metal reflect heat, and absorb comparatively little; that scratching or in any way roughening the surface of a metallic body increases its power of absorption, and blackening it with anything increases it still more.*

Experiment. Take, for instance, three circular pieces of metal, as tin, nine inches in diameter, and raised on a stand of a few inches high-one smooth, another scratched and roughened, the third blackened-the back of each being smeared with tallow, or some substance which melts at a low temperature; then placing a red-hot ball of iron at equal distances from any two of them, it will be found that the tallow on the blackened one will very soon melt, that on the roughened surface next, while the smooth surface would remain nearly at the temperature of the room; of course this experiment might be tried with different substances, and metals scratched and blackened in different degrees.

Another of Leslie's experiments. Take a cubical vessel, made of tin, one surface blackened, a second scratched, the third more roughened, and the last smooth; fill it with boiling water, and place the differential thermometer near it, and turning each side in succession towards it, it will be found that the quantity of heat radiated, or thrown off from the different surfaces, will be in the order mentioned above. Professor Leslie covered the surface of the vessel with thin plates or layers of different substances of different colours, and noted the number of degrees which the thermometer ose, and thus ascertained the radiating power of each par icular covering.

He then, instead of blackening or otherwise meddling with the faces of the tin vessel, made it perfectly smooth, and covered the bulb of the thermometer with the different

* See List of Apparatus at the end of the volume.

substances, and found by the way in which it was affected, that they absorbed heat much in the same way as they had before radiated it when on the tin vessel.

His experiment of heat reflected from parabolic reflectors is a very curious one, and they are well worth the expense of purchasing, in order to try the experiment, from the instruction it gives. A pair of these reflectors is a useful apparatus in a school.

Although heat is emitted from every point in the surface of a hot body in all directions, it is not emitted in all directions with the same intensity. The intensity of the heating ray is as the sine of the angle which it makes with the surface, and therefore those rays have the greatest heating power which are emitted at an angle of 90°.

As an instance of roughened bodies absorbing heat and then radiating it again, and of polished surfaces reflecting it-take the case of a blackened rough fender and polished fire-irons—the latter are generally nearer the fire than the fender, touch them and they will be found much the coolest; the fender having absorbed the heat, the irons reflected it. The different degrees in which bodies absorb heat depends also on colour.

Dr. Franklin observed, that when he laid pieces of differently coloured cloth upon snow, it melted more rapidly under the dark colours than the light. And black and red inks, for example, when exposed to the sun, become heated in different degrees from their absorbing the light which falls upon them, and consequently the heat in different degrees; while pure water seems to transmit all the rays equally, and is not sensibly heated by the passing light of the sun.

The teacher should also note the difference between the radiation of heat from the sun and that from any other bodies - that from the sun passing through air and glass, water, etc., the other not, or if so, in a very slight degree. The following experiment, attended with no expense, affords a good practical hint-two old tea-pots will serve, one of white metal, the other of black earthenware.

Fill them with boiling water, or with hot water from the same kettle after standing a given time, place a ther

-

mometer in them, and it will be found that it will stand much higher in the metal one than in the other; showing that for the purposes of making tea the metal one is the better, not radiating the heat so rapidly; but if placed before the fire the black one will absorb heat better than the other. A black earthen teapot loses heat by radiation, in the proportion of 100; while one of silver or other polished metal loses only as 12.

Thus hot water running in a blackened pipe or rough one, will give out its heat more rapidly than in a polished smooth one.

A solid, when changed into a fluid state, absorbs heatsome solids soften in melting, as wax, tallow, butter, and then become fluid; others, as ice, change at once.

In changing from a fluid to a state of vapour, heat also is absorbed; on the contrary, bodies in passing from vapour to fluid, and from fluid to solid, give out heat.

Water in freezing gives out heat, while in the melting of snow and ice heat is absorbed; hence the chilling cold felt in a thaw after there has been a great fall of snow; also the gradual melting, in consequence of the latent heat' in changing from snow into water.

