I have made two or three lanterns of different forms. Experience must determine which will be the most convenient.
Should there be a little delay in sending them, it will be the fault of the manufacturer. It is impossible to conceive the difficulty of getting any thing made in London which is not in the common routine of business; and I should be very sorry to send you any thing imperfectly executed.
With best respects to Mrs. Gray, I am, my dear Sir,
Very sincerely your obliged servant,
H. Davy.
The paper alluded to in the preceding letter, entitled, "On the Fire-damp of Coal Mines, and on methods of lighting the mine so as to prevent its explosion," was read before the Royal Society on the 9th of November, 1815.
In this memoir he communicates the results of some chemical experiments upon the nature of the fire-damp, and announces the existence of certain properties in that gas, which had previously escaped observation, and which leads to very simple methods of lighting the mines without danger.
He confirms the opinion of Dr. Henry, and other chemists, as to the fire-damp being light carburetted hydrogen gas, and analogous to the inflammable gas of marshes; but he found that the degree of its combustibility differed most materially from that of the other common inflammable gases, which it is well known will explode by the contact of both red-hot iron and charcoal; whereas well-burned charcoal, ignited to the strongest red heat, did not explode any mixture of the air and of the fire-damp; and a fire made of well-burned charcoal, that is to say, of charcoal that will burn without flame, was actually blown up to whiteness by an explosive mixture containing the fire-damp without producing its inflammation.[31] An iron rod also, at the highest degree of red heat, and even at the common degree of white heat, did not inflame explosive mixtures of the fire-damp; but when in brilliant combustion, it produced the effect.
He moreover found that the heat produced by the combustion of the fire-damp was much less than that occasioned by most other inflammable gases under similar circumstances; and hence its explosion was accompanied with comparatively less expansion: a circumstance of obvious importance in connection with the propagation of its flame.
Numerous experiments were likewise instituted by him with a view to determine the proportions of air with which the fire-damp required to be mixed, in order to produce an explosive atmosphere; and he found the quantity necessary for that purpose to be very considerable; 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 freely. The mixture which seemed to possess the greatest explosive power was that of seven or eight parts of air to one of gas.
On adding azote and carbonic acid in different proportions to explosive mixtures of fire-damp, it was observed that, even in very small quantities, these gases diminished the velocity of the inflammation, or altogether destroyed it. In this stage of the enquiry, the important fact was discovered, that explosive mixtures could not be fired in metallic tubes of certain lengths and diameters.[32] In exploding, for instance, a mixture of one part of gas from the distillation of coal, and eight parts of air, in a tube of a quarter of an inch in diameter and A foot long, more than a second was required before the flame reached from one end of the tube to the other; and not any mixture could be made to explode in a glass tube of one-seventh of an inch in diameter. In pursuing these experiments, he found that, by diminishing its diameter, he might in the same ratio shorten the tube without danger; and that the same principle of security was obtained by diminishing the length and increasing the number of the tubes, so that a great number of small apertures would not pass explosion when their depth was equal to their diameter. This fact led him to trials upon sieves made of wire-gauze, or metallic plates perforated with numerous small holes, and he found that it was impossible to pass explosions through them.[33]