HEAT.-V.

CONDUCTION - SPHEROIDAL STATE - CONVECTION - RADIANT HEAT-ABSORBING POWER-POWER OF TRANSMITTING

HEAT CONCLUSION.

this fact very clearly. Procure a large melting-pot containing several pounds of lead, and place it over a fire until the lead is not only melted, but quite red-hot. Having washed the hand so as to free it from grease, dip it into a vessel containing strong liquor ammonia, and it may then be plunged into the molten metal, or the lead may be ladled out by it, without any danger. The only sensation produced is one of cold. This experiment is one which few have the courage to attempt, but it is perfectly safe. The heat of the metal evaporates the liquid and drives out the ammoniacal gas from it, and thus the hand is entirely enveloped in a glove of vapour, which prevents contact with the lead. The cold felt arises from the rapid evaporation.

boiling in it. The fur is a nonconducting material, and protects the hand from the heat.

A red-hot poker likewise may be safely struck with the hand. This partly arises from the fact that a quick blow does not allow time for the metal to burn the hand, and partly from the fact that the moisture of the hand is converted into vapour, and prevents absolute contact with the heated iron. Some remarkable phenomena have been observed which illustrate this fact. If we take a silver vessel, and having raised it to a temperature a little above 212°, immerse it in a vessel of water, it will hiss from the sudden conversion of the water into steam, and will speedily be cooled down. If, however, wa heat the vessel to redness, and place it on the surface of water, no effect will at first be produced. It will quietly float for a time without any sound being heard. After a little while, however, a cloud of steam will be suddenly produced, and the usual hissing noise will be heard. A similar thing occurs if a highly heated silver weight be dipped into a vessel of water.

The reason of these apparently strange phenomena is that as soon as the heated metal touches the water, that portion which is nearest to it becomes suddenly converted into steam, and this keeps the silver from contact with the water. A layer of vapour is, in fact, interposed, which prevents actual contact. When, however, the silver is cooled down

These effects were first observed by Leidenfrost, but have since been carefully investigated by others. A simple way of showing them is to take a platinum or silver dish, and having placed a spirit-lamp under it so as to heat it to redness, drop with a pipe a little water into it. The liquid does not spread itself out and moisten the dish as it would at ordinary temperatures, but at once assumes a globular form, and rotates rapidly. Its evaporation, too, is very much less rapid than it would be if it boiled, and its temperature appears only to be about 95° or 100°. The liquid is said to have assumed the spheroidal state, and will remain in this condition if the source of heat is kept under the dish. If, however, it be removed, the heat will gradually diminish, till it is no longer sufficient to maintain the globule in the spheroidal state, and then the liquid will touch the metal, and be immediately thrown into a state of violent ebullition, a large amount of steam being given off.

A remarkable experiment may be tried which will illustrate

lead red-hot, as otherwise it may come into contact with the hand, and a severe burn will then be produced.

After conduction, the next mode in which heat is transmitted is by convection, or the setting up of currents in the liquid or gas to be heated. By this means each particle in succession is directly exposed to the source of beat, and thus has its temperature raised.

There are several ways in which convection may be illustrated:one of the best is to take a glass vessel filled with water (Fig. 25), and having dropped in a few fragments of litmus or a little cochineal, place a spirit-lamp under it, and watch the liquid. A stream will begin to rise directly over the lamp, its course being clearly shown by the coloured particles. This stream will rise to the top of the vessel, where it will spread out and forin a down current at the sides, and in this way all the liquid will in turn be exposed to the heat.

Another way in which this circulation may be shown is represented in Fig. 26. Two glass tubes are bent as there shown, the lower one being filled with coloured water; the other is filled with clear water, and is inverted into the funnel-shaped ends of the first. A spirit-lamp is now held to one side so as to warm the liquid there, and the coloured part of the liquid in that limb will at once begin to rise, and to descend in the other.

On this principle the hot-water apparatus frequently employed for warming large buildings is constructed (Fig. 27). A furnace and boiler are placed at the lower part of the building. From this a pipe, M, passes to a cistern, q, at the top, provided with a safety-valve,; and from it pipes lead to the stoves, a, b, c, d, e, f, in the various rooms. The water traverses these on its way back to the boiler, and gives up to them much of its heat. The water heated by the furnace becomes, of course, specifically lighter, and hence rises, while that which has been cooled by its passage through the pipes descends, and in this way a constant circulation is maintained.

Gases as well as liquids are heated by convection. The trade-winds are grand natural illustrations of this fact: the air having become heated by contact with the surface of the earth in tropical regions, expands, and rises, making way for the currents of colder air from the poles. In the higher regions of the atmosphere a current usually sets in the contrary direction to that on the earth's surface, and thus forms the return current. Land and sea breezes are further exemplifications of the same fact.

