In the commencement of his enquiry into this subject, Davy observed that the flame of hydrogen gas, the degree of rarefaction and the quantity of air being the same, burnt longer when it issued from a larger than a smaller jet,--a fact the very reverse of that which must have happened had the flame expired for want of oxygen; he moreover observed, that when the larger jet was used, the point of the glass tube became white-hot, and continued red-hot till the flame was extinguished: he therefore concluded, that the heat communicated to the gas by this tube was the cause of its protracted combustion, and that _flame expired in rarefied air, not for want of nourishment from oxygen, but for want of heat, and that if its temperature could be preserved by some supplementary aid, the flame might be kept burning_. The experiment by which he confirmed this theory was as beautiful as it was satisfactory. He burnt a piece of camphor in a _glass_ tube, under the receiver of an air-pump, so as to make the upper part of the tube red-hot; its inflammation was found to continue when the rarefaction was nine times; but by repeating the experiment in a _metallic_ tube, which could not be so considerably heated by it, it ceased after the rarefaction exceeded six times. It follows then that by artificially imparting heat,[55] bodies may be made to burn in a rarefied air, when under other circumstances they would be extinguished. [55] "It is upon this principle that, in the Argand lamp, the Liverpool lamp, and in the best fire-places, the increase of effect does not merely depend upon the rapid current of air, but likewise upon the heat preserved by the arrangement of the materials of the chimney, and communicated to the matters entering into inflammation." The art of making a good fire depends also upon the same principle of economising the heat. The following may be considered as an _experimentum crucis_, in proof of the fact that combustibility is neither increased nor diminished by rarefaction. He introduced the flame of hydrogen, in which was inserted a platinum wire, into a receiver of rarefied air, and he found that, as long as the metal remained at a dull red-heat, the flame continued to burn: now it so happens that the temperature, at which platinum approaches a red-heat, is precisely that at which hydrogen inflames under the ordinary pressure of the atmosphere; whence it follows, that its combustibility is not altered by rarefaction. The same law was found to apply to the flames of other bodies; those requiring the least heat for their combustion always sustaining the greater rarefaction without being extinguished.[56]