Fluids become vapour also at different temperatures, their boiling-points depending upon the pressure of the atmosphere, which varies with the altitude above the level of the sea, as well as from other causes; they may also be heated beyond their boiling-point in the atmosphere, by subjecting them to artificial pressure.

The following questions will suggest a few important things, on which the teacher who wishes to understand this subject may inform himself.

Why, as water in boiling becomes vapour, and, as it were, boils away, does its temperature not rise above 212°? When all converted into steam at 2120, what would take place if immediately condensed? What has become of all the heat required to convert the water into vapour, and how would it show itself when the steam is condensed?

If the steam were heated above 2120, how is its expansive force increased? Simply as the temperature, or in a higher ratio?

The disruption of vegetable substances produced by the passage of the electric fluid through a tree is caused by the intensity of the momentary heat converting the fluid of the wood into steam.

At what temperature does water vaporize?

What do you mean by saying that a liquid boils?

Describe the relation between the boiling power of a liquid and the pressure of the atmosphere above it-specify the effect on this boiling-point.

1. By artificially attenuating this atmosphere.

2. By artificially condensing

What is high-pressure steam?

Why, when a mass of ice is dissolved from the heat of a room, or in a vessel on the fire, does the temperature of the water not rise, so long as any ice remains undissolved (test this by placing a thermometer in melting ice), and why does it rise as soon as it is all melted?

Water being kept perfectly still, may be cooled many degrees below the freezing-point, but if shaken, ice would immediately be formed. The extent to which it freezes at once when shaken depends upon this, whether the quantity of heat given out on freezing is sufficient to raise the temperature of the rest higher than 32°. If, for instance, the mass is cooled to 10° below the freezing-point, then only 14th is immediately frozen, and in becoming solid it has given out sufficient heat to raise the temperature of the rest up to the point of freezing.

The circumstance of water, when cooled below 39° of Fahrenheit, expanding when further reduced in temperature, should be noticed this is shown from ice being lighter than water-from the bursting of water-pipes when frozen.

How beautiful the design of Providence in this arrangement, that when the surface water is near the freezingpoint, being lighter than that which is underneath, it cannot sink. If it had followed the general law, rivers would begin to freeze from the bottom, and become a solid mass of ice fish and all the other inhabitants of the water would be destroyed: ice is alsó a bad conductor.

Why can the human body bear to be brought in contact

with air at a much higher temperature than with a fluid with a fluid than with a solid, such as hot iron?

A fluid boils, when its temperature is raised to such a point that the elasticity of its vapour is sufficient to overcome the pressure which is acting upon it: whether from the cohesiveness of the substance itself, the pressure of the atmosphere, or any other artificial pressure.

This explains the principle of a vessel called Papin's Digester, made to extract all the nutritive matter from bones. It is a cylindrical vessel, capable of resisting great pressure; closed by a stopcock, which will resist a pressure of many atmospheres. Of course, in this, water may be heated far above the ordinary boiling-point, and from its greater heat, most animal substances are made to dissolve. The boiling-point is not changed by bodies mechanically mixed in a fluid as sand in water; but it is by all those chemically united with it. All soluble salts retard the boiling point of water, and substances such as starch, mechanically mixed with it, retard its cooling.*

The processes in the arts and manufactures carried on by distillation and evaporation should be noticed. The continual evaporation going on at all temperatures from every part of the surface of the globe - land and water, animal and vegetable-increasing the transparency of the atmosphere, sometimes when most charged with vapour it is most transparent-at others forming clouds, descending in rain to supply our rivers and springs, and to sustain the whole animal and vegetable world.

Formation of vapour absorbs heat, and therefore produces cold-instance a wet towel applied to the temples in case of headache - sometimes wrapped round a bottle containing anything which requires to be cooled-damping the mats in a doorway-a damp bed a very dangerous thing, for want of exercise to generate heat in the body, so as to counteract the cold in drying, etc. That evaporation pro

* The reader will see some interesting tables on the freezing and boiling points of liquids, etc., on the melting-points of solids, such as fat, metals, etc., at the end of the volume on Heat in Lardner's "Cyclopædia;" as also on their expansions at different temperatures. See pp. 214-216.