of a pound of water. This datum of experiment serves as a basis for all calculations relating to the mechanical theory of heat.

Here, then, there is a source of heat whose importance cannot escape our attention. Consider the innumerable falls of water which exist upon the earth, the waves of the ocean which resemble immense cataracts, incessantly renewed, and if we would represent to ourselves the enormous quantity of heat they produce, take as an example the falls of the Rhine at Schaffhausen. It has been calculated that this single water-fall creates in a day the heat required to melt 12,000 tons of ice.

Nor is heat created by the impact of heavy bodies alone. When any force has put a body in movement, it often happens that this movement is afterwards annihilated, that is to say, it stops without being communicated to other bodies, and heat becomes apparent. This is seen in the well-known experiment of the fire syringe. We exert a muscular effort on a piston; this compresses the air, and our force seems fruitlessly expended. But if it is not employed in communicating motion, it serves to produce heat; the compressed, air is heated sufficiently to kindle gun-cotton. We will lay it down then as a principle that heat may arise from the destruction of movement.

Our habitual sources of heat are chemical combinations. I take sulphuric acid, diluted with water, in which I have immersed a small balloon containing ether; I put zinc in this acid; a lively chemical action is produced, and the mixture is sufficiently heated to throw the ether into ebullition. The jet of vapor rushes out by a slender tube, and may be made more conspicuous by kindling it. The solution of zinc in an acid is therefore accompanied by a disengagement of heat.

The heat disengaged in a chemical reaction is often sufficiently intense to produce incandescence, and when the vivacity of action is very great, an explosion. I shall cite some examples recently discovered, without going however into detail. A leaf of paper is moistened with pyroligneous acid; we touch it with a glass rod coated with a mixture of sulphuric acid and hypermanganate of potassium, and the paper immediately takes fire. Again, we let fall some drops of the essence of anise on the same mixture placed in the bottom of a glass; there is now both incandescence and explosion.

To manifest to my auditors the connection which exists between the heat created by impact and that created by chemical action, I take an example, well known, but on account of its simplicity, serving better than the preceding for the purpose of explanation. The powdered iron, suitably prepared, takes fire when exposed to the air. What is it that occurs in this phenomenon? One of the elements of the air, oxygen, combines with the iron and forms a brown powder, which is called oxide of iron. If the iron be weighed before and after the combination, it will be found to have increased in weight; this proves the fixation of the oxygen in the iron. Now, we shall very well represent to ourselves the mechanism of the combination, by imagining that the particles of oxygen have been precipitated on the iron and become fixed, just as the stone which falls on the earth remains fixed to the soil. Heat, then, has been created by the impact, and the connection we sought is established.

The chemical sources of heat are so important in the arts, that I shall more particularly dwell upon them, with a view to point out the recent improvements of which they have been the object. It was only the combustion of charcoal, accelerated by the insufflation of a considerable quantity of air, which for a long time was made use of in industry; such is the fire of the smelting furnace, which can melt iron, but is incapable of melting platina. This metal, as precious as gold, could be melted by no known chemical process until quite recently, when the means were devised by M. H. Sainte-Claire Deville. At present we melt it very easily by the combustion of illuminating gas with pure oxygen. In this process the oxygen contained in a gasometer issues by a copper pipe terminated by

a tip of platina. This pipe is in the axis of a second and larger pipe of copper, whose extremity is also of platina. The illuminating gas issues forth, filling the interval comprised between the two pipes. We have thus a jet formed by a mixture. We kindle this jet and introduce it into a furnace composed of lime; the flame whirls with resonance in the midst of the furnace, heats it intensely, and issues by a lateral opening. It is by this opening that we introduce the platina under the form of thin laminæ. Each lamina disappears as if swallowed up, and a sparkling liquid trickles to the bottom of the furnace.

We stop the jet of flame and uncover the furnace; the liquid platina is so dazzling that we may extinguish the gas burners in the hall, and we are illuminated as by the electric light. We pour the liquid in a vase of limestone, and can see its perfect fluidity. By degrees it grows cool in the air, and finally becomes a solid; but it is so heated that it will remain a long time luminous. When there is need but of a moderate heat, the combustion of illuminating gas by the ordinary air is often preferable to that of coal, and the construction of apparatus for warming by gas is at present carried to great perfection. The principal improvement is due to the distinguished German chemist Bunsen, who has devised an excellent arrangement for completely burning the

gas.

