"There is a fourth lamp, by means of which any _blowers_ may be examined in air in which respiration cannot be carried on: that is, the _Charcoal lamp_. This consists of a small iron cage on a stand, containing small pieces of _very well burnt_ charcoal blown up to a red heat. This light will not inflame any mixtures of air with fire-damp.[30] "Of these inventions, the _Safe lamp_, which is the simplest, is likewise the one which affords the most perfect security, and requires no more care or attention than the common candle, and when the air in mines becomes improper for respiration, it is extinguished, and the workmen ought immediately to leave the place till a proper quantity of atmospheric air can be supplied by ventilation. "I have made many experiments on these lamps with the genuine fire-damp taken from a blower in the Hepburn Colliery, collected under the inspection of Mr. Dunn, and sent to me by the Reverend Mr. Hodgson. My results have been always unequivocal. "I shall immediately send models of the different lamps to such of the mines as are exposed to danger from explosion; and it will be the highest gratification to me to have assisted by my efforts a cause so interesting to humanity." [29] _Olefiant_ gas, when mixed with such proportions of common air as to render it explosive, is fired both by charcoal and iron heated to a dull-red heat. _Gaseous oxide of carbon_, which explodes when mixed with two parts of air, is likewise inflammable by red-hot iron, and charcoal. The case is the same with _sulphuretted hydrogen_. [30] "In addition to these four lamps, we learn from an Appendix to his Paper in the Philosophical Transactions, that in the beginning of his enquiries, he constructed a close lantern, which he called the _Fire-valve lantern_; in which the candle or lamp burnt with its full quantity of air, admitted from an aperture below, till the air began to be mixed with fire-damp, when, as the fire-damp increased the flame, a thermometrical spring at the top of the lantern, made of brass and steel, riveted together, and in a curved form, expanded, moved a valve in the chimney, diminished the circulation of air, and extinguished the flame. He did not, however, pursue this invention, after he had discovered the properties of the fire-damp, on which his Safety-lamp is founded." Contrary to the wish expressed by Sir Humphry Davy, the foregoing communication was inadvertently read at a public meeting of the Coal-trade, which was held at Newcastle on the 3rd of November: a circumstance which occasioned some embarrassment at the time, but is satisfactorily explained in the following letter from Sir Humphry. TO THE REVEREND DR. GRAY. 23, Grosvenor Street, Dec. 14, 1815. MY DEAR SIR,