This light is probably owing to that or some other animal matter. Sea water has been observed to become luminous from the substance of a fresh herring having been immersed in it; and certain insects, of the Medusa kind, are known to produce similar effects. But the strongest phosphorescence is produced by chemical compositions prepared for the purpose, the most common of which consists of oyster shells and sulphur, and is known by the name of Canton’s Phosphorus. EMILY. I am rather surprised, Mrs. B., that you should have said so much of the light emitted by phosphorescent bodies without taking any notice of that which is produced by burning bodies. MRS. B. The light emitted by the latter is so intimately connected with the chemical history of combustion, that I must defer all explanation of it till we come to the examination of that process, which is one of the most interesting in chemical science. Light is an agent capable of producing various chemical changes. It is essential to the welfare both of the animal and vegetable kingdoms; for men and plants grow pale and sickly if deprived of its salutary influence. It is likewise remarkable for its property of destroying colour, which renders it of great consequence in the process of bleaching. EMILY. Is it not singular that light, which in studying optics we were taught to consider as the source and origin of colours, should have also the power of destroying them? CAROLINE. It is a fact, however, that we every day experience; you know how it fades the colours of linens and silks. EMILY. Certainly. And I recollect that endive is made to grow white instead of green, by being covered up so as to exclude the light. But by what means does light produce these effects? MRS. B. This I cannot attempt to explain to you until you have obtained a further knowledge of chemistry. As the chemical properties of light can be accounted for only in their reference to compound bodies, it would be useless to detain you any longer on this subject; we may therefore pass on to the examination of heat, or caloric, with which we are somewhat better acquainted. HEAT and LIGHT may be always distinguished by the different sensations they produce, _Light_ affects the sense of sight; _Caloric_ that of feeling; the one produces _Vision_, the other the sensation of _Heat_. Caloric is found to exist in a variety of forms or modifications, and I think it will be best to consider it under the two following heads, viz. 1. FREE OR RADIANT CALORIC. 2. COMBINED CALORIC. The first, FREE or RADIANT CALORIC, is also called HEAT OF TEMPERATURE; it comprehends all heat which is perceptible to the senses, and affects the thermometer. EMILY. You mean such as the heat of the sun, of fire, of candles, of stoves; in short, of every thing that burns? MRS. B.