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Caradoc to her native land. Here she is taken to the arms of her mother Guendolen ; and the story ends, in uncertainty as. to whether our heroine remained in Britain, or returned to Rome. Presuming her identity with Martial's Claudia, there can be no real doubt; but the matter is not important to settle. Whatever the proportion of fact to fiction, we have in “Claudia" a most interesting poem, well sustained, and artistically wrought; fashioned, perhaps, on the model of Tennyson's narrative blank verse; but with a Christian feeling, and an insight into divine truth, that are emphatically the author's own.

PROFESSOR TYNDALL, ON HEAT.*

A QUARTER of a century ago, a western seer complained that “our age is retrospective. It builds the sepulchres of the fathers. It writes biographies, histories, and criticism. The foregoing generations beheld God and nature face to face; we, through their eyes.” The complaint sounds very antiquated now, and is felt to be unjust. Men are content no longer to take either their religion or their science on trust. They do not accept the report which is brought to them of God or nature; but they go out and look for themselves. And if there still remain a doubt, as may well be the case, of their fidelity in relation to God, there can be none of their fidelity to nature. All the prophets from Emerson to Sauerteig, must bear witness that the men of science no longer“ proceed in the small chink-lighted, or even oil-lighted, underground workshop of logic alone, but have come out thence, and looked, and wondered, and pondered. The result, as a matter of course is, that they have been rewarded for their docility and patience; and we, of this generation, have been permitted to witness some of the most remarkable discoveries that the world has ever known. Of these discoveries, none is likely to be more fruitful, certainly none more wonderful, than that which declares the great agencies of nature, heat, light, electricity, and magnetism, so unlike in their manifestation, to be essentially but one, And that, being one, they may be converted from one form to another. This doctrine was

* Heat considered as a Mode of. Motion. London : Longman & Co.

expounded a few years ago, in its most general form, by Mr. Grove, in his work on the “ Correlation of Forces.” In its more special relation to heat, it was the subject of twelve lectures by Professor Tyndall, at the Royal Institution; and to the volume containing these lectures, we propose to call the attention of our readers in the present paper. In many cases, we are glad to know, that the introduction is superfluous. But there are, in this age of over toil and large occupation, many whose opportunities for reading are but few, and who content themselves with an occasional perusal of a work of fiction, seeking for amusement and relaxation only. To such we beg to present a new friend, fascinating in speech, and wise in thought. We trust that he will win them from fiction to reality, and make them feel, not the idle curiosity that asks only for gratification, but the deep wonder that fills the mind seeking for true wisdom. Quite truly, our philosopher tells us, that, “presented rightly to the mind, the discoveries and generalization of modern science, constitute a poem more sublime than has ever yet been addressed to the intellect and imagination of man. The natural philosopher of to-day may dwell amid conceptions, which beggar those of Milton. So great and grand are they, that in the contemplation of them, a certain force of character is requisite to preserve us from bewilderment.” We will only say, that we think that the discoveries and generalization which he undertook to bring before us, he has presented rightly to the mind; and will proceed to give our report of his presentation.

In a very rhetorical passage, at the close of his last lecture, Professor Tyndall elaborates a statement made by Sir John Herschel thirty years ago, to the effect that “ the sun's rays are the ultimate source of almost every motion which takes place on the surface of the earth.” We have space only for a portion of this passage :-“He rears, as I have said, the whole vegetable world, and through it the animal; the lilies of the field are his workmanship, the verdure of the meadows, and the cattle upon a thousand hills. He forms the muscle, he urges the blood, he builds the brain. His fleetness is in the lion's foot; he springs in the panther, he soars in the eagle, he glides in the snake. He builds the forest, and hews it down; the power which raised the tree and which wields the axe being one and the same. His energy is poured freely into space, but our world is a haltingplace where this energy is conditioned. Here the Proteus works his spells; the self-same essence takes a million shapes and hues, and finally dissolves into its primitive and almost formless form. The sun comes to us as heat; he quits us as heat; and between his entrance and departure the multiform powers of our globe appear.” But before we can understand the method of his working, we must imagine what it is that he gives us; in other words, we must ask, what is heat? Until very recently, scientific men have taught us that it was a subtle fluid stored up in the inter-atomic spaces of bodies, or, in the words of a foreign chemist, “that substance whose entrance into our bodies causes the sensation of warmth, and its egress the sensation of cold.” We need not say that this was, and, among the uninformed on the subject, is still, the prevailing conception formed of heat. It is that which is most easily derived from a superficial observation of its phenomena, and commends itself to the mind as a simple and satisfactory explanation. The object of these lectures is to correct this opinion, and “to bring the rudiments of a new philosophy within the reach of a person of ordinary intelligence and culture." It is to verify, by a popular exposition, one of the guesses of the ancient Greeks, and a speculation of some of our own philosophers, that heat is not a substance, but simply motion. This theory, which is called the mechanical theory of heat, discards the idea of materiality, and asserts that heat is "an accident or condition of matter-namely, a motion of its ultimate particles.” It rejects the notion that some bodies have a greater capacity for storing up heat than others, and hide this substance within them till it is driven forth by friction, or by a blow. It maintains that heat is not called forth from a latent condition, but generated. It is communicated, not as a substance, but as a motion. That is to say, the particles of which a body is composed being supposed to be at rest among themselves, the body is said to be cool. When mechanical force is brought to bear on this body, so that its atoms are set in motion, it is said to be heated—to have received heat. An experiment of Sir H. Davy will illustrate, and help to prove, these statements. He took two blocks of ice, and, being careful to exclude all external sources of heat from them, rubbed them together until, by friction, he had liquefied the ice. The water so produced was found to be much higher in temperature than the ice from which it was obtained. Whence, then, had come the additional heat ? The believer in the material theory could not say it had been stored up in the ice, and that he had rendered the hidden heat sensible; for ice, which is solid water, has only one-half the capacity for heat which liquid water possesses. Davy came, therefore, to the conclusion that he had generated heat, and that it was only a kind of molecular motion.

