the nature of the case it must
ever remain an hypothesis, still in its special traits is so well
supported by analogy, that it certainly deserves our attention. It
was Kant who, feeling great interest in the physical description of
the earth and the planetary system, undertook the labour of studying
the works of Newton, and as an evidence of the depth to which he had
penetrated into the fundamental ideas of Newton, seized the notion
that the same attractive force of all ponderable matter which now
supports the motion of the planets, must also aforetime have been able
to form from matter loosely scattered in space the planetary system.
Afterwards, and independent of Kant, Laplace, the great author of the
_Mecanique Celeste_, laid hold of the same thought, and introduced
it among astronomers.
The commencement of our planetary system, including the sun, must,
according to this, be regarded as an immense nebulous mass which filled
the portion of space which is now occupied by our system, far beyond
the limits of Neptune, our most distant planet. Even now we perhaps
see similar masses in the distant regions of the firmament, as patches
of nebulæ, and nebulous stars; within our system also, comets, the
zodiacal light, the corona of the sun during a total eclipse, exhibit
remnants of a nebulous substance, which is so thin that the light
of the stars passes through it unenfeebled and unrefracted. If we
calculate the density of the mass of our planetary system, according to
the above assumption, for the time when it was a nebulous sphere, which
reached to the path of the outmost planet, we should find that it would
require several cubic miles of such matter to weigh a single grain.
The general attractive force of all matter must, however, impel these
masses to each other, and to condense, so that the nebulous sphere
became incessantly smaller, by which, according to mechanical laws, a
motion of rotation originally slow, and the existence of which must be
assumed, would gradually become quicker and quicker. By the centrifugal
force which must act most energetically in the neighbourhood of the
equator of the nebulous sphere, masses could from time to time be torn
away, which afterwards would continue their courses separate from the
main mass, forming themselves into single planets, or, similar to the
great original sphere, into planets with satellites and rings, until
finally the principal mass condensed itself into the sun. With regard
to the origin of heat and light, this view gives us no information.
When the nebulous chaos first separated itself from other fixed star
masses, it must not only have contained all kinds of matter which was
to constitute the future planetary system, but also, in accordance
with our new law, the whole store of force which at one time must
unfold therein its wealth of actions. Indeed in this respect an immense
dower was bestowed in the shape of the general attraction of all the
particles for each other. This force, which on the earth exerts itself
as gravity, acts in the heavenly spaces as gravitation. As terrestrial
gravity when it draws a weight downwards performs work and generates
_vis viva_, so also the heavenly bodies do the same when they draw
two portions of matter from distant regions of space towards each other.
The chemical forces must have been also present, ready to act; but as
these forces can only come into operation by the most intimate contact
of the different masses, condensation must have taken place before the
play of chemical forces began.
Whether a still further supply of force in the shape of heat was
present at the commencement we do not know. At all events, by aid of
the law of the equivalence of heat and work, we find in the mechanical
forces, existing at the time to which we refer, such a rich source of
heat and light, that there is no necessity whatever to take refuge in
the idea of a store of these forces originally existing. When through
condensation of the masses their particles came into collision,
and clung to each other, the _vis viva_ of their motion would
be thereby annihilated, and must reappear as heat. Already in old
theories, it has been calculated that cosmical masses must generate
heat by their collision, but it was far from anybody’s thought to make
even a guess at the amount of heat to be generated in this way. At
present we can give definite numerical values with certainty.
Let us make this addition to our assumption; that, at the commencement,
the density of the nebulous matter was a vanishing quantity, as
compared with the present density of the sun and planets; we can then
calculate how much work has been performed by the condensation; we can
further calculate how much of this work still exists in the form of
mechanical force, as attraction of the planets towards the sun, and as
_vis viva_ of their motion, and find by this how much of the force
has been converted into heat.
The result of this calculation is, that only about the 454th part
of the original mechanical force remains as such, and that the
remainder, converted into heat, would be sufficient to raise a mass
of water equal to the sun and planets taken together, not less than
twenty-eight millions of degrees of the centigrade scale. For the
sake of comparison, I will mention that the highest temperature which
we can produce by the oxyhydrogen blowpipe, which is sufficient to
fuse and vaporize even platina, and which but few bodies can endure,
is estimated at about two thousand centigrade degrees. Of the action
of a temperature of twenty-eight millions of such degrees we can
form no notion. If the mass of our entire system were pure coal,
by the combustion of the whole of it only the 3500th part of the
above quantity would be generated. This is also clear, that such a
development of heat must have presented the greatest obstacle to the
speedy union of the masses, that the larger part of the heat must have
been diffused by radiation into space, before the masses could form
bodies possessing the present density of the sun and planets, and that
these bodies must once have been in a state of fiery fluidity. This
notion is corroborated by the geological phenomena of our planet; and
with regard to the other planetary bodies, the flattened form of the
sphere, which is the form of equilibrium of a fluid mass, is indicative
of a former state of fluidity. If I thus permit an immense quantity of
heat to disappear without compensation from our system, the principle
of the conservation of force is not thereby invaded. Certainly for our
planet it is lost, but not for the universe. It has proceeded outwards,
and daily proceeds outwards into infinite space; and we know not
whether the medium which transmits the undulations of light and heat
possesses an end where the rays must return, or whether they eternally
pursue their way through infinitude.
The store of force at present possessed by our system, is also
equivalent to immense quantities of heat. If our earth were by a sudden
shock brought to rest on her orbit--which is not to be feared in the
existing arrangements of our system--by such a shock a quantity of heat
would be generated equal to that produced by the combustion of fourteen
such earths of solid coal. Making the most unfavourable assumption as
to its capacity for heat, that is, placing it equal to that of water,
the mass of the earth would thereby be heated 11,200 degrees; it would
therefore be quite fused and for the most part reduced to vapour. If,
then, the earth, after having been thus brought to rest, should fall
into the sun, which of course would be the case, the quantity of heat
developed by the shock would be four hundred times greater.
Even now, from time to time, such a process is repeated on a small
scale. There can hardly be a doubt that meteors, fire-balls, and
meteoric stones are masses which belong to the universe, and before
coming into the domain of our earth, moved like the planets round the
sun. Only when they enter our atmosphere do they become visible and
fall sometimes to the earth. In order to explain the emission of light
by these bodies, and the fact that for some time after their descent
they are very hot, the friction was long ago thought of which they
experience in passing through the air. We can now calculate that a
velocity of 3,000 feet a second, supposing the whole of the friction
to be expended in heating the solid mass, would raise a piece of
meteoric iron 1,000° C. in temperature, or, in other words, to a vivid
red heat. Now the average velocity of the meteors seems to be thirty or
forty times the above amount. To compensate this, however, the greater
portion of the heat is, doubtless, carried away by the condensed mass
of air which the meteor drives before it. It is known that bright
meteors generally leave a luminous trail behind them, which probably
consists of several portions of the red-hot surfaces. Meteoric masses
which fall to the earth often burst with a violent explosion, which
may be regarded as a result of the quick heating. The newly-fallen
pieces have been for the most part found hot, but not red-hot, which
is easily explainable by the circumstances, that during the short time
occupied by the meteor in passing through the atmosphere, only a thin,
superficial layer is heated to redness, while but a small quantity of
heat has been able to penetrate to the interior of the mass. For this
reason the red heat can speedily disappear.
Thus has the falling of the meteoric stone, the minute remnant of
processes which seems to have played an important part in the formation
of the heavenly bodies, conducted us to the present time, where we
pass from the darkness of hypothetical views to the brightness of
knowledge. In what we have said, however, all that is hypothetical is
the assumption of Kant and Laplace, that the masses of our system were
once distributed as nebulæ in space.
On account of the rarity of the case, we will still further remark,
in what close coincidence the results of science here stand with the
earlier legends of the human family, and the forebodings of poetic
fancy. The cosmogony of ancient nations generally commences with chaos
and darkness.
Neither is the Mosaic tradition very divergent, particularly when we
remember that that which Moses names heaven is different from the blue
dome above us, and is synonymous with space, and that the unformed
earth, and the waters of the great deep, which were afterwards divided
into waters above the firmament, and waters below the firmament,
resembled the chaotic components of the world.
Our earth bears still the unmistakable traces of its old fiery fluid
condition. The granite formations of her mountains exhibit a structure,
which can only be produced by the crystallization of fused masses.
Investigation still shows that the temperature in mines, and borings,
increases as we descend; and if this increase is uniform, at the depth
of fifty miles, a heat exists sufficient to fuse all our minerals. Even
now our volcanoes project, from time to time, mighty masses of fused
rocks from their interior, as a testimony of the heat which exists
there. But the cooled crust of the earth has already become so thick,
that, as may be shown by calculations of its conductive power, the heat
coming to the surface from within, in comparison with that reaching the
earth from the sun, is exceedingly small, and increases the temperature
of the surface only about one-thirtieth of a degree centigrade; so that
the remnant of the old store of force which is enclosed as heat within
the bowels of the earth, has a sensible influence upon the processes
at the earth’s surface, only through the instrumentality of volcanic
phenomena. These processes owe their power almost wholly to the action
of other heavenly bodies, particularly to the light and heat of the
sun, and partly also, in the case of the tides, to the attraction of
the sun and moon.
