I promised to continue in this letter my ideas respecting the materiality of Heat, Light, and Electricity, and shall therefore begin without further preface.
It is perhaps more difficult to establish the materiality of light than that of heat, yet many of its phenomena scarcely admit another explication, and the authority of Newton is powerful in my favor. Perhaps its decomposition is one of the best arguments that can be urged, as it would be difficult to conceive the sevenfold nature of a motion. The difficulty is, if light be a substance, constantly emitted from the sun, how comes it that long ere this period the earth has not acquired such a reservoir as to be almost independent of its aid. If light be constantly proceeding from a candle, what becomes of it when the candle is extinguished? Perhaps, assuming that different bodies have different capacities for light as well as for heat, this last problem is not difficult of solution. Bodies become visible, not by the light they absorb but by that which they reflect: a body reflecting the blue rays of the sun and absorbing all the rest appears blue; if on the contrary the red rays be reflected and the others absorbed, it will appear red; black objects absorb, white objects reflect, the whole of the light which falls upon them. While a candle is burning in an apartment, light is diffused through the air, and the quantity produced is greater than that absorbed by surrounding objects. When however the candle is extinguished, this superfluous light is soon imbibed from the air, which appears to have no capacity for its retention, and the room appears dark. Should not then, it will be asked, these bodies become themselves luminous from the access of light they have absorbed? It appears to me that they do so. Many bodies after having been exposed some hours to the sun become phosphorescent, and on extinguishing a candle, after some time we begin again to distinguish objects in a room, not perhaps merely because our eyes are then more accustomed to the low degree of light, but because the objects, possessing then a greater quantity of light than air, are beginning to part with it in their turn.
The other question, with respect to the light accumulated on the globe, corroborates and elucidates this one. Bodies may be said during the day to be absorbing light; yet they are far from absorbing all that falls upon them; a great part is consumed in radiation. By what means do the Moon and the Planets become visible? By the light of the Sun that they reflect back to us. Thus it is undoubtedly with our Earth, and probably the quantity expended by day and by night in reflexion to the different planets may be equal to that absorbed from the Sun, and may supply that light which is perhaps never entirely banished from the sky.
Sound we know to be an action, and its vibrations are diffused in every direction; but light always moves in straight lines, a circumstance strongly in favor of its materiality. The angle of reflexion also is equal to the angle of incidence; on the same principle that a ball, striking on a hard substance, rebounds in an angle precisely opposite to its original direction.
I will not here dwell on the fact that light and heat are so generally, though not inseparably, connected, that the analogy of their nature appears more than probable. Perhaps indeed these two substances are never completely divorced; and the light of the Moon, cold as it is, may not be more inferior to that of the Sun in warmth than in lustre. I shall leave these hints to the discussion of the Attic Society, and will in my next letter proceed to a consideration of the nature of Electricity, as connected with that of Light and Heat; if they are not already too much fatigued with the lucubrations of their most obedient