This experiment calls our attention also to one of the most common means of producing heat, viz., by friction. The schoolboy knows that he can heat a brass button by rubbing it on his coat sleeves, and he has heard that savages obtain fire by rubbing together two pieces of dry wood. Dr. Tyndall mentions other instances more or less familiar, as the cleaning of a knife upon a board, the use of a saw in dividing wood. He shows that what is true of solid applies also to liquid. So that the water at the foot of a cataract is warmer than at the point from which it falls; "the sea is rendered warmer through the agitation produced by a storm, the mechanical dash of its billows being ultimately converted into heat." In one of his experiments, Dr. Tyndall boils water by friction.

Whenever friction is overcome, heat is produced, and the heat produced is the measure of the force expended in overcoming the friction. The heat is simply the primitive force in another form, and, if we wish to avoid this conversion, we must abolish the friction. We usually put oil upon the surface of a hone, we grease

saw, and are careful to lubricate the axles of our railway carriages. What are we really doing in these cases?

Let us get general notions first; we shall come to particulars afterwards. It is the object of a railway engineer to urge his train bodily from one place to another. ... He wishes to apply the force of his steam, or of his furnace, which gives tension to the steam, to this particular purpose. It is not his interest to allow any portion of that force to be converted into another form of force which would not further the attainment of his object. He does not want his axles heated, and hence he avoids as much as possible expending his power in heating them. In fact, he has obtained his force from heat, and it is not his object to re-convert the force thus obtained into its primitive force ; for, for every degree of temperature generated by the frction of his axles, a definite amount would be withdrawn from the urging force of his engine. There is no force lost absolutely. Could we gather up all the heat generated by the friction, and could we apply it all mechanically, we should, by it, be able to impart to the train the precise amount of speed which it had lost by the friction. Thus, every one of those railway porters whom you see moving about with his can of yellow grease, and opening the little boxes which surround the carriage axles, without knowing it, illustrating a principle which forms the very solder of nature. In so doing, he is unconsciously affirming both the convertibility and indestructibility of force. He is practically asserting that mechanical energy may be converted into heat, and that when so converted, it cannot still exist as mechanical energy, but that for every degree of heat developed a strict and proportional equivalent of the locomotive force of the engine disappears. A station is approached, say at the rate of thirty or forty miles an hour; the brake is applied, and smoke and sparks issue from the wheel on which it presses. The train is brought to rest—How? Simply by converting the entire moving force which it possessed, at the moment the brake was applied, into heat.” (Pp. 8—10.)

Heat is generated by compression and percussion, as well as by friction. One example will serve to introduce us to another branch of this subject :- A lead bullet is placed upon an anvil, and struck with a sledge hammer. The bullet is flattened and heated to a considerable degree. What has become of the force of the hammer? It cannot be lost, for force is indestructible. It is converted into heat, which is but another form assumed by the force; "and could we gather up all the heat generated by the shock of the sledge, and apply it, without loss, mechanically, we should be able, by means of it, to lift the hammer to the height from which it fell." The words in italics are very important. They suggest that there is a method by which heat can be calculated in amount, and expressed arithmetically in terms of mechanical energy. This is true. We have not space to explain the patient processes by which this result has been attained; we must content onrselves with saying that the mechanical equivalent of heat is thus expressed : the quantity of heat necessary to raise one pound of water one degree is able to produce mechanical energy sufficient to raise a one pound weight 772 feet, and conversely. By means of this rule, the amount of heat generated by a rifle-bullet on striking a target, may be calculated, if its weight and velocity are known. But if this may be calculated, so may the heat generated by the stoppage of any body, its weight and velocity being ascertained ; and this leads to a very interesting and surprising result. “For example; knowing, as we do, the weight of the earth, and the velocity with which it moves through space, a simple calculation would enable us to determine the exact amount of heat which would be developed, supposing the earth to be stopped in her orbit, we could tell the number of degrees which this amount of heat would impart to a globe of water equal to the earth in size. Mayer and Helmholtz have made this calculation, and found that the quantity of heat generated by this colossal shock, would be quite sufficient not only to fuse the entire earth, but to reduce it in great part to vapour. Thus, by the simple stoppage of the earth in its orbit, “the elements’ might be caused 'to melt with fervent heat.' The amount of heat thus developed, would be equal to that derived from the combustion of fourteen globes of coal, each equal to the earth in magnitude. And if, after the stoppage of its motion, the earth should fall into the sun, as it assuredly would, the amount of heat generated by the blow, would be equal to that developed by the combustion of 5,600 worlds of solid carbon." (P. 43.)

We hope that we have by these explanations and illustrations

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