Most varied and numerous are the changes which we owe to the light
and heat of the sun. The sun heats our atmosphere irregularly, the
warm rarefied air ascends, while fresh cool air flows from the sides
to supply its place: in this way winds are generated. This action is
most powerful at the equator, the warm air of which incessantly flows
in the upper regions of the atmosphere towards the poles: while just
as persistently, at the earth’s surface, the trade wind carries new
and cool air to the equator. Without the heat of the sun all winds
must, of necessity, cease. Similar currents are produced by the same
cause in the waters of the sea. Their power may be inferred from the
influence which in some cases they exert upon climate. By them the warm
water of the Antilles is carried to the British Isles, and confers upon
them a mild, uniform warmth and rich moisture; while, through similar
causes, the floating ice of the North Pole is carried to the coast
of Newfoundland, and produces cold. Further, by the heat of the sun,
a portion of the water is converted into vapour which rises in the
atmosphere, is condensed to clouds, or falls in rain and snow upon the
earth, collects in the form of springs, brooks, and rivers, and finally
reaches the sea again, after having gnawed the rocks, carried away the
light earth, and thus performed its part in the geologic changes of the
earth; perhaps, besides all this it has driven our water-mill upon its
way. If the heat of the sun were withdrawn, there would remain only a
single motion of water, namely, the tides, which are produced by the
attraction of the sun and moon.
How is it now, with the motions and the work of organic beings? To
the builders of the automata of the last century, men and animals
appeared as clockwork which was never wound up, and created the force
which they exerted out of nothing. They did not know how to establish
a connection between the nutriment consumed and the work generated.
Since, however, we have learned to discern in the steam-engine this
origin of mechanical force, we must inquire whether something similar
does not hold good with regard to men. Indeed, the continuation of
life is dependent on the consumption of nutritive materials: these
are combustible substances, which, after digestion and being passed
into the blood, actually undergo a slow combustion, and finally enter
into almost the same combinations with the oxygen of the atmosphere
that are produced in an open fire. As the quantity of heat generated
by combustion is independent of the duration of the combustion and
the steps in which it occurs, we can calculate from the mass of the
consumed material how much heat, or its equivalent work is thereby
generated in an animal body. Unfortunately, the difficulty of the
experiments is still very great; but within those limits of accuracy
which have been as yet attainable, the experiments show that the heat
generated in the animal body corresponds to the amount which would be
generated by the chemical processes. The animal body therefore does not
differ from the steam-engine, as regards the manner in which it obtains
heat and force, but does differ from it in the manner in which the
force gained is to be made use of. The body is, besides, more limited
than the machine in the choice of its fuel; the latter could be heated
with sugar, with starch-flour, and butter, just as well as with coal
or wood; the animal body must dissolve its materials artificially, and
distribute them through its system; it must, further, perpetually renew
the used-up materials of its organs, and as it cannot itself create
the matter necessary for this, the matter must come from without.
Liebig was the first to point out these various uses of the consumed
nutriment. As material for the perpetual renewal of the body, it seems
that certain definite albuminous substances which appear in plants, and
form the chief mass of the animal body, can alone be used. They form
only a portion of the mass of nutriment taken daily; the remainder,
sugar, starch, fat, are really only materials for warming, and are
perhaps not to be superseded by coal, simply because the latter does
not permit itself to be dissolved.
If, then, the processes in the animal body are not in this respect to
be distinguished from inorganic processes, the question arises, whence
comes the nutriment which constitutes the source of the body’s force?
The answer is, from the vegetable kingdom; for only the material of
plants, or the flesh of plant-eating animals, can be made use of for
food. The animals which live on plants occupy a mean position between
carnivorous animals, in which we reckon man, and vegetables, which
the former could not make use of immediately as nutriment. In hay and
grass the same nutritive substances are present as in meal and flour,
but in less quantity. As, however, the digestive organs of man are not
in a condition to extract the small quantity of the useful from the
great excess of the insoluble, we submit, in the first place, these
substances to the powerful digestion of the ox, permit the nourishment
to store itself in the animal’s body, in order in the end to gain it
for ourselves in a more agreeable and useful form. In answer to our
question, therefore, we are referred to the vegetable world. Now when
what plants take in and what they give out are made the subjects of
investigation, we find that the principal part of the former consists
in the products of combustion which are generated by the animal.
They take the consumed carbon given off in respiration, as carbonic
acid, from the air, the consumed hydrogen as water, the nitrogen in
its simplest and closest combinations as ammonia; and from these
materials, with the assistance of small ingredients which they take
from the soil, they generate anew the compound combustible substances,
albumen, sugar, oil, on which the animal subsists. Here, therefore,
is a circuit which appears to be a perpetual store of force. Plants
prepare fuel and nutriment, animals consume these, burn them slowly
in their lungs, and from the products of combustion the plants again
derive their nutriment. The latter is an eternal source of chemical,
the former of mechanical forces. Would not the combination of both
organic kingdoms produce the perpetual motion? We must not conclude
hastily: further inquiry shows, that plants are capable of producing
combustible substances only when they are under the influence of the
sun. A portion of the sun’s rays exhibits a remarkable relation to
chemical forces,--it can produce and destroy chemical combinations;
and these rays, which for the most part are blue or violet, are called
therefore chemical rays. We make use of their action in the production
of photographs. Here compounds of silver are decomposed at the place
where the sun’s rays strike them. The same rays overpower in the green
leaves of plants the strong chemical affinity of the carbon of the
carbonic acid for oxygen, give back the latter free to the atmosphere,
and accumulate the other, in combination with other bodies, as woody
fibre, starch, oil, or resin. These chemically active rays of the sun
disappear completely as soon as they encounter the green portions of
the plants, and hence it is that in daguerreotype images the green
leaves of plants appear uniformly black. Inasmuch as the light coming
from them does not contain the chemical rays, it is unable to act upon
the silver compounds.
Hence a certain portion of force disappears from the sunlight, while
combustible substances are generated and accumulated in plants; and
we can assume it as very probable, that the former is the cause of
the latter. I must indeed remark, that we are in possession of no
experiments from which we might determine whether the vis viva of the
sun’s rays which have disappeared, corresponds to the chemical forces
accumulated during the same time; and as long as these experiments are
wanting, we cannot regard the stated relation as a certainty. If this
view should prove correct, we derive from it the flattering result,
that all force, by means of which our bodies live and move, finds
its source in the purest sunlight; and hence we are all, in point
of nobility, not behind the race of the great monarch of China, who
heretofore alone called himself Son of the Sun. But it must also be
conceded that our lower fellow-beings, the frog and leech, share the
same ethereal origin, as also the whole vegetable world, and even the
fuel which comes to us from the ages past, as well as the youngest
offspring of the forest with which we heat our stoves and set our
machines in motion.
You see, then, that the immense wealth of ever-changing meteorological,
climatic, geological, and organic processes of our earth are almost
wholly preserved in action by the light and heat-giving rays of the
sun; and you see in this a remarkable example, how Proteus-like the
effects of a single cause, under altered external conditions, may
exhibit itself in nature. Besides these, the earth experiences an
action of another kind from its central luminary, as well as from its
satellite the moon, which exhibits itself in the remarkable phenomenon
of the ebb and flow of the tide.
Each of these bodies excites, by its attraction upon the waters of the
sea, two gigantic waves, which flow in the same direction round the
world, as the attracting bodies themselves apparently do. The two waves
of the moon, on account of her greater nearness, are about three and a
half times as large as those excited by the sun. One of these waves has
its crest on the quarter of the earth’s surface which is turned towards
the moon, the other is at the opposite side. Both these quarters
possess the flow of the tide, while the regions which lie between have
the ebb. Although in the open sea the height of the tide amounts to
only about three feet, and only in certain narrow channels, where the
moving water is squeezed together, rises to thirty feet, the might of
the phenomena is nevertheless manifest from the calculation of Bessel,
according to which a quarter of the earth covered by the sea possesses,
during the flow of the tide, about 25,000 cubic miles of water more
than during the ebb, and that therefore such a mass of water must, in
six and a quarter hours, flow from one quarter of the earth to the
other.
The phenomena of the ebb and flow, as already recognized by Mayer,
combined with the law of the conservation of force, stand in remarkable
connection with the question of the stability of our planetary system.
The mechanical theory of the planetary motions discovered by Newton
teaches, that if a solid body in absolute vacuo, attracted by the sun,
move around him in the same manner as the planets, this motion will
endure unchanged through all eternity.
Now we have actually not only one, but several such planets, which
move around the sun, and by their mutual attraction create little
changes and disturbances in each other’s paths. Nevertheless Laplace,
in his great work, the _Mecanique Celeste_, has proved that in
our planetary system all these disturbances increase and diminish
periodically, and can never exceed certain limits, so that by this
cause the external existence of the planetary system is unendangered.
But I have already named two assumptions which must be made: first,
that the celestial spaces must be absolutely empty; and secondly, that
the sun and planets must be solid bodies. The first is at least the
case as far as astronomical observations reach, for they have never
been able to detect any retardation of the planets, such as would
occur if they moved in a resisting medium. But on a body of less mass,
the comet of Encke, changes are observed of such a nature: this comet
describes ellipses round the sun which are becoming gradually smaller.
