Estimation of Sensible Heat.

415 body, is deemed the coldest, simply because it conducts or carries off heat the fastest. Were a similar experiment made in a hothouse, or in India, while the temperature of every object around was at 98°, or that of the living body, then not the slightest difference would be felt in any of the substances. Or, lastly, were the experiment made in a room where, by any means, the general temperature were raised considerably above blood-heat, then the carpet would be deemed the coolest instead of the warmest, and the other things would appear hotter in the same order in which they appeared colder in the winter room. Were a bunch of wool and a bar of iron exposed to the severest cold of Siberia, or that of an artificial frigorific mixture, a man might touch the first with impunity (it would merely be felt as rather cold); but if he grasped the second, his hand would be instantly frozen. Were the two substances, on the contrary, transferred to an oven, and heated as far as the wool would bear, he might again touch the wool with impunity (it would then be felt as rather hot), but the iron would burn his flesh. The writer once entered a place where there was no fire, but where the temperature from hot air admitted, was sufficiently high to boil the fish, &c., of which, in another room, he afterwards partook at dinner. He breathed the hot air without uneasiness. He could bear to touch woollen cloth in the hot room, but no substance more solid.

605. Those who indulge in Turkish baths, in which the air is sometimes heated to 170° or 180°, can sit or tread only on wood or some badly conducting material. The contact of any metal with the skin produces a painful sense of burning. For this reason, the bather is wrapped in a blanket, and is provided with a pair of wooden slippers, in order to protect his feet against the great heat of the marble floor.

Liquid mercury, by reason of its great conducting power, feels cool to the hand plunged into it; but a bar of the metal in a solid state cannot be touched without instantly destroying the life of the part. In passing to the liquid state by contact with the skin, the metal, which has a temperature of 40° below zero F., absorbs and removes all the heat of the part with which it is in contact. The part dies, and finally separates by mortification.

The physical condition of a substance at a very low temperature will greatly affect its conducting power. Solid carbonic acid, which is a white snowy-like solid, at a temperature considerably below that of frozen mercury (— 106°), may be held in the palm of the hand

416

Influence of Clothing.

and even pressed, without any particular sense of coldness or pain. There is merely a slight pricking sensation. It is rapidly evolving a gaseous body, namely carbonic acid, which, as a gas, is a nonconductor, and thus direct contact with the skin is prevented. If a small quantity of ether is poured over it, the solid acid is instantly dissolved, producing a cold of 166° below zero, and if this comes in contact with the skin, its vitality would be instantly destroyed.

The non-conducting power of glass, and the effect of suddenly cooling one side, is singularly illustrated by the action of the liquefied gas. If some solid carbonic acid is placed on the surface of thick plate glass, and a small quantity of ether poured upon it, the glass instantly flies to pieces, owing to the sudden and intense cold produced.

606. It is a vulgar error, then, to suppose that there is a positive warmth in the materials of clothing. The thick cloak which guards a Spaniard against the cold of winter, is also in summer used by him as a protection against the direct rays of the sun; and while in England, flannel is our warmest article of winter dress, yet we cannot more effectually preserve ice in summer than by wrapping the vessel containing it in many folds of the softest flannel. That which prevents the heat from penetrating, also prevents it from escaping.

In every case where a substance of lower temperature than the living body touches it, a thin surface of the substance immediately in contact shares the heat of the bodily part touched-the hand generally; and while, in a good conductor, the heat so received quickly passes inwards, or away from the surface, leaving this in a state to absorb more, in the tardy conductor the heat first received tarries at the surface, which consequently soon acquires nearly the same temperature as the hand, and, therefore, however cold the interior of the substance may be, it does not cause a strong sensation of cold. The hand on a good conductor warms it deeply; on a slow conductor, only superficially. The following cases further illustrate the same principle :-Wrap a layer of paper tightly round a brass or iron cylinder about an inch in diameter, and round a wooden cylinder of the same dimensions, and hold them over the flame of a spirit or gas lamp; the paper on the wood will begin to burn immediately, while that on the metal will resist the heat for some time; or, if pieces of paper be laid separately on a wooden plank and on a plate of steel, and a burning coal be then placed on each, the paper on the wood will begin to burn long before that on the plate. The explanation is, that the paper in contact with the

The principle of the Safety Lamp.

417 good conductor, imparts to it so rapidly the heat received from the coal, that it remains at too low a temperature to inflame, and will even cool to blackness the part of the coal which is in contact with it; while on the tardy conductor, the paper becomes almost immediately as hot as the coal, and burns. It is because water open to the atmosphere cannot be heated beyond 212°, that it may be made to boil in an egg-shell or a vessel made of paper, held over a lamp, without the containing substance being destroyed; but as soon as the water is dried up, the paper will burn and the shell will be calcined. The heat is here transmitted through the substance to the water, and is carried off in vapour.

As a remarkable instance of the useful application of these principles regarding the conduction of heat by metals, may be mentioned the Miner's Safety Lamp, invented by Sir Humphry Davy.

