P. We had. You had made a few observations on heat, or caloric.

T. Yes; and our observations had gone to this, that whatever caloric is, it is not one of those elementary bodies which can be combined with others for the production of a third, differing from the two in combination. It can be added to others, and being added, it acts upon them; but even this addition is not permanent. One of the properties of heat is called its tendency to equilibrium, so that, if a body be heated above the temperature of either the surrounding atmosphere, or of other bodies in contact with it, the cooling process, as it is termed, instantly commences; that is, the heat is given out, till the body is no hotter than the atmosphere, or the other bodies.

P. Is this always done in the same time, or in the same proportion?

T. No; and this reminds me of the concluding remark in our last conversation. Heat is said to radiate, when it is given forth to the surrounding atmosphere; that is, heat is given out, as it were, in rays, as from a centre. The earth, for instance, having been warmed by the sun, gives out its extra heat to the atmosphere by radiation. Heat is also communicated by conduction; that is, when a body receives heat by communication with the source of heat, for the time and occasion. And experiments show that the receiving and conducting powers of bodies vary. Some receive and lose it more quickly, some more slowly, than others. The first are called good conductors, the second bad ones. Thus, metals conduct heat rapidly. Put one end of a knitting-needle in the fire. It not only quickly receives it, but the heat passes so rapidly to the portion not in contact with the fire, that you would not be able to hold it. Glass is a bad conductor. Put a glass needle (so to call it) into the fire, and you might hold it at one end, though the opposite end were actually melting. The heat travels very quickly in the metallic rod, very slowly in the glass rod. It is the same with wood. The stick you put into the fire burns to destruction at one end, while you feel not the least alteration in the temperature of the part held in your hand. So again with water. In a saucepan on the fire,

the portion at the bottom is converted into vapour before you feel the upper part warmed. The vapour, being confined, ascends in small bubbles, communicating heat in the ascent, till the water is filled to its utmost capacity. It boils, and then it cannot be made hotter. The pressure of the atmosphere on its surface prevents it doing this sooner than till there is the quantity of heat indicated on the thermometer by 212°, which is therefore called boiling heat. Let the pressure be lessened, -as by carrying the apparatus up a high mountain, where a column of air, being shorter, of course weighs less; or by artificially creating a vacuum over the water, and boiling commences at a much lower point. There are some medicinal substances whose virtue resides in their extracts, that is, in those parts which water can hold in solution. The water being evaporated, the substance remains. Thus, Spanish juice, as bought in the shops, is the extract of the root of the liquorice plant. First, the water extracted as much of the juices as possible, and then, being driven off by evaporation, the juices were left behind in a mass, which is moulded into the forms in which it is sold. Now, some plants have their virtue lessened by being exposed to a heat up to boiling-point. They are directed, therefore, to be prepared in vacuo: a proper apparatus is employed to cause a vacuum of air over the water, which thus boils and evaporates with a less degree of heat; and the injury which the preparation would otherwise sustain, is prevented, or, at all events, greatly lessened. I have seen water in a tumbler-glass, with a piece of ice at the bottom. By means of a tube, a current of steam has been introduced, say an inch or two below the surface. That heat is communicated by the ascent of the steam-bubbles, is made visible in this experiment. In a very short time, the water boils at the surface.

But the heat does not descend. Water is a bad conductor. The ice at the bottom is seen to be unmelted, while the water at the top actually boils. This is a law very advantageous for man. The bottom of a river is in contact with the earth, which will be about its own temperature. There is therefore scarcely any movement upwards. If the atmosphere at the surface is much colder, the surface rapidly parts with its heat, and when 32° are reached,

the water freezes. If it froze first at the bottom, the whole would become a mass of ice. Whereas, there is only surfaceice, more or less thick, but leaving the under mass as it was. This freezing takes place more or less rapidly, according to atmospheric temperature. I once saw water steaming, and ice at the same time forming on the surface. The indication on the thermometer was about 20° below zero, (a cipher,) which itself is 32° below freezing-point. The water at the surface was, of course, about 32°. Ice, therefore, was formed so rapidly that the process was visible. It was beautiful, in this natural experiment, to see the spiculæ of the solidifying water darting in every direction, and soon forming a film on the surface. The steam was caused by the rapid evaporation of the water, which was above 50° warmer than the superincumbent atmosphere, and the vapour, ascending into such cold air, was condensed as it arose. I witnessed, therefore, at once, the three forms of water. Its fluid form, in the water itself; its solid form, in the ice which was so rapidly darting into view; and its vapour form, in the steam gently rising, as it seemed, from the surface. But proceed now with the questions you may wish to propose.