If this kind of motion, which certainly corresponds to that through a
resisting medium, be actually due to the existence of such a medium,
a time will come when the comet will strike the sun; and a similar
end threatens all the planets, although after a time, the length of
which baffles our imagination to conceive of it. But even should the
existence of a resisting medium appear doubtful to us, there is no
doubt that the planets are not wholly composed of solid materials which
are inseparably bound together. Signs of the existence of an atmosphere
are observed on the Sun, on Venus, Mars, Jupiter, and Saturn. Signs
of water and ice upon Mars; and our earth has undoubtedly a fluid
portion on its surface, and perhaps a still greater portion of fluid
within it. The motions of the tides, however, produce friction, all
friction destroys _vis viva_, and the loss in this case can only
affect the _vis viva_ of the planetary system. We come thereby to
the unavoidable conclusion, that every tide, although with infinite
slowness, still with certainty, diminishes the store of mechanical
force of the system; and as a consequence of this, the rotation of
the planets in question round their axes must become more slow; they
must therefore approach the sun, or their satellites must approach
them. What length of time must pass before the length of our day is
diminished one second by the action of the tide cannot be calculated,
until the height and time of the tide in all portions of the ocean are
known. This alteration, however, takes place with extreme slowness,
as is known by the consequences which Laplace has deduced from the
observations of Hipparchus, according to which, during a period of
2000 years, the duration of the day has not been shortened by the
one-three-hundredth part of a second. The final consequence would be,
but after millions of years, if in the mean time the ocean did not
become frozen, that one side of the earth would be constantly turned
towards the sun, and enjoy a perpetual day, whereas the opposite side
would be involved in eternal night. Such a position we observe in our
moon with regard to the earth, and also in the case of the satellites
as regards their planets; it is, perhaps, due to the action of the
mighty ebb and flow to which these bodies, in the time of their fiery
fluid condition, were subjected.
I would not have brought forward these conclusions, which again
plunge us in the most distant future, if they were not unavoidable.
Physico-mechanical laws are, as it were, the telescopes of our
spiritual eye, which can penetrate into the deepest night of time, past
and to come.
Another essential question as regards the future of our planetary
system has reference to its future temperature and illumination.
As the internal heat of the earth has but little influence on the
temperature of the surface, the heat of the sun is the only thing which
essentially affects the question. The quantity of heat falling from the
sun during a given time upon a given portion of the earth’s surface
may be measured, and from this it can be calculated how much heat in a
given time is sent out from the entire sun. Such measurements have been
made by the French physicist Pouillet, and it has been found that the
sun gives out a quantity of heat per hour equal to that which a layer
of the densest coal ten feet thick would give out by its combustion;
and hence in a year a quantity equal to the combustion of a layer of
seventeen miles. If this heat were drawn uniformly from the entire mass
of the sun, its temperature would only be diminished thereby one and
one-third of a degree centigrade per year, assuming its capacity for
heat to be equal to that of water. These results can give us an idea of
the magnitude of the emission, in relation to the surface and mass of
the sun; but they cannot inform us whether the sun radiates heat as a
glowing body, which since its formation has its heat accumulated within
it, or whether a new generation of heat by chemical processes takes
place at the sun’s surface. At all events the law of the conservation
of force teaches us that no process analogous to those known at the
surface of the earth, can supply for eternity an inexhaustible amount
of light and heat to the sun. But the same law also teaches that the
store of force at present existing, as heat, or as what may become
heat, is sufficient for an immeasurable time. With regard to the store
of chemical force in the sun, we can form no conjecture, and the
store of heat there existing can only be determined by very uncertain
estimations. If, however, we adopt the very probable view, that the
remarkably small density of so large a body is caused by its high
temperature, and may become greater in time, it may be calculated that
if the diameter of the sun were diminished only the ten-thousandth
part of its present length, by this act a sufficient quantity of heat
would be generated to cover the total emission for 2100 years. Such a
small change besides it would be difficult to detect even by the finest
astronomical observations.
Indeed, from the commencement of the period during which we possess
historic accounts, that is, for a period of about 4000 years, the
temperature of the earth has not sensibly diminished. From these old
ages we have certainly no thermometric observations, but we have
information regarding the distribution of certain cultivated plants,
the vine, the olive tree, which are very sensitive to changes of the
mean annual temperature, and we find that these plants at the present
moment have the same limits of distribution that they had in the times
of Abraham and Homer; from which we may infer backwards the constancy
of the climate.
In opposition to this it has been urged, that here in Prussia the
German knights in former times cultivated the vine, cellared their
own wine and drank it, which is no longer possible. From this the
conclusion has been drawn, that the heat of our climate has diminished
since the time referred to. Against this, however, Dove has cited the
reports of ancient chroniclers, according to which, in some peculiarly
hot years, the Prussian grape possessed somewhat less than its usual
quantity of acid. The fact also speaks not so much for the climate of
the country as for the throats of the German drinkers.
But even though the force store of our planetary system is so immensely
great, that by the incessant emission which has occurred during the
period of human history it has not been sensibly diminished, even
though the length of the time which must flow by, before a sensible
change in the state of our planetary system occurs, is totally
incapable of measurement, still the inexorable laws of mechanics
indicate that this store of force, which can only suffer loss and not
gain, must be finally exhausted. Shall we terrify ourselves by this
thought? Men are in the habit of measuring the greatness and the wisdom
of the universe by the duration and the profit which it promises to
their own race; but the past history of the earth already shows what
an insignificant moment the duration of the existence of our race
upon it constitutes. A Nineveh vessel, a Roman sword awakes in us the
conception of grey antiquity. What the museums of Europe show us of the
remains of Egypt and Assyria we gaze upon with silent astonishment, and
despair of being able to carry our thoughts back to a period so remote.
Still must the human race have existed for ages, and multiplied itself
before the pyramids of Nineveh could have been erected. We estimate the
duration of human history at 6000 years; but immeasurable as this time
may appear to us, what is it in comparison with the time during which
the earth carried successive series of rank plants and mighty animals,
and no men; during which in our neighbourhood the amber-tree bloomed,
and dropped its costly gum on the earth and in the sea; when in
Siberia, Europe and North America groves of tropical palms flourished;
where gigantic lizards, and after them elephants, whose mighty remains
we still find buried in the earth, found a home? Different geologists,
proceeding from different premises, have sought to estimate the
duration of the above creative period, and vary from a million to nine
million years. And the time during which the earth generated organic
beings is again small when we compare it with the ages during which the
world was a ball of fused rocks. For the duration of its cooling from
2000° to 200° centigrade, the experiments of Bishop upon basalt show
that about 350 millions of years would be necessary. And with regard
to the time during which the first nebulous mass condensed into our
planetary system, our most daring conjectures must cease. The history
of man, therefore, is but a short ripple in the ocean of time. For a
much longer series of years than that during which man has already
occupied this world, the existence of the present state of inorganic
nature favourable to the duration of man seems to be secured, so that
for ourselves and for long generations after us, we have nothing
to fear. But the same forces of air and water, and of the volcanic
interior, which produced former geological revolutions, and buried one
series of living forms after another, act still upon the earth’s crust.
They more probably will bring about the last day of the human race than
those distant cosmical alterations of which we have spoken, and perhaps
force us to make way for new and more complete living forms, as the
lizards and the mammoth have given place to us and our fellow-creatures
which now exist.
Thus the thread which was spun in darkness by those who sought a
perpetual motion has conducted us to a universal law of nature, which
radiates light into the distant nights of the beginning and of the
end of the history of the universe. To our own race it permits a long
but not an endless existence; it threatens it with a day of judgment,
the dawn of which is still happily obscured. As each of us singly
must endure the thought of his death, the race must endure the same.
But above the forms of life gone by, the human race has higher moral
problems before it, the bearer of which it is, and in the completion of
which it fulfils its destiny.
FOOTNOTES:
[Footnote 34: Translated from _Über die Erhaltung der Kraft_
(Berlin, 1847).]
XXXII
LOUIS PASTEUR
1822-1895
_Louis Pasteur was born at Dôle, France, December 27, 1822, the son
of a tanner. Educated at Arbois, Besançon, and the École Normale,
he was appointed assistant professor of chemistry at the last-named
institution. His first important work was in demonstrating the
asymmetry of molecules. In 1863 he investigated fermentation and showed
that it was caused by the growth of bacteria and later proved that it
was also the cause of putrefaction, a suggestion which Lister employed
in developing antiseptic surgery. In 1865 Pasteur discovered the
bacillus which caused the silkworm disease. Taking up the principle of
inoculation he applied it to small-pox and later extended it to other
infectious diseases. He died September 28, 1895._
INOCULATION FOR HYDROPHOBIA[35]
Gentlemen:--Your Congress meetings are the place for the discussion
of the gravest problems of medicine; they serve also to point out the
great landmarks of the future. Three years ago, on the eve of the
London Congress, the doctrine of micro-organisms, the ætiological cause
of transmissible maladies, was still the subject of sharp criticisms.
Certain refractory minds continued to uphold the idea that “disease is
in us, from us, by us.”
It was expected that the decided supporters of the theory of the
spontaneity of diseases would make a bold stand in London; but no
opposition was made to the doctrine of “exteriority,” or external
causes, the first cause of contagious diseases, and those questions
were not discussed at all.
It was there seen, once again, that when all is ready for the final
triumph of truth, the united conscience of a great assembly feels it
instinctively and recognises it.
All clear-sighted minds had already foreseen that the theory of the
spontaneity of diseases received its death-blow on the day when it
became possible reasonably to consider the spontaneous generation of
microscopic organisms as a myth, and when, on the other hand, the
life-activity of those same beings was shown to be the main cause of
organic decomposition and of all fermentation.
From the London Congress, also, dates the recognition of another very
hopeful progress; we refer to the attenuation of different viruses,
to the production of varying degrees of virulence for each virus, and
their preservation by suitable methods of cultivation; to the practical
application, finally, of those new facts in animal medicine.
New microbic prophylactic viruses have been added to those of
fowl-cholera and of splenic fever. The animals saved from death by
contagious diseases are now counted by hundreds of thousands, and the
sharp opposition which those scientific novelties met with at the
beginning was soon swept away by the rapidity of their onward progress.