The Bunsen burner is essentially formed of two concentric pipes; the gas is conducted into the inner one; the external pipe being open at the two extremities, the atmospheric air naturally enters, mingles with the gas, and it is this mixture which is kindled. The flame is but slightly luminous, but very hot; if we prevent the access of air, the flame becomes brilliant, because the carbon of the gas is not immediately burned by the oxygen of the air, and it remains for some time as a solid dust raised to a very high temperature. It is the presence of the free carbon which enables the flame to be illuminative; the form of the burner for giving light is such that the carbon is not burned so soon as the hydrogen of the gas, while in the burner of Bunsen it is burned at the same time. A single one of these burners, of a suitable size, is sufficient to melt silver.

At present the Bunsen burners are of the greatest service in our laboratories; they are employed for heating the tubes for chemical analysis, and quite recently an arrangement has been contrived which secures for this mode of heating the greatest regularity. The mixture of air and gas issues by some sixty small holes pierced in a cylinder of fire-proof earth, and all these small flames raise the cylinder to a red heat, in such sort that the caloric is uniformly diffused in all directions. Some hundred jets of this sort, suitably disposed around a glass tube, raise all its points to the same temperature without risk of fracture or distortion of the tube. Is it possible to attain a temperature sufficient for the fusion of platina by burning simply a mixture of air and coal-gas? It is the presence of nitrogen, an element of the air altogether inert, which hinders the temperature of combustion of such a mixture from being as high as that of the mixture of gas and pure oxygen. The nitrogen appropriates a part of the heat created by the chemical combination, and moreover it embarrasses the contact of the oxygen and the combustible. The employment of air would nevertheless be much preferable to that of pure oxygen, when an industrial interest is in question, on account of the dearness of the latter and the difficulty of its prepararation. Hence it has been sought to solve this problem; and, by applying to the blow-pipe of M. Schlæsing the principle of the ventilator of M. Demontdésir, M. Wiesnegg has succeeded in melting platina by means of a mixture of air and coal gas.

In the blow-pipe of M. Wiesnegg, as in that of M. Schlæsing, compressed air arrives by a small orifice at the bottom of a tube, and the gas penetrates into this tube by a lateral tubulure in advance of the jet of air. The mixture is kindled at the outlet from the tube. But in the blow-pipe of M. Wiesnegg, the air being very strongly compressed, issues with great velocity; it briskly draws in the gas, and holes being pierced around the orifice of efflux, the atmospheric

air is itself drawn in, and penetrates into the blow-pipe, which considerably augments the total quantity of air mingled with the gas. In order to evince this fact of the aspiration of the surrounding air, we kindle the jet and direct it into a small furnace of brick, analogous to the furnace of lime, which has served us for melting platina. The flame issues with resonance by a small aperture, and the walls of the furnace are rapidly raised to a red heat. We now let fall powder of iron around the holes of the blow-pipe; this powder becomes heated in the furnace and issues with the flame in brilliant sparks. The necessary accessory of the blow-pipe consists in a powerful bellows, which impels the jet of air under a pressure of two atmospheres. For this purpose a pump compresses the atmosphere in a reservoir, while a tube of resistant caoutchouc conveys the compressed air from this reservoir into the blow-pipe.

In this way the inconveniences of the nitrogen contained in the air are lessened. In the blow-pipe of M. Wiesnegg, the air is so intimately mingled with the gas that the inertia of the nitrogen offers the least possible opposition to the rapidity of the chemical combination. Now, it is on this rapidity that depends the temperature of the flame. The more rapid the molecular movements which create heat, the higher the point to which the temperature is raised; because the environing bodies have not time within a certain limit to absorb that heat.

I limit myself to these applications of the chemical sources of heat, and pass to a source of quite another kind to that which has furnished us the highest known temperatures, and which can reduce to vapor the diamond itself. None of the preceding methods enable us to modify this substance; it is the most refractory of which we have any knowledge.

