But, besides this confirmation, further proof is afforded by the fact, that in passing rays of solar light through media that are _transparent to heat_, but not to _light_, the heat rays may be _separated_ from the luminous rays, and _vice versa_. Black glass, and black mica, which are nearly _opaque to light_, are _transparent to heat_ to the extent of ninety degrees out of a hundred. While pale green glass, coloured by oxide of copper, and covered with a coating of water, or a thin coating of alum, will be perfectly _transparent to light_, but will be almost quite _opaque to heat_. These remarks apply, in a greater or less degree, to various other substances. 543. _In what respects are light and heat similar?_ Both heat and light have been referred to minute vibratory motions which occur, under exciting causes, in a very subtile elastic medium. They are both united in the sun's rays. They are both subject to laws of absorption, radiation, reflection, and refraction. They are both essential to life, whether animal or vegetable. Both may be developed in their greatest intensity by electricity. They are both imponderable. [Verse: "When I consider thy heavens, the work of thy fingers, the moon and the stars which thou hast ordained:"] 544. _In what respects are light and heat dissimilar?_ Heat frequently exists without light. Light is usually attended with heat. Light may be instantly extinguished, but Heat can only be more gradually reduced, by diffusion. The solar rays deliver heat to the earth by day, and the heat remains with the earth when the light has departed. Heat diffuses itself in all directions. Light travels only in straight lines. The colours that absorb and radiate both light and heat do not act in the same degree upon them both. Black, which does not radiate light, is a good _radiator of heat_, &c., &c. The oxy-hydrogen _light_ emits a most intense heat, but glass which will transmit the rays of light, will afford no passage to the rays of the _heat_. Heat is latent in all bodies, but no satisfactory proof has been found that light is latent in substances. These are only a few of the analogies and distinctions that exist between the two mysterious agents, light and heat. But they are sufficient to supply the starting points of investigation. The importance of the heat that attends the solar rays may be illustrated by the experiments performed a few years ago, by Mr. Baker, of Fleet-street, London, who made a large burning lens, three feet and a half in diameter, and employed another lens to reduce the rays of the first to a focus of half an inch in diameter. The heat produced was so great that iron plates, gold, and stones were _instantly melted_; and sulphur, pitch, and resinous bodies, _were melted under water_. 545. _What is the point of heat at which bodies become luminous?_