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flame will pass through gauze saturated with oil and covered with cannel coal dust.
In cleaning lamps which have had coal dust burnt into a cinder on them, it is very difficult to get it off, and in doing so the gauze is sometimes damaged. Accidents occur frequently from damaged gauze, and too great care cannot be bestowed in examining the gauze before the lamp is used.
Before proceeding to the consideration of mining lamps, it will be well to say something on the nature of flame, as all the safety belonging to them depends on their power to prevent flame passing from the inside of the lamp to the outside. Flame, according to Sir Humphrey Davy, “is gaseous matter heated so highly as to become luminous, and that to a degree of temperature beyond the white heat of solid bodies, as shewn by the circumstance that air not luminous will communicate this degree of heat. This is proved by the simple experiment of holding a fine wire of platinum about th of an inch from the exterior of the middle of the flame of a spirit lamp, and concealing the flame by an opaque body, the wire will become white hot in a space where there is no visible light. When an attempt is made to pass flame through a fine mesh of wire gauze, at the common temperature, the gauze cools each portion of the elastic matter that passes through it, so as to reduce its temperature below that degree at which it is luminous, and the diminution of temperature must be proportional to the smallness of the mesh and the mass of the metal. The power 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 on combustion, will pass through a metallic tissue that will interrupt the flame of less inflammable substances, or those that produce little heat or combustion. Or the tissue being the same, and impermeable to all flames at common temperatures, the flames of the most combustible substances, and of those that produce most heat, will most readily pass through it when heated, and each will pass through it at a different degree of temperature. In short, all the circumstancus which apply to the effect of cooling mixtures upon flame, will apply to cooling perforated surfaces."* The same author also states that when rapid currents of explosive mixtures are made to act on wire gauze, it is of course much more rapidly heated; and therefore the same mesh which arrests the mixtures of explosive mixtures at rest, will suffer them to pass when in rapid motion; but by increasing the cooling surface, by diminishing the size or increasing the depth of the aperture, all flames, however rapid their motion, may be arrested." Sir H. Davy, in one of his latest papers, says “I have had lamps made of thick twilled gauze of wires of 7th of an inch 16 to the warp and 30 to the weft, which being rivetted to the screw caunot be displaced; from its flexibility it cannot be broken, and from its strength it cannot be crushed, except by a very strong blow. Since the discovery of Sir Humphrey Davy in 1816, when he produced a light enclosed in a cylinder of wire gauze 8 inches high, and not more than 2 inches in diameter, the wire being when twilled not less than 7 th of an inch in thickness, and 30 in the warp, and 16 to 18 in the weft; and when plain not less than both of an inch in diameter, and from 28 to 30 both warp and woof, - little has been done in the way of improving it. In common use throughout the United Kingdom, the lamp is pretty much the same as when it came out of the hands of the inventor. For simplicity, utility, and cheapness, the lamp will in the opinion of most practical men be preferred to any yet introduced. The inventor especially cautioned workmen against allowing the gauze cylinder to be heated to dull redness, and moving it against a strong current of air. He saw what we all now know after 30 years' experience, that the locked Davy Lamp with a good shield was a safe instrument in the hands of acareful man who knew the principles on which it was constructed. It never was his idea that it was to be opened and closed, like the door of a common lantern in the open air-placed on the sloping floor of a mine with dry coal-dust on the outside, and oil spilled on the gauze in the inside-moved swiftly against a strong current of an explosive mixture-placed under a falling mass of coal so as to be crushed into all kinds of shapes
-or, when injured to have the light extinguished by being blown out;—yet it is well-known that the lamp has been subjected to all these varieties of hard usage, and very unreasonably been expected to prove "a safety lamp."
I shall now proceed slightly to notice several lamps which have been more or less in use:
1st. That invented by the late George Stephenson, a man equally eminent in practical as Sir H. Davy was in theoretical science, and who without doubt produced a mining lamp before the latter philosopher. This lamp in its complete state when it was known in the north by the name of the “Geordie lamp,” was like a common Davy with a wide glass cylinder in the inside and close to the gauze. It is now in considerable use in some parts of South Yorkshire, but it has never been in great use, owing to the glass being liable to break, and from its going out in an impure atmosphere sooner than a Davy. So long as the glass remains entire the lamp must be safer than a Davy, and when broken it must be as good as that lamp; but still the opinion amongst workmen has been that when the glass was broken the lamp was
not safe. Its diameter, 21 inches, is too much to be thoroughly safe.
2nd. The Upton and Roberts' Lamp. This has the gauze in the inside instead of the outside. Like the Geordie last named it is liable to go out in a foul atmosphere sooner than the Davy. It is a question whether this alleged fault is not a positive advantage, although I know it is strongly objected to by the miners.
3rd. The Solar Mining Lamp. This has never been in general use. It consists in placing a solar lamp in the inside of a common Davy. was of a most dangerous width, being nearly 3 inches in diameter,-a very serious fault with a mining lamp.
4. The Clannys' Lamp. This is a lamp with the gauze placed within the glass. It has come into considerable use in the north, but many parties object to the glass on the outside being liable to be broken, and the light going out in an impure atmosphere.
5. The Muessler, or Belgian Lamp. This is in considerable use in Belgium, but it has not progressed much in England. It is a light simply enclosed in a glass, and consequently not nearly so safe in general use as the lights protected by both glass
I shall now direct your attention to the explosive mixtures of fire-damp. It is a very fortunate circumstance that fire-damp takes a much greater heat to explode it than pure hydrogen, olefiant, or sulphuretted hydrogen gases, or than carbonic oxide. Sir H. Davy found that the ratio of the combustibility of the different gaseous matters was to a certain extent as the masses of the heated matter required to inflame them. Thus, an iron wire of moth of an inch heated cherry red will not inflame olefiant gas, but it will inflame hydrogen gas; and a wire of įth heated to the same degree, will inflame olefiant gas; but a wire of both 'must be heated to whiteness to inflame hydrogen, though at a low red heat it will inflame bi-phosphuretted hydrogen gas; but wire of wth, heated even to whiteness will not inflame mixtures of fire-damp.
A tissue which will not interrupt the flame of hydrogen when red hot, will still intercept that of olefiant gas; and a heated tissue which would communicate explosion from a mixture of oefiant gas and air, will stop an explosion from a mixture of fire-damp or carburetted hydrogen.
Sir H. Davy states—“I first began with a minute chemical examination of the substance with which I had to contend. The analysis of various specimens of fire-damp shewed me that the pure inflammable part of it was light carburetted hydrogen gas, as Dr. Henry had before stated; hydrogen or pure inflammable air combined with charcoal or carbon. I made numerous experiments on the circumstances under which it explodes, and the degree of its inflammability. I found that it required to be mixed with very large quantities of atmospheric air to produce explosion; even when mixed with three or nearly four times its bulk of air, it burnt quietly in the atmosphere, and extinguished a taper. When mixed with between five and six times its volume of air, it exploded feebly; it exploded with most energy when mixed with seven oreighttimes its volume of air, and mixtures of fire-damp and air retained their explosive power when the proportions were one of gas to fourteen of air. When the air was in a larger quantity, the flame of a taper was merely enlarged in the mixture, an effect which was still perceived in thirty parts of air and one of gas. *
“On mixing one part of carbonic acid or fixed air with seven parts of an admixture of fire-damp, or
* Davy on Flame, p. 10.