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?
The point of heat at which the eye begins to discover luminosity has been estimated at 1,000 deg.