Conceive a sheet of zinc and one of copper plunged into sulphuric acid, diluted with water. We know that the zinc combines with the elements of the liquid, producing heat. If we unite the two sheets by a metallic wire, the latter becomes heated, which indicates that it is the seat of a peculiar modification. The cause of this modification we name electricity, and we say that the assemblage formed of the acid, the metals and the wire, is traversed by the electric current. Now, if we measure the heat produced in the acid and in the wire, we find it to be, for a certain weight of zinc dissolved, the same as if the metal were simply dissolved in the acid without the wire, which gives passage to the current. The sole difference which exists between these two modes of operating consists in the heat being differently distributed; in the act of the dissolution of the zinc in the acid without an electric current, the heat is only produced at the place of the chemical action; when there is a current, this heat is produced simultaneously in all the parts of the circuit traversed by the current. In order to exhibit the heat disengaged in the electric circuit, a battery has been arranged outside of the apartment occupied by my audience; that battery being an assemblage of sheets of zinc and acidulated water in which the chemical combination is effected, while the metallic wire which serves to close the circuit extends to myself for the performance of the experiments. At this moment the wire is divided, and I hold in my hand its two extremities; I touch with them the two ends of a fine wire of platina, 50 centimetres in length, so that the circuit is now closed. The current passes, and we see that the wire of platina is heated to a white red; it in fact melts, and no doubt therefore can remain of the disengagement of heat which I announced. The two wires with which I touched the platina were of copper, and their diameter was about two millimetres; these also have become heated, but the elevation of their temperature was slight, simply because of their thickness.

I shall not seek on this occasion to explain how electricity effects the distribution of heat in the circuit of the battery; I propose merely to mention this means of producing heat, the discovery of which we owe to Volta, and which dates but a half century ago. It has been seen that this source of heat is of chemical

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origin, and consequently the connection which it has with the sources previously spoken of is sufficiently established.

It remains for me to show how the Voltaic circuit realizes the highest temperature known. We attach to the extremities of our two copper wires cylindrical pieces of charcoal; we then bring these cylinders into contact with one another. The circuit is now closed. If we separate these two pieces of charcoal, a mass of dazzling light fills the interval between them and re-establishes the continuity of the circuit. What is this light, which has received the name of the Voltaic arch? It is a volume of incandescent particles of charcoal, which acts in like manner with the platina wire of the preceding experiment. This. mass of particles is vehemently heated by the passage of the current. In order that its enormous temperature may be appreciated, we project it on a tablet by means of lenses, having first enclosed it in a suitable box, that the eyes of the spectators may be sheltered from its blinding brightness. By this expedient, we are enabled to view all the details of the Voltaic arch. On the tablet we may see the reversed image of the sticks of charcoal, themselves heated to white-red, as well as that of the arch, which appears as a violet flame. Let us place a sheet of platina in this flame; it melts rapidly, and the fused platina collects in a sparkling globule on one of the pieces of charcoal. Thus, the heat is at least as strong as in the furnace which, an instant ago, we heated by a chemical process. But it is much stronger, and if we placed a diamond instead of platina in the Voltaic arch, it would be seen to become soft and begin to melt. Were we to conduct this operation in a vacuum, the vapor of the diamond and that of the charcoal of the apparatus would be deposited on the walls of the vase. This experiment was made for the first time by Despretz, at the Sorbonne, with a battery of adequate power.

The sources of heat which I have thus far noticed are at the disposal of man, who can regulate them at his pleasure; these are artificial sources. It remains to speak of the natural sources of those whose power the Creator has regulated, in order to constitute the universal harmony of nature.

It is impossible to explain in this short discourse, by what admirable laws heat is incessantly generated by animals. It suffices to recall the fact that this heat has a chemical origin, in order to comprehend that it is referable to the same fundamental principle with the others. In effect, the carbon and hydrogen furnished by our aliments are placed in presence of the atmospheric oxygen by the act of respiration, and their chemical combination is effected in the blood, attended with the customary disengagement of heat. I should add, however, that animals further create heat by another process purely mechanical. When a man, for instance, goes down stairs, his body is displaced and falls, as it were, from a small height; he displaces it anew, again falls, and continues doing so. Now, each of these little descents creates heat, like the fall of every heavy body which does not rebound. One of our most distinguished savants, M. Hirn de Colmar, has succeeded in measuring the heat thus produced, and finds that it very competently satisfies the general law.