The third way in which heat is communicated from one body to another is by means of radiation. In conduction and convection the particles of matter to be heated were brought into close contact with the source of heat: we shall now find, however, that heat can pass from one body to another without actual contact, and even without altering the temperature of the medium through which it passes. A striking illustration of the latter fact is seen in the experiment of setting light to various substances by condensing the sun's rays on them through a lens of ice. The heat passes through it in sufficient quantities to inflame the substances, and yet the ice is unmelted.

When we stand a little distance from a fire we at once experience a sensation of warmth, no particles of matter appear to pass, and yet the influence of the fire is felt. Rays of heat are given off by the burning fuel, which create in us the feeling of

Fig. 30.

warmth. The presence of the air is evidently not necessary for their passage, since we experience the heat of the sun, whose rays must pass through space. We may also prove this fact experimentally by letting two charcoal points connected with a powerful battery touch under an exhausted receiver. Rays of heat will be given off despite the absence of the air, and their presence will at once be felt.

Now we find that radiant heat obeys the same laws as light does, the rays being given off in all directions, and, in a uniform medium, always travelling in a straight line. This may easily be shown by suspending a heated body in the air, and then holding a thermo-electric pile at equal distances on each side of it. If, however, a plate of metal be interposed between the pile and the source of heat, the rays will at once be intercepted, and the needle will return to zero. The power of radiant heat diminishes, as in the case of light, with the square of the dis

tance.

If we take a heated body, such as a cubical vessel, M, filled with boiling water (Fig. 28), and place it in front of a concave mirror, we shall find that the rays of heat are reflected from its surface, in the same way as those of light are. Let a differential

thermometer be placed in the focus of the mirror, a screen, A, being placed so as to keep off the direct rays from M. The indicating bubble will at once show the increase of temperature; if the bulb be moved at all out of the focus, the bubble will return to its place, clearly showing that the rays have been reflected and brought to a focus. By means of a small mirror we can easily prove that in the case of reflected heat the angle of incidence is always equal to the angle of reflection. An ordinary sheet of

M

Fig. 28.

137

parts. In doing so we find that at the violet end of the spectrum there is but little heat; even in the yellow, though that is the most luminous part, there is not much. At the red portion the heat is greater, but its intensity is greatest when the pile is moved altogether beyond the visible spectrum, so that the most intense portion of the heat is altogether non. luminous. The thermal spectrum, in fact, overlaps the visible one.

When we commence to try experiments on the radiation of heat, we soon find that different surfaces possess different powers of throwing off rays of heat. This is easily shown by means of a "Leslie " cube (Fig. 29), which consists simply of a tin or pewter cube with an opening on one side, by which it can be filled with boiling water. One side may be covered. with a layer of gold-leaf, another with glass, a third with lampblack, while the fourth is left

blank. Each side is now turned in succession towards the thermo-electric pile, and the exact deflection of the needle noted. Other substances may then be laid on the sides of the cube, and in this way a table showing the radiating power of different bodies may be drawn up.

When the gilded face is towards the pile, little effect will be produced; if the pewter be a little tarnished, a greater deflection will be produced when that side is turned to the pile. When the glass side is presented, the intensity will be much more, while with the lamp-black it will be most of all. lamp-black is the best radiator, its power is represented by 100, and then the power of gold and other brilliant metals will be between 12 and 15.

As

Heureux, disait Mentor, le peu

ple qui est conduit par un sage roi,

:

Happy, said Mentor, the people who are governed by a wise king. (4.) In interrogative sentences the noun generally precedes the verb, which must be immediately followed by a pronoun corresponding in gender, number, and person with the noun :La mort est-elle un mal? La vie Is death an evil? Is life a benest est-elle un bien? Literally, Death, is it an evil? etc.

(5.) When the sentence commences with one of the following words-où, where; que, what; combien, how much; quand, when-the noun may be placed immediately after the verb, or in accordance with the last rule :

This must also be the case when the régime direct is shorter than the régime indirect.

(8.) When, however, the qualifying or explanatory words render the direct regimen longer than the indirect, the régime indirect is placed first:

Avez-vous donné à mon frère les Have you given my brother the livres que vous lui aviez promis? books which you had promised him! the meaning would otherwise be doubtful:(9.) The indirect regimen precedes the direct regimen when

Tâchez de ramener par la dou- Try to bring back by mildness ceur ces esprits égarés, these erring spirits.

Any other construction would render the sentence equivocal. (10.) In English the name of the possessor frequently precedes the name of the object possessed; and the two are con nected by means of 's (the old Saxon genitive termination). In French the order is always different. The name of the object precedes that of the possessor, and the connecting link is a preposition

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