Will the circle of practical applications of those new notions be
limited in future to the prophylaxis of animal distempers? We must
never think little of a new discovery, nor despair of its fecundity;
but more than that, in the present instance, it may be asserted that
the question is already solved in principle. Thus, splenic fever is
common to animals and man, and we make bold to declare that, were it
necessary to do so, nothing could be easier than to render man also
proof against that affection. The process which is employed for animals
might, almost without a change, be applied to him also. It would simply
become advisable to act with an amount of prudence which the value of
the life of an ox or a sheep does not call for. Thus, we should use
three or four vaccine-viruses instead of two, of progressive intensity
of virulence, and choose the first ones so weak that the patient
should never be exposed to the slightest morbid complication, however
susceptible to the disease he might be by his constitution.
The difficulty, then, in the case of human diseases, does not lie in
the application of the new method of prophylaxis, but rather in the
knowledge of the physiological properties of their viruses. All our
experiments must tend to discover the proper degree of attenuation
for each virus. But experimentation, if allowable on animals, is
criminal on man. Such is the principal cause of the complication of
researches bearing on diseases exclusively human. Let us keep in mind,
nevertheless, that the studies of which we are speaking were born
yesterday only, that they have already yielded valuable results, and
that new ones may be fairly expected when we shall have gone deeper
into the knowledge of animal maladies, and of those in particular which
affect animals in common with man.
The desire to penetrate farther forward in that double study led me to
choose rabies as the subject of my researches, in spite of the darkness
in which it was veiled.
The study of rabies was begun in my laboratory four years ago, and
pursued since then without other interruption than what was inherent
to the nature of the researches themselves, which present certain
unfavourable conditions. The incubation of the disease is always
protracted, the space disposed of is never sufficient, and it thus
becomes impossible at a given moment to multiply the experiments as
one would like. Notwithstanding those material obstacles, lessened by
the interest taken by the French Government in all questions of great
scientific interest, we now no longer count the experiments which we
have made, my fellow workers and myself. I shall limit myself to-day to
an exposition of our latest acquisitions.
The name alone of a disease, and of rabies above all others, at once
suggests to the mind the notion of a remedy.
But it will, in the majority of cases, be labour lost to aim in the
first instance at discovering a mode of cure. It is, in a manner,
leaving all progress to chance. Far better to endeavour to acquaint
oneself, first of all, with the nature, the cause, and the evolution of
the disease, with a glimmering hope, perhaps, of finally arriving at
its prophylaxis.
To this last method we are indebted for the result that rabies is no
longer to-day to be considered as an insoluble riddle.
We have found that the virus of rabies develops itself invariably in
the nervous system, brain, and spinal cord, in the nerves, and in the
salivary glands; but it is not present at the same moment in every
one of those parts. It may, for example, develop itself at the lower
extremity of the spinal cord, and only after a time reach the brain.
It may be met with at one or at several points of the encephalon whilst
being absent at certain other points of the same region.
If an animal is killed whilst in the power of rabies, it may require
a pretty long search to discover the presence here or there in the
nervous system, or in the glands, of the virus of rabies. We have been
fortunate enough to ascertain that in all cases, when death has been
allowed to supervene naturally, the swelled-out portion, or bulb, of
the medulla oblongata nearest to the brain, and uniting the spinal
cord with it, is always rabid. When an animal has died of rabies (and
the disease always ends in death), rabid matter can with certainty be
obtained from its bulb, capable of reproducing the disease in other
animals when inoculated into them, after trephining, in the arachnoid
space of the cerebral meninges.
Any street dog whatsoever, inoculated in the manner described with
portions of the bulb of an animal which has died of rabies, will
certainly develop the same disease. We have thus inoculated several
hundreds of dogs brought without any choice from the pound. Never once
was the inoculation a failure. Similarly also, with uniform success,
several hundred guinea-pigs, and rabbits more numerous still.
Those two great results, the constant presence of the virus in the
bulb at the time of death, and the certainty of the reproduction
of the disease by inoculation into the arachnoid space, stand out
like experimental axioms, and their importance is paramount. Thanks
to the precision of their application, and to the well-known daily
repetition of those two criteria of our experiments, we have been
able to move forward steadily and surely in that arduous study. But,
however solid those experimental bases, they were, nevertheless,
incapable in themselves of giving us the faintest notion as to some
method of vaccination against rabies. In the present state of science
the discovery of a method of vaccination against some virulent malady
presupposes:
1. That we have to deal with a virus capable of assuming diverse
intensities, of which the weaker ones can be put to vaccinal or
protective uses.
2. That we are in possession of a method enabling us to reproduce those
diverse degrees of virulence at will.
At the present time, however, science is acquainted with one sort of
rabies only--viz., dog rabies.
Rabies, whether in dog, man, horse, ox, wolf, fox, etc., comes
originally from the bite of a mad dog. It is never spontaneous,
neither in the dog nor in any other animal. There are none seriously
authenticated among the alleged cases of so-called spontaneous rabies,
and I add that it is idle to argue that the first case of rabies of
all must have been spontaneous. Such an argument does not solve the
difficulty, and wantonly calls into question the as yet inscrutable
problem of the origin of life. It would be quite as well, against the
assertion that an oak tree always proceeded from another oak tree, to
argue that the first of all oak trees that ever grew must have been
produced spontaneously. Science, which knows itself, is well aware that
it would be useless for her to discuss about the origin of things;
she is aware that, for the present at any rate, that origin is placed
beyond the ken of her investigations.
In fine, then, the first question to be solved on our way towards the
prophylaxis of rabies is that of knowing whether the virus of that
malady is susceptible of taking on varying intensities, after the
manner of the virus of fowl-cholera or of splenic fever.
But in what way shall we ascertain the possible existence of varying
intensities in the virus of rabies? By what standard shall we measure
the strength of a virus which either fails completely or kills? Shall
we have recourse to the visible symptoms of rabies? But those symptoms
are extremely variable, and depend essentially on the particular point
of the encephalon or of the spinal cord where the virus has in the
first instance fixed and developed itself. The most caressing rabies,
for such do exist, when inoculated into another animal of the same
species, give rise to furious rabies of the intensest type.
Might we then perhaps make use of the duration of incubation as a
means of estimating the intensity of our virus? But what can be more
changeful than the incubative period? Suppose a mad dog were to bite
several sound dogs: one of them will take rabies in one month or six
weeks, another after two or three months or more. Nothing, too, is more
changeful than the length of incubation according to the different
modes of inoculation. Thus, other circumstances the same, after bites
or hypodermic inoculation rabies occasionally develops itself, and at
other times aborts completely; but inoculations on the brain are never
sterile, and give the disease after a relatively short incubation.
It is possible, nevertheless, to gauge with sufficient accuracy the
degree of intensity of our virus by means of the time of incubation,
on condition that we make use exclusively of the intra-cranial mode
of inoculation; and secondly, that we do away with one of the great
disturbing influences inherent to the results of inoculation made
by bites, under the skin or in the veins, by injecting the right
proportion of material.
The duration of incubation, as a matter of fact, may depend largely
on the quantity of efficient virus--that is to say, on the quantity
of virus which reaches the nervous system without diminution or
modification. Although the quantity of virus capable of giving rabies
may be, so to speak, infinitely small, as seen in the common fact of
the disease developing itself after rabid bites which, as a rule,
introduce into the system a barely appreciable weight of virus, it
is easy to double the length of incubation by simply changing the
proportion of those very small quantities of inoculated matter. I may
quote the following examples:--
On May 10, 1882, we injected into the popliteal vein of a dog ten drops
of a liquid prepared by crushing a portion of the bulb of a dog, which
had died of ordinary canine madness, in three or four times its volume
of sterilised broth.
Into a second dog we injected 1/100th of that quantity, into a third
1/200th. Rabies showed itself in the first dog on the eighteenth day
after the injection, on the thirty-fifth day in the second dog, whilst
the third one did not take the disease at all, which means that, for
the last animal, with the particular mode of inoculation employed, the
quantity of virus injected was not sufficient to give rabies. And yet
that dog, like all dogs, was susceptible of taking the disease, for it
actually took it twenty-two days after a second inoculation, performed
on September 3, 1882.
I now take another example bearing on rabbits, and by a different mode
of inoculation. This time, after trephining, the bulb of a rabbit
which had died of rabies after inoculation of an extremely powerful
virus is triturated and mixed with two or three times its volume of
sterilised broth. The mixture is allowed to stand a little, and then
two drops of the supernatant liquid are injected after trephining into
a first rabbit, into a second rabbit one-fourth of that quantity, and
in succession into other rabbits, 1/16th, 1/64th, 1/128th, and 1/152nd
of that same quantity. All those rabbits died of rabies, the incubation
having been eight days, nine and ten days for the third and fourth,
twelve and sixteen days for the last ones.
Those variations in the length of incubation were not the result of
any weakening or diminution of the intrinsic virulence of the virus
brought on possibly by its dilution, for the incubation of eight days
was at once recovered when the nervous matter of all those rabbits was
inoculated into new animals.
Those examples show that, whenever rabies follows upon bites or
hypodermic inoculations, the differences in respect of length of
incubation must be chiefly ascribed to the variations, at times within
considerable limits, of the ever-undeterminate proportions of the
inoculated viruses which reach the central nervous system.
If, therefore, we desire to make use of the length of incubation as a
measure of the intensity of the virulence, it will be indispensable
to have recourse to inoculation on the surface of the brain, after
trephining, a process the action of which is absolutely certain,
coupled with the use of a larger quantity of virus than what is
strictly sufficient to give rise to rabies. By those means the
irregularities in the length of incubation for the same virus tend to
disappear completely, because we always have the maximum effect which
that virus can produce; that maximum coincides with a minimum length of
incubation.
