Part of the light which enters bodies is retained within them, and proceeds no farther; but so loosely in some kinds of bodies, that a small degree of heat is sufficient to expel it again, so as to make the body visible in the dark: but the more heat is applied, the sooner is all the light expelled. This is a strong argument for the materiality of light. _Bolognian phosphorus_ is a substance which has this property; but a composition made by Mr. Canton, of calcined oyster-shells and sulphur, in a much greater degree. However, white paper, and most substances, except the metals, are possessed of this property in a small degree. Some bodies, especially phosphorus, and animal substances tending to putrefaction, emit light without being sensibly hot. The _colours_ of vegetables, and likewise their _taste_ and _smell_, depend upon light. It is also by means of light falling on the leaves and other green parts of plants, that they emit dephlogisticated air, which preserves the atmosphere fit for respiration. It is light that imparts colour to the skins of men, by means of the fluid immediately under them. This is the cause of _tanning_, of the _copper colour_ of the North Americans, and the _black_ of the Negroes. Light also gives colour to several other substances, especially the solutions of mercury in acids. LECTURE XXXIII. _Of Magnetism._ Magnetism is a property peculiar to iron, or some ores of it. The earth itself, owing probably to the iron ores contained in it, has the same property. But though all iron is acted upon by magnetism, _steel_ only is capable of having the power communicated to it. Every magnet has two poles, denominated _north_ and _south_, each of which attracts the other, and repels that of the same kind with itself. If a magnet be cut into two parts, between the two poles, it will make two magnets, the parts that were contiguous becoming opposite poles. Though the poles of a magnet are denominated _north_ and _south_, they do not constantly, and in all parts of the earth, point due north or south, but in most places to the east or west of them, and with a considerable variation in a course of time. Also a magnet exactly balanced at its center will have a declination from an horizontal position of about 70 degrees. The former is called the _variation_, and the latter the _dipping_ of the magnetic needle.