“In reflecting on the circumstances of the two species of combustion I was led to imagine that the cause of the superiority of the light of the _stream_ of coal gas might be owing to the _decomposition_ of a part of the gas towards the interior of the flame, where the air was in smallest quantity, and the deposition of solid charcoal, which, first by its _ignition_, and afterwards by its combustion, increased in a high degree the intensity of the light.” The principle of the increase of the brilliancy and density of flame by the production and ignition of solid matter explains the appearance of the different parts of the flames of burning bodies, and of the blow-pipe flame; it also explains the intensity of the light of those flames in which fixed solid matter is produced in combustion, _e.g._ phosphorus and zinc in oxygen, potassium in chlorine; and, on the other hand, the feebleness of the light of flames in which gaseous and volatile matter is alone produced, _e.g._ hydrogen in oxygen, phosphorus in chlorine. Davy’s theory has not been unchallenged, but all subsequent research, when pushed sufficiently far, has shown that, as regards all ordinary illuminating flames, _i.e._ carbonaceous flames--_e.g._ coal-gas, oil, paraffin, candle--the presence of solid incandescent carbon is a prime cause of their luminosity. It had been observed that the rarefaction of a mixture of inflammable gases diminishes its combustibility: Davy proved that this diminution was not the result of the removal of pressure _per se_, but of the cooling effect thus indirectly produced. Hence, the lower the temperature of ignition of a gaseous mixture the more it may be rarefied without becoming uninflammable. In like manner he shows that by heating the gaseous mixture it may be caused to explode at a lower temperature, and that when gases combine by sudden compression, the combination is caused by the heat evolved. Also that the power of an indifferent gas to prevent the explosion of a gaseous mixture depends upon its power of abstracting heat, and that the higher the temperature of ignition of the explosive mixture the less is the amount of indifferent gas required to stop the explosion. He proved that it was quite possible to effect the gradual combination of gases without flame--that is, without the production of heat sufficient to raise the products to incandescence; and he discovered the singular fact that platinum would induce the combination of many inflammable gases and vapours, and on this circumstance based the construction of his flameless lamp. * * * * * In the early summer of 1818, he thus wrote to his mother:--