We have thus, finally, become possessed of a method enabling us to
investigate the possible existence of different degrees of virulence,
and to compare them with one another. The whole secret of the method,
I repeat, consists in inoculating on the brain, after trephining, a
quantity of virus which, although small in itself, is still greater
than what is simply necessary to reproduce rabies. We thus disengage
the incubation from all disturbing influences and render its duration
dependent exclusively on the activity of the particular virus used,
that activity being in each case estimated by the minimum incubation
determined by it.
This method was applied in the first instance to the study of canine
madness, and in particular to the question of knowing whether
dog-madness was always one and the same, with perhaps the slight
variations which might be due to the differences of race in diverse
dogs.
We accordingly got hold of a number of dogs affected with ordinary
street rabies, at all times of the year, at all seasons of the same
year or of different years, and belonging to the most dissimilar canine
races. In each case the bulbar portion of the medulla oblongata was
taken out from the recently dead animal, triturated and suspended in
two or three times its volume of sterilised liquid, making use all
along of every precaution to keep our materials pure, and two drops
of this liquid injected after trephining into one or two rabbits.
The inoculation is made with a Pravaz syringe, the needle of which,
slightly curved at its extremity, is inserted through the dura-mater
into the arachnoid space. The results were as follows: all the rabbits,
from whatever sort of dog inoculated, showed a period of incubation
which ranged between twelve and fifteen days, without almost a single
exception. Never did they show an incubation of eleven, ten, nine, or
eight days, never an incubation of several weeks or of several months.
Dog-rabies, the ordinary rabies, the only known rabies, is thus
sensibly one in its virulence, and its modifications, which are very
limited, appear to depend solely on the varying aptitude for rabies
of the different known races. But we are going now to witness a deep
change in the virulence of dog-rabies.
Let us take one, any one, of our numerous rabbits, inoculated with the
virus of an ordinary mad dog, and, after it has died, extract its bulb,
prepare it just as described, and inject two drops of the bulb-emulsion
into the arachnoid space of a second rabbit, whose bulb will in turn
and in time be injected into a third rabbit, the bulb of which again
will serve for a fourth rabbit, and so on.
There will be evidence, even from the first few passages, of a marked
tendency towards a lessening of the period of incubation in the
succeeding rabbits. Just one example:
Towards the end of the year 1882 fifteen cows and one bull died of
rabies on a farm situated in the neighbourhood of the town of Melun.
They had been bitten on October 2 by the farm dog, which had become
mad. The head of one of the cows, which had died on November 15, was
sent to my laboratory by M. Rossignol, a veterinary surgeon in Melun.
A number of experiments were made on dogs and rabbits, and showed that
the following parts, the only encephalic (or those pertaining to the
brain) ones tested, were rabid: the bulb, the cerebellum, the frontal
lobe, the sphenoidal lobe. The rabbits trephined and inoculated with
those different parts showed the first symptoms of rabies on the
seventeenth and eighteenth days after inoculation. With the bulb of
one of those rabbits two more were inoculated, of which one took rabies
on the fifteenth day, the other on the twenty-third day.
We may notice, once for all, that when rabies is transferred from one
animal to another of a different species, the period of incubation is
always very irregular at first in the individuals of the second species
if the virus had not yet become fixed in its maximum virulence for the
first species. We have just seen an example of that phenomenon, since
one of the rabbits had an incubation of fifteen days, the other of
twenty-three days, both having received the same virus and all other
circumstances remaining apparently the same for them.
The bulb of the first one of those last rabbits which died was
injected into two more rabbits, still after trephining. One of them
took rabies on the tenth day, the other on the fourteenth day. The
bulb of the first one that died was again injected into a couple of
new rabbits, which developed the disease in ten days and twelve days
respectively. A fifth time two new animals were inoculated from the
first one that died, and they both took the disease on the eleventh day
after inoculation: similarly, a sixth passage was made, and gave an
incubation of eleven days, twelve days for the seventh passage, ten and
eleven for the eighth, ten days for the ninth and tenth passages, nine
days for the eleventh, eight and nine days for the twelfth, and so on,
with differences of twenty-four hours at the most, until we got to the
twenty-first passage, when rabies declared itself in eight days, and
subsequently to that always in eight days up to the fiftieth passage,
which was only effected a few days ago. That long experimental series
which is still going on was begun on November 15, 1882, and will be
kept up for the purpose of preserving in our rabies virus that maximum
virulence which it has come to now for some considerable time, as it is
easy to calculate.
Allow me to call your attention to the ease and safety of the
operations for trephining and then inoculating the virus. Throughout
the last twenty months we have been able without a single interruption
in the course of the series to carry the one initial virus through a
succession of rabbits which were all trephined and inoculated every
twelfth day or so.
Guinea-pigs reach more rapidly the maximum virulence of which they are
susceptible. The period of incubation is in them also variable and
irregular at the beginning of the series of successive passages, but
it soon enough fixes itself at a minimum of five days. The maximum
virulence in guinea-pigs is reached after seven or eight passages only.
It is worth noting that the number of passages required before reaching
the maximum virulence, both in guinea-pigs and in rabbits, varies with
the origin of the first virus with which the series is begun.
If now this rabies with maximum virulence be transferred again into the
dog from guinea-pig or rabbit, there is produced a dog-virus which in
point of virulence goes far beyond that of ordinary canine madness.
But, a natural query--of what use can be that discovery as to the
existence and artificial production of diverse varieties of rabies,
every one of them more violent and more rapidly fatal than the habitual
madness of the dog? The man of science is thankful for the smallest
find he can make in the field of pure science, but the many, terrified
at the very name of hydrophobia, claim something more than mere
scientific curiosities. How much more interesting it would be to become
acquainted with a set of rabies viruses which should, on the contrary,
be possessed of attenuated degrees of virulence! Then, indeed, might
there be some hope of creating a number of vaccinal rabies viruses
such as we have done for the virus of fowl-cholera, of the microbe of
saliva, of the red evil of swine (swine-plague), and even of acute
septicæmia. Unfortunately, however, the methods which had served for
those different viruses showed themselves to be either inapplicable
or inefficient in the case of rabies. It therefore became necessary
to find out new and independent methods, such, for example, as the
cultivation _in vitro_ of the mortal rabies virus.
Jenner was the first to introduce into current science the opinion that
the virus which he called the grease of the horse, and which we call
now more exactly horse-pox, probably softened its virulence, so to
speak, in passing through the cow and before it could be transferred
to man without danger. It was therefore natural to think of a possible
diminution of the virulence of rabies by a number of passages through
the organisms of some animal or other, and the experiment was worth
trying. A large number of attempts were made, but the majority of the
animal species experimented on exalted the virulence after the manner
of rabbits and guinea-pigs; fortunately, however, it was not so with
monkey.
On December 6, 1883, a monkey was trephined and inoculated with the
bulb of a dog, which had itself been similarly inoculated from a child
who had died of rabies. The monkey took rabies eleven days later, and
when dead served for inoculation into a second monkey, which also took
the disease on the eleventh day. A third monkey, similarly inoculated
from the second one, showed the first symptoms on the twenty-third
day, etc. The bulb of each one of the monkeys was inoculated, after
trephining, into two rabbits each time. The rabbits inoculated from the
first monkey developed rabies between thirteen and sixteen days, those
from the second monkey between fourteen and twenty days, those from
the third monkey between twenty-six and thirty days, those from the
fourth monkey both of them after the twenty-eighth day, those from the
fifth monkey after twenty-seven days, those from the sixth monkey after
thirty days.
It cannot be doubted after that, that successive passages through
monkeys, and from the several monkeys to rabbits, do diminish the
virulence of the virus for the latter animals; they diminish it for
dogs also. The dog inoculated with the bulb of the fifth monkey gave
an incubation of no less than fifty-eight days, although it had been
inoculated in the arachnoid space.
The experiments were renewed with fresh sets of monkeys and led
to similar results. We were therefore actually in possession of a
method by means of which we could attenuate the virulence of rabies.
Successive inoculations from monkey to monkey elaborate viruses which,
when transferred to rabbits, reproduce rabies in them, but with a
progressively lengthening period of incubation. Nevertheless, if one of
those rabbits be taken as the first for inoculations through a series
of rabbits, the rabies thus cultivated obeys the law which we have seen
before, and has its virulence increased at each passage.
The practical application of those facts gives us a method for the
vaccination of dogs against rabies. As a starting point, make use of
one of the rabbits inoculated from a monkey sufficiently removed from
the first animal of the monkey series for the inoculation--hypodermic
or intra-venous--of that rabbit’s bulb not to be mortal for a new
rabbit. The next vaccinal inoculations are made with the bulbs of
rabbits derived by successive passages from that first rabbit.
In the course of our experiments we made use, as a rule, for
inoculation, of the virus of rabbits which had died after an incubation
of four weeks, repeating three or four times each the vaccinal
inoculations made with the bulbs of rabbits derived in succession
from one another and from the first one of the series, itself coming
directly from the monkey. I abstain from giving more details, because
certain experiments which are actually going on allow me to expect that
the process will be greatly simplified.
You must be feeling, gentlemen, that there is a great blank in my
communication; I do not speak of the micro-organism of rabies. We have
not got it. The process for isolating it is still imperfect, and the
difficulties of its cultivation outside the bodies of animals have not
yet been got rid of, even by the use, as pabulum, of fresh nervous
matter. The methods which we employed in our study of rabies ought all
the more perhaps, on that account, to fix attention. Long still will
the art of preventing diseases have to grapple with virulent maladies
the micro-organic germs of which will escape our investigations. It is,
therefore, a capital scientific fact that we should be able, after all,
to discover the vaccination process for a virulent disease without yet
having at our disposal its special virus and whilst yet ignorant of how
to isolate or to cultivate its microbe.