607. Davy found that fine iron wire woven into a metallic gauze, having from 700 to 800 meshes in the square inch, operated as a powerfully conducting surface, and rapidly cooled flames below their igniting or combustible

Fig. 163.

b

of the gauze, and be inflamed above it, at d, continuing to burn without igniting the jet of gas below the gauze, the heat of the flame being so reduced by the conducting power of the fine metallic wires as to cool it below the temperature required for ignition. Here, then, in spite of the presence of an inflammable, and even explosive, mixture of coal-gas and air below the gauze, and between it and the jet, there is no communication of the flame, which cannot pass down. If, however, by continued combustion, the wire gauze becomes red-hot, the mixture of gas and air below it will be ignited, the heat of the flame not being cooled by conduction under these circumstances. So, if the miner continues tc work with the wire gauze of his lamp red-hot, the flame may tra

118

Insulation of Flame by Wire-gauze.

verse the gauze, and lead to an explosion. A heat of full redness is, however, required for this purpose.

608. The following experiments will serve as further illustrations of the power of conduction in iron wire gauze. Place a piece of camphor in the middle of the gauze, and hold it over the flame of a spirit-lamp. The camphor will take fire, and burn below, but not above, although there is a large quantity of highly inflammable camphor vapour escaping from it. Tow soaked with alcohol or ether may be substituted for the camphor with similar results. Into a small cylinder of wire gauze, closed with gauze at the bottom, but open at the top, throw a mass of tow soaked in alcohol and inflamed. The cylinder may now be placed in a saucer containing alcohol, when the whole of the spirit will be gradually drawn through the meshes and consumed within the cylinder, but there will be no inflammation of that which is on the outside. Let the same cylinder be fitted closely to a wire-stand supporting a wax taper, so that when the taper is kindled it may be completely enclosed by the gauze cylinder placed over it, and can receive air only through the meshes. If a jar of hydrogen, or coal gas, or a mixture of either with air, be brought over the cylinder inverted, and very gradually lowered, the inflammable gas will traverse the gauze and burn with flame, but without explosion, in the interior of the cage, but there will be no kindling of the explosive mixture on the outside. In this manner the whole of the inflammable gas may be consumed without being inflamed in the jar, whereas, if any such mixtures were brought over the lighted taper not covered by the gauze cylinder, there would be instantaneous inflammation and explosion.*

609. The Miner's Safety Lamp.-The safety lamp is an oil-lamp in which the flame is surrounded with a cylindrical cage of fine wire gauze (see fig. 164, which represents it entire, and fig. 165, in section). The wire gauze is double at the top to guard it against the heat of the flame. There is a framework of strong iron wire to support the cage, and by an ingenious arrangement, a brass tube

* The efficiency of wire gauze shields as a protection against flame, was shown in some remarkable experiments performed by Aldini. An avenue of fagots was kindled, and a fireman clothed in asbestos and armed with a large iron wire-gauze shield, ran through the flames, repelling them before him by means of the shield. The non-conducting power of the iron wire temporarily protected his person from the scorching effect of the flame As an experiment the result was successful. (See Art. 58, p. 23.)

The Miners' Safety Lamp-its use.

419

passes up through the oil, in which a wire bent at the top works stiffly, so that the wick may be trimmed without rendering it neces

sary to remove the cage. The lamp when properly used, not only protects the miner from the danger of explosion, but serves as an indicator of the safety or danger of the atmosphere in which he is working. If the air is pure, the lamp burns with a bright clear flame, the light somewhat reduced by its having to traverse the gauze. If a small quantity of fire-damp is present, the flame becomes elongated and smoky. In larger and dangerous quantities the flame is extinguished, and the mixed gas and air burn inside the gauze cylinder without igniting the mixture outside. Taken into pure air, this inner combustion ceases, and the wick is re-kindled. remain with the mixed gases burning inside, the gauze may become red hot, and then the flame will traverse and cause an explosion. There are other sources of danger under these circumstances. A small particle of coal-dust falling upon the heated wire of the cage may be kindled into flame on the outside, and the fire-damp exploded.

Fig. 165.

If allowed to

Blowers or strong currents of air may act like a blow-pipe on the flame, and cause the point of the flame to heat to redness and traverse the wire gauze. A blast of gunpowder will have a similar effect, but the use of gunpowder in fiery mines is always attended with danger. The safety-lamp under such circumstances can afford no protection.

The efficiency of the lamp and the correctness of the principles on which it is constructed have been established by numerous experiments on mixtures of coal-gas and air. The writer has frequently lowered it into the most explosive proportions of fire-damp and air, with no other effect than that of extinguishing the light, and causing a volume of flame to appear in the interior of the cylinder. The awful destruction of life which takes place yearly from coal-mine explosions may be generally traced, when traceable at all, to carelessness in removing the cage, or making holes in the wire gauze for additional light, the use of tobacco, lucifer-matches,