P. We find, on looking at our notes, that we shall not be able, in the present conversation, to go through all the subjects to which we wish to refer. One or two of them, however, seem to be, as it were, at the foundation, and if explained, will prepare for what is to come. If you please, therefore, we will add them to the miscellanies by which we have now been occupied.

T. What are they?

P. Why, first, as you just now spoke of seeing at once the three forms in which water may exist, and pointed out their connexion with heat, so I suppose we may conclude, that water only furnishes an instance of what holds good in reference to matter universally?

T. It is supposed, I should say, fully believed, that this is the case. And that those instances which seem to be exceptive, are so only from the imperfection of our instruments, or the limited range of our powers. It is, at all events, well known that these forms owe their existence, as such, to heat,

either by its reception or its removal. When heat is withdrawn from below a certain limit, the substance exists in the solid form. I should say here, though, that bodies vary very greatly among themselves as to the quantity of heat necessary to produce these changes of form. A piece of stone is solid in almost all temperatures; though it is possible, by galvanic heat, to evaporate the diamond past recovery: an experiment, therefore, not often tried. Quicksilver is fluid at the ordinary temperature, and remains so, long after water is frozen. The cold must be very intense to freeze quicksilver. And yet, at a certain degree, which has been ascertained by the thermometer, it becomes solid as lead. So much heat, therefore, varying in different bodies, there is the solid form. So much more, likewise varying, there is the fluid form. At another accession, it becomes aëriform. In all these operations, heat is the great power employed. Steam is water with heat added. Ice is water with heat taken away. And, were we to examine all these variations, we should find them exhibiting proofs of the greatest wisdom and benevolence in reference to the uses of man. Some substances he needs most in a permanently solid form, and in that form they exist. We should have uncomfortable houses, if stone melted as easily as ice. And that we are able so easily to liberate the gas from the solid coal in which it naturally exists, and which, in ordinary circumstances, it does not leave, is the reason that we can give our streets at night almost the brightness of day. We can easily convert water into steam; and let steam-engines in manufactories, on vessels, on the railway-road, declare the advantage. But what could we do if all bodies became liquid and gaseous at the same temperature? If the iron holding the water was liquefied as soon as the water, we could have no containing vessels, any more than we could if iron became steam when water did. I again say, in all these arrangements, see the wisdom and goodness of their Author, and his provisions for the well-being and improvement of

man.

By the way, before we conclude, I would mention au instance of the pleasure of chemical research, as explaining what might otherwise seem inexplicable. And the human

mind is so formed that it likes to know things. Much has been said lately of gun-cotton. You have read about it in the papers, I dare say, and recollect its preparation.

P. Yes; cotton treated with sulphuric and nitric acids. It seems very much unlike gunpowder, which is a composition of charcoal, saltpetre, and sulphur.

T. Yes. But look to elements; and to elements the exploding heat brings them. In the gunpowder,-(I speak generally, and not now with scientific precision; general expressions will suffice for my present objects,)—in gunpowder, then, you have carbon in the charcoal; and in the saltpetre and sulphur, the elements of the nitric and sulphuric acids. Now, what more or less have you in the gun-cotton? In the cotton you have carbon: (what is burnt cotton but charcoal from cotton instead of wood?) in the two acids, you have the same elements that are in gunpowder furnished by sulphur and saltpetre. There is not so much difference, after all. Chemical research brings you to the same elements. But, in our next conversation, we will go on with the important subject of heat or caloric.

CHRONICLES OF THE KINGS OF NORWAY. [OUR fathers were either very credulous, so that their chronicling poets thought they would swallow anything, or keen in the perception of the meaning and application of satire. Chapter xxxvii. of Saga vi. bears this title, as the argument of the piece, "King Harald sends a Warlock in a transformed shape to Iceland." Perhaps popular superstitions might furnish the form of the composition, while its signification was understood to refer to the characters of the inhabitants represented as being found in the different parts of the island by the messenger, or spy, whom the King sent on this exploring expedition. At all events the tale is an amusing one, and illustrative of popular belief as in still darker times it had been."-ED. Y. I.]

KING HARALD told a warlock to hie to Iceland in some altered shape, and to try what he could learn there to tell him: and he set out in the shape of a whale. And when he