As soon as the method for the vaccination of dogs was firmly
established, and we had in our possession a large number of dogs which
had been rendered refractory to rabies, I had the idea of submitting
to a competent committee those of the facts which appeared destined in
future to serve as a basis for the vaccination of dogs against rabies.
That course was suggested to me in prevision of the later practical
application of the method, by the recollection of the opposition with
which Jenner’s discovery met at its beginning.
I spoke of my project to M. Fallières, the Minister of Public
Instruction, who was pleased to approve of it and gave commission to
the following gentlemen to control the facts which I had summarily
communicated to the Academy of Sciences in its sitting of May 19 last:
Messrs. Béclard, Paul Bert, Bouley, Aimeraud, Villemin, Vulpian. M.
Bouley was appointed president, Dr. Villemin, secretary, and the
commission at once set to work. I have the pleasure of informing
you that it has just sent in a first report to the Minister. I was
acquainted with it here, and the following are in a few words, the
facts related in that first report on rabies. I had given to the
commission nineteen vaccinated dogs in succession--that is to say,
dogs which had been rendered refractory by preventive inoculations.
Thirteen only of them had after their vaccination been already
submitted to the test-inoculation on the brain.
The nineteen dogs were, for the sake of comparison, divided into
sets along with nineteen more control dogs brought from the pound
without any sort of selection. To begin with, two refractory dogs
and two control dogs were on June 1 trephined and inoculated under
the dura-mater, on the surface of the brain, with the bulb of a dog
affected with ordinary street rabies.
On June 3 another refractory dog and another control dog were bitten by
a furious street mad dog.
The same furious mad dog was on June 4 made to bite still another
refractory and another control dog. On June 6 the furious dog which
had been utilised on June 3 and 4 died. The bulb was taken out and
inoculated, after trephining, into three refractory dogs and three
control dogs. On June 10 another street mad dog, having been secured,
was, by the commission, made to bite one refractory and one control
dog. On June 16 the commission had two new dogs, a refractory one and
a control one, bitten by one of the control dogs of June 1, which had
been seized with rabies on June 14 in consequence of the inoculation
after trephining which it had received on June 1.
On June 19 the commission got three refractory and three control dogs
inoculated before their own eyes in the popliteal vein with the bulb
of an ordinary street mad dog. On June 20 they had inoculated in
their presence, and still in a vein, ten dogs altogether, six of them
refractory and four just brought from the pound.
On June 28, the Commission hearing that M. Paul Simon, a veterinary
surgeon, had a furious biting mad dog, had four of their dogs, two
refractory and two control dogs, taken to his place and bitten by the
mad dog.
The Rabies Commission have, therefore, experimented on thirty-eight
dogs altogether--namely, nineteen refractory dogs and nineteen control
dogs susceptible of taking the disease. Those of the dogs which have
not died in consequence of the operations themselves are still under
observation, and will long continue to be. The commission, reporting
up to the present moment on their observations as to the state of the
animals tried and tested by them, find that out of the nineteen control
dogs six were bitten, of which six three have taken rabies. Seven
received intra-venous inoculations, of which five have died of rabies.
Five were trephined and inoculated on the brain; the five have died of
rabies.
On the other hand, not one of the nineteen vaccinated dogs has taken
rabies.
In the course of the experiments, on July 13, one of the refractory
dogs died in consequence of a black diarrhœa which had begun in the
first days of July. In order to ascertain whether rabies had anything
to do with it as the cause of death, its bulb was at once inoculated,
after trephining, into three rabbits and one guinea-pig. All four
animals are still to-day in perfect health, a certain proof that the
dog died of some common malady, and not of rabies.
The second report of the Commission will be concerned with the
experiments made as to the refractoriness to rabies of twenty dogs to
be vaccinated by the Commission themselves.
(_M. Pasteur then announced that he had just received that same
morning the first report addressed to M. Fallières by the Official
Commission on Rabies. It states that twenty-three refractory dogs were
bitten by ordinary mad dogs, and that not one of them had taken rabies.
On the other hand, within two months after the bites, 66 per cent. of
the normal dogs similarly bitten had already taken the disease._)
_November 1, 1886.--New Communication on Rabies._--On October 26,
1885, I acquainted the Academy with a method of prophylaxis of rabies
after bites. Numerous applications on dogs had justified me in trying
it on man. As early as March 1, 350 persons bitten by dogs undoubtedly
mad, and several more by dogs simply suspected of rabies, had already
been treated at my laboratory by Dr. Grancher. And in consideration
of the happy results obtained it appeared to me that it had become
necessary to found an establishment for anti-rabic vaccinations.
To-day, October 31, 1886, 2,490 persons have received the preventive
inoculations in Paris alone. The treatment was in the first instance
uniform for the great majority of the patients, notwithstanding the
different conditions presented by them as to age, sex, the number of
bites received, their seat, their depth, and the time which had elapsed
since the occurrence of the accident. It lasted ten days, the patient
receiving every day an injection prepared from the spinal marrow of a
rabbit, beginning with that of fourteen days’ and ending with that of
five days’ desiccation.
Those 2,490 cases are subdivided according to nationality in the
following manner:
Russia 191
Italy 165
Spain 107
England 80
Belgium 57
Austria 52
Portugal 25
Roumania 22
United States 18
Holland 14
Greece 10
Germany 9
Turkey 7
Brazil 3
India 2
Switzerland 2
France and Algeria 1,726
The number of French persons has been considerable, amounting to 1,726,
and it will be enough to confine ourselves to the category formed by
them as a basis for discussing the degree of efficacy of the method.
Out of the total 1,726 cases treated, the treatment has failed ten
times--namely, in the following cases:
The children: Lagut, Peytel, Clédière, Moulis, Astier, Videau.
The woman: Leduc, seventy years old.
The men: Marius Bouvier (thirty years), Clergot (thirty), and Norbert
Magnevon (eighteen).
I leave out of count two other persons, Louise Pelletier and Moermann,
whose deaths must be attributed to their tardy arrival at the
laboratory, Louise Pelletier thirty-six days, and Moermann forty-three
days after they had been bitten.
We have therefore ten deaths for 1,726 cases, or 1 in 170; such are,
for France and Algeria, the results of the first year’s application of
the method.
Those statistics, taken as a whole, demonstrate the efficacy of the
treatment, as proved further by the relatively large number of deaths
which occurred amongst bitten persons who had not been vaccinated.
FOOTNOTES:
[Footnote 35: From Address delivered August 10, 1884 at the Copenhagen
meeting of the International Medical Congress.]
XXXIII
JAMES CLERK MAXWELL
1831-1879
_James Clerk Maxwell, born November 13, 1831, attended Edinburgh
University 1847-1850. Entering Cambridge, he graduated second wrangler
in 1854. He then taught for four years in Marischal College, Aberdeen,
and in 1860 was called to King’s College, London, where he remained for
the following eight years. He early revealed his mathematical genius
and before he was nineteen had the honor of reading several pages
before the Royal Society of Edinburgh. He developed by mathematics the
theory that electricity was a condition of stress or strain in the
ether, a wave moving in the same medium as light and traveling at the
same rate of speed. The theory was substantiated by the experiments of
Hertz, a pupil of Helmholtz, who in 1887 proved the existence of the
waves which now bear his name. Maxwell died at Cambridge, November 5,
1879._
THE MAXWELL AND HERTZ THEORY OF ELECTRICITY AND LIGHT[36]
It was at the moment when the experiments of Fresnel were forcing
the scientific world to admit that light consists of the vibrations
of a highly attenuated fluid filling interplanetary spaces that the
researches of Ampère were making known the laws of the mutual action
of currents and were so enunciating the fundamental principles of
electro-dynamics.
It needed but one step to the supposition that that same fluid, the
ether, which is the medium of luminous phenomena, is at the same
time the vehicle of electrical action. In imagination Ampère made
this stride; but the illustrious physicist could not foresee that the
seducing hypothesis with which he was toying, a mere dream for him, was
ere long to take a precise form and become one of the vital concerns of
exact science.
A dream it remained for many years, till one day, after electrical
measurements had become extremely exact, some physicist, turning over
the numerical data, much as a resting pedestrian might idly turn over
a stone, brought to light an odd coincidence. It was that the factor
of transformation between the system of electro-statical units and the
system of electro-dynamical units was equal to the velocity of light.
Soon the observations directed to this strange coincidence became so
exact that no sane head could longer hold it a mere coincidence. No
longer could it be doubted that some occult affinity existed between
optical and electrical phenomena. Perhaps, however, we might be
wondering to this day what this affinity could be were it not for the
genius of Clerk Maxwell.
DISPLACEMENT CURRENTS
The reader is aware that solid bodies are divided into two classes,
conductors through which electricity can move in the form of a galvanic
current, and nonconductors, or dielectrics. The electricians of former
days regarded dielectrics as quite inert, having no part to play but
that of obstinately refusing passage to electricity. Had that been so,
any one non-conductor might be replaced by any other without making
any difference in the phenomena; but Faraday found that that was not
the case. Two condensers of the same form and dimensions put into
connection with the same source of electricity do not take the same
charge, though the thickness of the isolating plate be the same, unless
the matter of that plate be chemically the same. Now Clerk Maxwell had
too deeply studied the researches of Faraday not to comprehend the
importance of dielectrics and the imperative obligation to recognize
their active part.
