PREFACE.

IN terminating the second year of their labours, the Editors of this Journal proceed, consistently with the plan which they formerly announced, to draw a retrospective outline of the progress of those principal branches of Science and Art, which properly belong to the object of their work; and if it be found that during the last twelvemonth no very brilliant discoveries have been added to the general stock, it will, nevertheless, be allowed to have been enriched with many useful and important inventions, and to have received a considerable increase of facts and observations. Although therefore the march of science has not been rapid, it has been sure, and we have nothing to apprehend from those retrograde movements which are apt to succeed too rapid strides.

Regarding the Philosophical Transactions as the standard of English Science, we shall first notice the contents of the volume for 1817, and afterward advert to the novelties which have been brought before the public by other channels of information, and from other sources.

Among the chymical papers, Sir Humphry Davy's researches on flame obviously stand fore

most; they not only contain many new philosophical facts, and tend to elucidate some recondite chymical phenomena, but what is of more importance, they develope principles applicable to the purposes of common life; among them are those upon which the security of the miner's lamp depends, and which we have elsewhere frequently adverted to. The question, What is flame? is for the first time satisfactorily answered in this paper. It is aëriform matter, heated so highly as to be luminous; and when luminous, its temperature is considerably beyond that which is commonly called a white heat; so that air may be made hot enough to impart a white heat to solid bodies, and yet not become luminous itself; as may be easily shown by holding a piece of thin platinum wire over the chimney of an Argand lamp fed with spirit of wine, or even by the common expedient of lighting a piece of paper, by exposing it to the current of hot air which rushes out of a common lamp glass. Such being the nature of flame, it is further obvious, that if we cool it by any means, we must at the same time extinguish it; and this is accordingly done, by passing it through the metallic apertures of fine wire-gauze, or any other substance which has considerable conducting and radiating powers in regard to heat, or which, in other words, is capable of producing a cooling effect. So a piece of wire-gauze placed in the centre of the flame of a candle, cuts it as it were in half, the upper part

being extinguished by the cooling power of the gauze, while the lower part remains luminous, because of a temperature sufficiently high.

The power, therefore, of a metallic or other tissue to prevent explosion, will depend upon the heat required to produce the combustion as compared with that acquired by the tissue; and the flame of the most inflammable substances, and of those that produce most heat in combustion, will pass through a metallic tissue that will interrupt the flame of less inflammable substances, or those that produce little heat in combustion; so that different flames will pass through at different degrees of temperature.

It fortunately happens that the fire-damp of coal mines requires a very high temperature for its inflammation, and, consequently, even a coarse tissue will have sufficient cooling powers to prevent its explosion, and security is proportionally easily attainable.

That flame may be extinguished simply by cooling, Sir Humphry ingeniously shows by putting a coil of cold platinum wire close to a small flame of a spirit lamp: it goes out in consequence of the heat carried off by the wire: which is not the case if the wire be previously heated: or to descend to a more common illustration-when we blow out a candle, the extinction of the flame is produced by the cooling power of the current of air projected into the flame, and the hottest flames are least easily blown out.

There is therefore nothing mysterious, recondite, or difficult to be understood in the operation of the safety-lamp. The flame being surrounded by wire-gauze, nothing can enter or pass out of the cage in a state of inflammation; and when the fire-damp gets in, it burns without being able to communicate with the exterior inflammable atmosphere.

Another interesting subject discussed in this paper, relates to the nature of the light of flames, and their form. When pure gaseous matter is burned, the light is very feeble, and the density of a common flame is proportional to the quantity of solid charcoal first deposited and afterward burned. The flame of pure hydrogen is pale blue, and emits very little light; but if we throw into it metallic filings, small pieces of platinum wire, powdered charcoal, or any other solid matter, its light becomes increased by the ignition of this extraneous addition. It is precisely thus with the flames of candles, lamps, and carburetted hydrogen, or as it is now emphatically called, gas.

The inflammable element is pure hydrogen; the whiteness and intensity of the light being produced by a quantity of ignited carbonaceous matter given off by the decomposition of the inflammable matter, and heated white hot. The form of flame is conical, because the greatest heat is in the centre of the explosive mixture. In looking steadfastly at flame, the part where the combustible matter is volatilized is seen, and it

appears dark, contrasted with the part in which it begins to burn: that is, where it is so mixed with air as to become explosive. When the wick becomes clogged with charcoal, it cools the flame by radiation, and prevents a proper quantity of air from mixing with its central part; hence the charcoal thrown off from the top of the flame is only red hot, and much escapes unconsumed.

The facts stated by Sir H. Davy, in the first section of this paper, show that the luminous appearances of shooting stars and meteors cannot be owing to any inflammation of elastic fluids, but must depend upon the ignition of solid bodies. Dr. Halley calculated the height of a meteor at ninety miles; and the great American meteor which threw down showers of stones, was estimated at seventeen miles high.

The velocity of motion in these bodies must in all cases be immensely great; and the heat produced by the compression of the most rarefied air from the velocity of motion, must be probably sufficient to ignite the mass; and all the phenomena may be explained, if falling stars be supposed to be small bodies moving round the earth in very eccentric orbits, which become ignited only when they pass with immense velocity through the upper regions of the atmosphere, and if the meteoric bodies which throw down stones with explosions be supposed to be similar bodies containing com bustible or elastic matter.