When the Lavoiserian theory was first advanced, it was generally thought that the light and heat were furnished by the oxygen : hence, whatever might be the combustible body, the greatest light and heat would be produced, the greater the quantity of oxygen which entered into combi nation in a given time ; and the intensity inversely as the • space in which the combustion took place. It has since been held, and with good reason, that the inflammable body also contributes light and heat.
There does not appear to be any just theory of the pro duction of light and heat by combustion, but that founded on the change of specific heat between the materials of combustion and the body resulting from the combustion. We cannot, however, expect to derive much practical be nefit from such a theory, till we are in possession of a cor rect table of the specific heat of bodies.
Since chemists are sufficiently acquainted with four in flammable gases to obtain them in a state of purity, name ly, hydrogen, carburetted hydrogen, carbonic oxide, and olefiant gas, we might, by a few experiments, get some idea of the relative quantities of light afforded by carbon and hydrogen. If we suppose these gases to consist of pure hydrogen, and still retaining their respective densities, the:; the intensity and quantity of light would be directly as their densities. In as much, therefore, as their light differs from the ratios of their densities, may be attributed the re lative quantities of light afforded by the bodies of which they are composed.
Two small gazometers will be necessary for these ex periments, the one to contain hydrogen gas, and the other the inflammable gas to be compared with it. Let the pres sure of each be exactly the same, and let the gas from each pass through exactly the same sized aperture, at the time • it is burnt. The flames must now be compared with each other, by making shadows in them fall upon a white sur face; then remove the strongest light backward, till the shadows are of the same intensity. The squares of the distances of the flames, from their respective shadows, will express the ratio of the illuminating powers of the two flames. If, for instance, hydrogen were compared with olefiant gas, and if the carbon of the latter gas contributed as much to the illumination as the hydrogen, then the ra tio of the squares of the distances of the flames from the shadows, when the shadows were of the same intensity, would be as I to 11.85. If, however,•the flame of the ole
fiant gas will not require to be shifted so far back, in order to make the shadows equal, then it will show that the car bon of this gas has not afforded the same light as so much hydrogen would have done. If now the comparison be made between hydrogen and carburetted hydrogen, the ratio of the squares of the distances, if the latter gas were all hydrogen, would be as I to 7.5. But the distance of the flame of the carburctted hydrogen gas will probably fall short of the %/7.5, owing to the carbon it contains ; but it contains a less proportion of carbon on the whole than olefiant gas, and therefore ought to produce more light, in proportion to its density, than olefiant gas. In these in stances we have presumed, and with good ground, that a given weight of hydrogen, in its combination with oxygen, affords more light and heat than any other inflammable bo dy. In these and all other instances of combustion, the ab solute quantity of light and heat will be the same, whatever may be the density of the combustible body and the oxygen; but the intensity may be much increased by diminishing the time of burning the same quantity of matter, and the space in which the combustion takes place. Hence we ac cumulate light and heat by means of bellows, and other means of furnishing oxygen, with great facility. We should also get a proportionate effect by increasing the density of the oxygen. If hydrogen and oxygen were in creased in their density by artificial pressure, and present ed to each other for combustion, the intensity of the light and heat would be in the complicate ratio of their increas ed density. If each were compressed into half the space, then the effect of their combustion would be four times the intensity of that in their natural state. In this way much greater intensity of both light and heat may be produced than we have hitherto heard of. The carburetted hydrogen would much exceed olcfiant gas in producing light, if its density were equal to the latter gas, because it contains more hydrogen than olefiant gas. And if pure hydrogen were of the density of olefiant gas, the intensity of its light would be nearly twelve times greater than when burnt in its ordinary state, and it would be to olefiant gas as about 7 to 3.