Besides, if light is but an electric phenomenon, when it traverses a
thickness of glass electrical events must take place in that glass. And
what can be the nature of those events? Maxwell boldly answers, they
are, and must be, currents.
All the experience of his day seemed to contradict this. Never had
currents been observed except in conductors. How was Maxwell to
reconcile his audacious hypothesis with a fact so well established
as that? Why is it that under certain circumstances those supposed
currents produce manifest effects, while under ordinary conditions they
can not be observed at all?
The answer was that dielectrics resist the passage of electricity not
so much more than conductors do, but in a different manner. Maxwell’s
idea will best be understood by a comparison.
If we bend a spring, we meet a resistance which increases the more
the spring is bended. So, if we can only dispose of a finite force, a
moment will come when the motion will cease, equilibrium being reached.
Finally, when the force ceases the spring will in flying back restore
the whole of the energy which has been expended in bending it.
Suppose, on the other hand, that we wish to displace a body plunged
into water. Here again a resistance will be experienced, but it will
not go on increasing in proportion as the body advances, supposing it
to be maintained at a constant velocity. So long as the motive force
acts, equilibrium will never, then, be attained; nor when the force
is removed will the body in the least tend to return, nor can any
portion of the energy expended be restored. It will, in fact, have been
converted into heat by the viscosity of the water.
The contrast is plain; and we ought to distinguish elastic resistance
from viscous resistance. Using these terms, we may express Maxwell’s
idea by saying that dielectrics offer an elastic resistance, conductors
a viscous resistance, to the movements of electricity. Hence, there
are two kinds of currents; currents of displacement which traverse
dielectrics and ordinary currents of conduction which circulate in
conductors.
Currents of the first kind, having to overcome an elastic resistance
which continually increases, naturally can last but a very short time,
since a state of equilibrium will quickly be reached.
Currents of conduction, on the other hand, having only a viscous
resistance to overcome, must continue so long as there is any
electromotive force.
Let us return to the simile used by M. Cornu in his notice in the
Annuaire du Bureau des Longitudes for 1893. Suppose we have in a
reservoir water under pressure. Lead a tube plumb downward into the
reservoir. The water will rise in the tube, but the rise will stop
when hydrostatic equilibrium is attained--that is, when the downward
pressure of the water in the tube above the point of application of the
first pressure on the reservoir, and due to the weight of the water,
balances that first pressure. If the pipe is large, there will be no
friction or loss of head, and the water so raised can be used to do
work. That represents a current of displacement.
If, on the other hand, the water flows out of the reservoir by a
horizontal pipe, the motion will go on till the reservoir is emptied;
but if the tube is small and long there will be a great loss of energy
and considerable production of heat by friction. That represents a
current of conduction.
Though it would be vain, not to say idle, to attempt to represent all
details, it may be said that everything happens just as if the currents
of displacement were acting to bend a multitude of little springs.
When the currents cease, electrostatic equilibrium is established,
and the springs are bent the more, the more intense is the electric
field. The accumulated work of the springs--that is, the electrostatic
energy--can be entirely restored as soon as they can unbend, and so it
is that we obtain mechanical work when we leave the conductors to obey
the electrostatic attractions. Those attractions must be due to the
pressure exercised on the conductors by the bent springs. Finally, to
pursue the image to the death, the disruptive discharge may be compared
to the breaking of the springs when they are bent too much.
On the other hand, the energy employed to produce conduction currents
is lost, being wholly converted into heat, like that spent in
overcoming the viscosity of fluids. Hence it is that the conducting
wires become heated.
From Maxwell’s point of view it seems that all currents are in closed
circuits. The older electricians did not so opine. They regarded the
current circulating in a wire joining the two poles of a pile as
closed; but if in place of directly uniting the two poles we place them
in communication with the two armatures of a condenser, the momentary
current which lasts while the condenser is getting charged was not
considered as a current round a closed circuit. It went, they thought,
from one armature through the wire, the battery, the other wire, to
the other armature, and there it stopped. Maxwell, on the contrary,
supposed that in the form of a current of displacement it passes
through the nonconducting plate of the condenser, and that precisely
what brings it to cessation is the opposite electromotive force set up
by the displacement of electricity in this dielectric.
Currents become sensible in three ways--by their heating effects, by
their actions on other currents and on magnets, and by the induced
currents to which they give rise. We have seen why currents of
conduction develop heat and why currents of displacement do not.
But Maxwell’s hypothetical currents ought at any rate to produce
electro-magnetic and inductive effects. Why do these effects not
appear? The answer is, that it is because a current of displacement
can not last long enough. That is to say, they can not last long in
one direction. Consequently in a dielectric no current can long exist
without alteration. But the effects ought to and will become observable
if the current is continually reversed at sufficiently short intervals.
THE NATURE OF LIGHT
Such, according to Maxwell, is the origin of light. A luminiferous wave
is a series of alternating currents produced in dielectrics, in air, or
even in the interplanetary void, and reversed in direction a million
of million of times per second. The enormous induction due to these
frequent alternations sets up other currents in the neighboring parts
of the dielectric, and so the waves are propagated.
Calculation shows that the velocity of propagation would be equal to
the ratio of the units, which we know is the velocity of light.
Those alternative currents are a sort of electrical oscillation. Are
they longitudinal, like those of sound, or are they transversal, like
those of Fresnal’s ether? In the case of sound the air undergoes
alternative condensations and rarefactions. The ether of Fresnal, on
the other hand, behaves as if it were composed of incompressible layers
capable only of slipping over one another. Were these currents in open
paths, the electricity carried from one end to the other would become
accumulated at one extremity. It would thus be condensed and rarefied
like air, and its vibrations would be longitudinal. But Maxwell only
admits currents in closed circuits; accumulation is impossible, and
electricity behaves like the incomprehensible ether of Fresnel, with
its transversal vibrations.
EXPERIMENTAL VERIFICATION
We thus obtain all the results of the theory of waves. Yet this was not
enough to decide the physicists to adopt the ideas of Maxwell. It was a
seductive hypothesis; but physicists consider hypotheses which lead to
no distinct observational consequences as beyond the borders of their
province. That province, so defined, no experimental confirmation of
Maxwell’s theory invaded for twenty-five years.
What was wanted was some issue between the two theories not too
delicate for our coarse methods of observation to decide. There was but
one line of research along which any _experimentum crucis_ was to
be met with.
The old electro-dynamics makes electro-magnetic induction take place
instantaneously; but according to Maxwell’s doctrine it propagates
itself with the velocity of light.
The point was then to measure, or at least to make certain, a velocity
of propagation of inductive effects. This is what the illustrious
German physicist Hertz has done by the method of interferences.
The method is well known in its application to optical phenomena. Two
luminous rays from one identical center interfere when they reach the
same point after pursuing paths of different lengths. If the difference
is one, two, or any whole number of wave lengths, the two lights
re-enforce one another so that if their intensities are equal, that of
their combination is four times as great. But if the difference is an
odd number of half wave lengths, the two lights extinguish one another.
Luminiferous waves are not peculiar in showing this phenomenon;
it belongs to every periodic change which is propagated with
definite velocity. Sound interferes just as light does, and so must
electro-dynamic induction if it is strictly periodic and has a definite
velocity of propagation. But if the propagation is instantaneous there
can be no interference, since in that case there is no finite wave
length.
The phenomenon, however, could not be observed were the wave length
greater than the distance within which induction is sensible. It is
therefore requisite to make the period of alternation as short as
possible.
ELECTRICAL EXCITERS
We can obtain such currents by means of an apparatus which constitutes
a veritable electrical pendulum. Let two conductors be united by a
wire. If they have not the same electric potential the electrical
equilibrium is disturbed and tends to restore itself, just as the molar
equilibrium is disturbed when a pendulum is carried away from the
position of repose.
A current is set up in the wire, tending to equalize the potential,
just as the pendulum begins to move so as to be carried back to the
position of repose. But the pendulum does not stop when it reaches that
position. Its inertia carries it farther. Nor, when the two electrical
conductors reach the same potential, does the current in the wire
cease. The equilibrium instantaneously existing is at once destroyed by
a cause analogous to inertia, namely self-induction. We know that when
a current is interrupted it gives rise in parallel wires to an induced
current in the same direction. The same effect is produced in the
circuit itself, if that is not broken. In other words, a current will
persist after the cessation of its causes, just as a moving body does
not stop the instant it is no longer driven forward.
When, then, the two potentials become equal, the current will go on and
give the two conductors relative charges opposite to those they had
at first. In this case, as in that of the pendulum, the position of
equilibrium is passed, and a return motion is inevitable. Equilibrium,
again instantaneously attained, is at once again broken for the same
reason; and so the oscillations pursue one another unceasingly.
Calculation shows that the period depends on the capacity of the
conductors in such a way that it is only necessary to diminish that
capacity sufficiently (which is easily done) to have an electric
pendulum capable of producing an alternating current of extremely short
period.
All that was well enough known by the theoretical researches of Lord
Kelvin and by the experimentation of Federson on the oscillatory
discharge of the Leyden jar. It was not that which constituted the
originality of Hertz.
But it is not enough to construct a pendulum; it is further requisite
to set it into oscillation. For that, it is necessary to carry it off
from equilibrium and to let it go suddenly, that is to say, to release
it in a time short as compared to the period of its oscillation.
For if, having pulled a pendulum to one side by a string, we were to
let go of the string more slowly than the pendulum would have descended
of itself, it would reach the vertical without momentum, and no
oscillation would be set up.
In like manner, with an electric pendulum whose natural period is, say,
a hundred-millionth of a second, no mechanical mode of release would
answer the purpose at all, sudden as it might seem to us with our more
than sluggish conceptions of promptitude. How, then, did Hertz solve
the problem?