I might also speak of the heat disengaged by vegetables, at certain epochs, when their organs are the theatre of intense chemical reactions; at the same time, I should say that it is much rather their rôle to consume heat than to produce it, and that in this the functions of vegetable life are made to compensate those of animal life. But I am about to transport my audience into other regions, in essaying to lift a corner of the veil which renders them so mysterious.

The grand and most wonderful source of heat is the sun. The genius of man, bursting at a bound its terrestrial shackles, has long since overleaped the distance which separates us from that marvellous luminary. It has measured, it has weighed it, and we are to-day very remote from the time when men bowed in awe before it as before a divinity. Taking as a guide the observations conducted by

M. Pouillet, we find that the sun disengages in a year a quantity of heat capable of melting a covering of ice 1,500 leagues in thickness, which might envelop a globe one million four hundred thousand times larger than the earth.

How are we to explain this enormous production of heat? Is it the result of a combustion analogous to those which take place on our hearths? To be convinced of the impossibility of such an origin, it suffices to know that if the sun were a globe of charcoal burning in oxygen, it would be consumed in 5,000 years.

The new theory of heat has led to an hypothesis which satisfies the mind up to a certain point. The universe is filled with bodies called asteroids, which gravitate under the control of undiscovered laws. It is they which produce the shooting stars and meteorites. Now, it is easily conceivable that such bodies may fall regularly upon the sun and create heat by the impact. It has been calculated what would be the mass of asteroids capable of thus producing the solar heat, and it has been found that it would form in a year a simple stratum of 20 metres (21.872 yards) at the surface of the sun It would, at this rate, require more than a million five hundred thousand centuries for the diameter of the solar disk to appear doubled. Our instruments of astronomy are not sufficiently sensitive to enable us to observe an augmentation so slow; thus the hypothesis does not stand in opposition to facts. We should not forget, however, that all this is conjecture, nor can we plume ourselves on having discovered the cause of the phenomena which have been observed.

After the solar heat, it remains to speak of the terrestrial heat, of which we have striking manifestations in volcanic eruptions, in the geysers, those gigantic eruptions of boiling water which are met with in Iceland, to say nothing of the tranquil indications of artesian wells. The laws of these phenomena are not in general completely known, though that of the geysers has been artificially reproduced upon the ingenious theory of Bunsen. What shall it be said is the origin of this terrestrial heat? Everything would lead us to believe that the earth was primitively an incandescent fluid mass, and thus its condition would be analogous to that of the sun. An incessant fall of cosmical matter would maintain its heat and gradually enlarge its mass. Some time or other this supply has failed, and the globe, in cooling, undergone solidification at the surface. Then only did it become the earth.

Thus it will be seen that the generation of heat by the destruction of movement will serve to explain the production of heat in chemical combinations, in the Voltaic circuit, in the organs of living beings-nay, it will furnish no improbable hypothesis of the origin of solar and terrestrial heat. Are we not tempted hence to conclude that heat is likewise a movement? We thereby associate the connection we have observed with a great and more general principle than the preceding, that of the conservation of energy. In virtue of this principle, if movement ceases in one body, it commences in a neighboring body, so that nothing is lost; all the phenomena of the material world result from an exchange of movement between bodies, one gaining what the other loses.* There is nothing which seems to oppose itself to this generalization; it offers us a picture of what we learn by the senses, and by accepting it as a law, we yield to the sentiment of unity which the Creator has implanted in our souls. But we should be circumspect; we must not allow ourselves to be swayed by imagination, nor surrender reason to the seductive creations of our own invention.

The generation of heat may be a transformation of movement, but what is the intermediary of that transformation? In order to raise the veil which conceals from us the mystery of creation, is it sufficient to say that matter transmits

*The statement requires limitation. It cannot be said that the motion or energy of a cannon ball is all transferred to the side of the ship which it penetrates-a large part is expended in making the hole, another portion in producing the noise of the percussion, and the remainder in generating heat.—J. H.