engraving to represent a heated body; the star-like lines emanating from it may denote that heat is escaping, or radiating from it in all directions, quite independent of conduction or of convection.

Fig. 32.

A few simple experiments of the chemical laboratory will serve as an introduction to the grand natural phenomenon of radiation.

A plate of metal, either dull or bright, is an excellent conductor of heat; if, for example, a lighted coal be placed upon any part of its surface, this will soon become equally warm throughout its whole extent.

At the distance of four or five feet in front of a clear glowing fire, hold a large bright tin-plate, in any position, and it will not become warm; turn it about, present its surface at different angles to the fire, and at last a certain angle will be found, at which, although the plate remain cold, heat will instantly dart from its surface, and may be sensibly felt upon the face or hands.

Thus, in chemical language, heat is said to radiate from the glowing fire, through the intervening non-conducting air, to impinge upon the metal plate, and although it is an excellent conductor of mere heat of contact, as that of the lighted coal in the first experiment, it is not a conductor of this radiant heat which reaches it from

the distant fire; it is not even a retainer of this radiant heat; but it is an admirable reflector of this imponderable element, and accordingly it instantly darts upon surrounding objects, whilst the metal plate or reflecting surface remains perfectly cold.

Place the bright tin-plate flat upon a table, fold half a sheet of fine emery-paper smoothly round a small book, or a flat piece of cork, and with this rub the surface of the tin-plate all over, in the direction of its length; then do the same in that of its breadth; and thus effectually destroy all brightness or polish.

Now hold the tin-plate in front of the fire, as at first; it will soon become hot, and it will reflect but little heat; thus, by mere mechanical alteration of its surface, the metal is instantly rendered a retainer of radiant heat, and deprived of its power of reflecting such agent.

Select another tin-plate, brush one of its bright surfaces over with weak glue; then sift fine sand over this, and allow it to dry; thus the bright polish is covered, and not destroyed; and upon holding this sanded side in front of the fire, as already directed, the tin-plate will become hot, but no heat will be reflected.

Similar will be the result, if the tin-plate be painted with a mixture of weak glue and lamp-black, whiting, yellow ochre, Venetian red, Brunswick green, or any pigment that may be chosen.

In like manner, if a large flat surface of paving stone, or an unglazed earthen paving-tile, be held in front of the fire, no heat will be reflected from them; but they will absorb heat, and soon become warm.

The following striking experiments will afford farther illustrations concerning the extraordinary manner in

which the mere alteration of the lustre of a metallic surface affects its habitudes with radiant heat.

Select a large tin-plate, equally bright and polished on both sides, and with a strong pair of scissors, or small shears, cut it across to form two pieces similar in size; leave one of these without any further preparation, but blacken one side of the other with lamp-black, mixed with thin glue, and allow it to dry.

Provide a strip of wood, one foot long, one inch thick, and four inches wide, and with a saw, make two cuts or grooves across this, each of these being an inch from each of the ends of the wood, and each sufficiently deep to hold a tin-plate firm and upright when inserted on its shortest edge.

Place each plate in its groove, the black surface of the one being opposed to the bright surface of the other; attach a marble to the centre of the exterior surface of each with a lump of pomatum, then hold a red-hot heater of an "Italian iron" exactly between the plates, as shown in the annexed engraving.

The plates are thus exposed to the same degree of heat that is radiating from the red-hot iron; but they have different habitudes with it; the bright surface will reflect

nearly all, and will not become sufficiently hot to melt the pomatum, and so cause the marble to drop; whilst the black surface will absorb nearly all, and will soon become sufficiently hot for such purpose.

By experimenting with great accuracy, the chemist has discovered, that as the unmetallic surface will absorb heat with the most facility, so will it part with such heat, or cool, or radiate such heat with the most facility; the following experiment will prove this fact sufficiently well for our present purpose.

Take the tin canisters that were used in the experiment at page 113, cleanse the adhering oil from the one by washing it thoroughly with a strong solution of pearlash, then rinse it with water, and polish its exterior as bright as possible; paint the exterior of the other with the black mixture already described, and allow it to dry. Pour an equal quantity of cold water into each, place on their respective covers, and insert the thermometers, that the bulbs may dip beneath the water, the bulb of the oily thermometer having been cleansed as above directed.

Lay a soft brick flat upon the table, and in its centre, with a knife or chisel make a hole about three-quarters of an inch deep, and sufficiently large to admit the end of the iron-heater of a tea-urn; make the heater red-hot,

and then drop it into the hole, and place a canister to stand at each end of the brick.

They will be equally distant from the radiant heat of the iron-indeed, more accurately so than the tin plates were in the last experiment-but still the black surface will heat the water the fastest, as an inspection of the thermometers will prove.

Let it heat to 100 degrees;-using a fresh heater, if necessary; then remove the canister from the brick, and place it upon the table; remove the bright canister also, throw away its almost cold water, and fill it with water heated to the same degree as that contained in the black canister, and place it likewise upon the table;-watch the thermometers, and they will indicate that the water in the black canister is cooling, or radiating heat, more rapidly than that in the bright canister, which will remain sensibly warm when the other is cold.

These results are directly contrary to those which might be anticipated to ensue; we might think that by coating a metal plate with black paint, a bad conductor, it would prevent the heat from coming into contact with the metal beneath, and so keep it cold; and, on the other hand, the bad conductor would keep in the heat of the water; but the chemist proves that unmetallic, or earthy substances, though extremely bad conductors, are most excellent absorbers and radiators of heat.

Another curious and instructive experiment concerning this power of surfaces in radiating heat may be made by covering one of the sides of a cubical tin-plate canister with lamp-black, another with thin jelly, a third with writing-paper, and leaving the fourth bright and polished: then, upon filling the canister with boiling water, and approaching the sides in succession with the hands, or a