To return to our electric pendulum, a gap of a few millimeters is
made in the wire which joins the two conductors. This gap divides our
apparatus into two symmetrical parts, which are connected to the two
poles of a Ruhmkorff coil. The induced current begins to charge the
two conductors, and the difference of their potential increases with
relative slowness.
At first the gap prevents a discharge from the conductors; the air in
it plays the rôle of insulator and maintains our pendulum in a position
diverted from that of equilibrium.
But when the difference of potential becomes great enough, a spark will
jump across. If the self-induction is great enough and the capacity
and resistance small enough, there will be an oscillatory discharge
whose period can be brought down to a hundred-millionth of a second.
The oscillatory discharge would not, it is true, last long by itself;
but it is kept up by the Ruhmkorff coil, whose current is itself
oscillatory with a period of about a hundred-thousandth of a second,
and thus the pendulum gets a new impulse as often as that.
The instrument just described is called a resonance exciter. It
produces oscillations which are reversed from a hundred million to a
thousand million times per second. Thanks to this extreme frequency,
they can produce inductive effects at great distances. To make these
effects sensible another electric pendulum is used, called a resonator.
In this the coil is suppressed. It consists simply of two little
metallic spheres very near to one another, with a long wire connecting
them in a roundabout way.
The induction due to the exciter will set the resonator in vibration
the more intensely the more nearly the natural periods of vibration
are the same. At certain phases of the vibration the difference of
potential of the two spheres will be just great enough to cause the
sparks to leap across.
PRODUCTION OF THE INTERFERENCES
Thus we have an instrument which reveals the inductive waves which
radiate from the exciter. We can study them in two ways. We may either
expose the resonator to the direct induction of the exciter at a great
distance, or else make this induction act at a small distance on a long
conducting wire which the electric wave will follow and which in its
turn will act at a small distance on the resonator.
Whether the wave is propagated along a wire or across the air,
interferences can be produced by reflection. In the first case it
will be reflected at the extremity of the wire, which it will again
pass through in the opposite direction. In the second case it can be
reflected on a metallic leaf which will act as a mirror. In either case
the reflected ray will interfere with the direct ray, and positions
will be found in which the spark of the resonator will be extinguished.
Experiments with a long wire are the easier and furnish much valuable
information, but they cannot furnish an _experimentum crucis_,
since in the old theory, as in the new, the velocity of the electric
wave in a wire should be equal to that of light. But experiments on
direct induction at great distances are decisive. They not only show
that the velocity of propagation of induction across air is finite,
but also that it is equal to the velocity of the wave propagated along
a wire, conformably to the ideas of Maxwell.
SYNTHESIS OF LIGHT
I shall insist less on other experiments of Hertz, more brilliant
but less instructive. Concentrating with a parabolic mirror the wave
of induction that emanates from the exciter, the German physicist
obtained a true pencil of rays of electric force, susceptible of
regular reflection and refraction. These rays, were the period but
one-millionth of what it is, would not differ from rays of light.
We know that the sun sends us several varieties of radiations, some
luminiferous, since they act on the retina, others dark, infra-red, or
ultraviolet, which reveal themselves in chemical and calorific effects.
The first owe the qualities which render them sensible to us to a
physiological chance. For the physicist, the infra-red differs from red
only as red differs from green; it simply has a greater wave length.
That of the Hertzian radiations is far greater still, but they are mere
differences of degree, and if the ideas of Clerk Maxwell are true, the
illustrious professor of Bonn has effected a genuine synthesis of light.
CONCLUSION
Nevertheless, our admiration for such unhoped-for successes must not
let us forget what remains to be accomplished. Let us endeavor to take
exact account of the results definitely acquired.
In the first place, the velocity of direct induction through air is
finite; for otherwise interferences could not exist. Thus the old
electro-dynamics is condemned. But what is to be set up in its place?
Is it to be the doctrine of Maxwell, or rather some approximation to
that, for it would be too much to suppose that he had foreseen the
truth in all its details? Though the probabilities are accumulating, no
complete demonstration of that doctrine has ever attained.
We can measure the wave length of the Hertzian oscillations. That
length is the product of the period into the velocity of propagation.
We should know the velocity if we knew the period; but this last is
so minute that we cannot measure it; we can only calculate it by a
formula due to Lord Kelvin. That calculation leads to figures agreeable
to the theory of Maxwell; but the last doubts will only be dissipated
when the velocity of propagation has been directly measured. (See Note
I.)
But this is not all. Matters are far from being as simple as this
brief account of the matter would lead one to think. There are various
complications.
In the first place, there is around the exciter a true radiation of
induction. The energy of the apparatus radiates abroad, and if no
source feeds it, it quickly dissipates itself and the oscillations
are rapidly extinguished. Hence arises the phenomenon of multiple
resonance, discovered by Messrs. Sarasin and De la Rive, which at first
seemed irreconcilable with the theory.
On the other hand, we know that light does not exactly follow the
laws of geometrical optics, and the discrepancy, due to diffraction,
increases proportionately to the wave length. With the great waves
of the Hertzian undulations these phenomena must assume enormous
importance and derange everything. It is doubtless fortunate, for the
moment at least, that our means of observation are as coarse as they
are, for otherwise the simplicity which struck us would give place to
a dedalian complexity in which we should lose our way. No doubt a good
many perplexing anomalies have been due to this. For the same reason
the experiments to prove a refraction of the electrical waves can
hardly be considered as demonstrative.
It remains to speak of a difficulty still more grave, though doubtless
not insurmountable. According to Maxwell, the coefficient of
electrostatic induction of a transparent body ought to be equal to the
square of its index of refraction. Now this is not so. The few bodies
which follow Maxwell’s law are exceptions. The phenomena are plainly
far more complex than was at first thought. But we have not yet been
able to make out how matters stand, and the experiments conflict with
one another.
Much, then, remains to be done. The identity of light with a vibratory
motion in electricity is henceforth something more than a seductive
hypothesis; it is a probable truth. But it is not yet quite proved.
NOTE I.--Since the above was written another great step
has been taken. M. Blondlot has virtually succeeded, by ingenious
experimental contrivances, in directly measuring the velocity of a
disturbance along a wire. The number found differs little from the
ratio of the units; that is, from the velocity of light, which is
300,000 kilometers per second. Since the interference experiments made
at Geneva by Messrs. Sarasin and De la Rive have shown, as I said
above, that induction is propagated in air with the same velocity as an
electric disturbance which follows a conducting wire, we must conclude
that the velocity of the induction is the same as that of light, which
is a confirmation of the ideas of Maxwell.
M. Fizeau had formerly found for the velocity of electricity a number
far smaller, about 180,000 kilometers. But there is no contradiction.
The currents used by M. Fizeau, though intermittent, were of small
frequency and penetrated to the axis of the wire, while the currents of
M. Blondlot, oscillatory and of very short period, remained superficial
and were confined to a layer of less than a hundredth of a millimeter
in thickness. One may readily suppose the laws of propagation are not
the same in the two cases.
NOTE II.--I have endeavored above to render the explanation
of the electrostatic attractions and of the phenomena of induction
comprehensible by means of a simile. Now let us see what Maxwell’s idea
is of the cause which produces the mutual attractions of currents.
While the electrostatic attractions are taken to be due to a multitude
of little springs--that is to say, to the elasticity of the ether--it
is supposed to be the living force and inertia of the same fluid which
produce the phenomena of induction and electro-dynamical effects.
The complete calculation is far too extended for these pages, and I
shall again content myself with a simile. I shall borrow it from a well
known instrument--the centrifugal governor.
The living force of this apparatus is proportional to the square of the
angular velocity and to the square of the distance of the balls.
According to the hypothesis of Maxwell, the ether is in motion in
galvanic currents, and its living force is proportional to the square
of the intensity of the current, which thus correspond, in the parallel
I am endeavoring to establish, to the angular velocity of rotation.
If we consider two currents in the same direction, the living force,
with equal intensity, will be greater the nearer the currents are to
one another. If the currents have opposite directions, the living force
will be greater the farther they are apart.
In order to increase the angular velocity of the regulator and
consequently its living force, it is necessary to supply it with
energy and consequently to overcome a resistance which we call its
inertia.
In the same way, in order to increase the intensity of a current, we
must augment the living force of the ether, and it will be necessary to
supply it with energy and to overcome a resistance which is nothing but
the inertia of the ether and which we call the induction.
The living force will be greater if the currents are in the same
direction and near together. The energy to be furnished the counter
electromotive force of induction will be greater. This is what we
express when we say that the mutual action of two currents is to be
added to their self-induction. The contrary is the case when their
directions are opposite.
If we separate the balls of the regulator, it will be necessary, in
order to maintain the angular velocity, to furnish energy, because with
equal angular velocity the living force is greater the more the balls
are separated.
In the same way, if two currents have the same direction and are
brought toward one another, it will be necessary, in order to maintain
the intensity to supply energy, because the living force will be
augmented. We shall, therefore, have to overcome an electromotive
force of induction which will tend to diminish the intensity of the
currents. It would tend on the contrary to augment it, if the currents
had the same direction and were carried apart, or if they had opposite
directions and were brought together.
Finally, the centrifugal force tends to increase the distance between
the balls, which would augment the living force were the angular
velocity to be maintained.
In like manner, when the currents have the same direction, they attract
each other--that is to say, they tend to approach each other, which
would increase the living force if the intensity were maintained.
If their directions are opposed they repel one another and tend to
separate, which would again tend to increase the living force were the
intensity kept constant.