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272 TRANSLATION OF “INQUISITIO DE MAGNETE.”
iron, and likewise with the magnet itself, placed on the top of St. Paul's in London (one of the highest churches in Europe), to see whether their attractive power was diminished in consequence of their distance from the ground; but there was no difference at all.
I. The Table of Presence. OBSERVE first, all bodies of every kind which generate light; as stars, fiery meteors, flame, wood, metals, and other bodies ignited, sugar in scraping and breaking, the glowworm, spray of salt-water beaten and thrown about, the eyes of some animals, some kinds of rotten wood, a great mass of snow. The air itself may perhaps have a feeble light suited to the eyes of those animals which see at night. Iron and tin when put into aqua-fortis for solution boil up, and without
any fire conceive a strong heat; but whether they emit any light is a point for inquiry. The oil of lamps sparkles in hard frosts; on a clear night a feeble light is sometimes visible about a sweating horse ; and sometimes likewise, though seldom, about men's hair, in the shape of a small lambent flame; as happened to Lucius Marcius in Spain. A woman's stomacher was lately observed to shine, but only on being rubbed ; this however had been dyed green, a dye in which alum is an ingredient, and it slightly crackled while it glittered. Inquire whether alum glitters on being scraped or broken; but I suppose it requires a stronger fracture than sugar, as being a more stubborn body. Some stockings have been observed to shine on being pulled off, either from sweat or alum dye. Other instances.
1 Livy, xxv. 39.
II. The Table of Absence in the next Degree. Observe likewise what those bodies are which do not emit light, and yet have a great resemblance to those which do. Boiling water gives no light; neither does air though violently heated. Mirrors and diamonds, which reflect light so wonderfully, give none of their own.
Other instances. Observe likewise accurately in this kind of instances respecting those that are migratory, that is, where light is present and absent, as it were in passing. An ignited coal gives light, but if it be strongly compressed it at once loses it. The crystalline moisture of the glowworm, at the death of the worm, though broken and divided into parts, retains its light for a short time; but this soon dies away.
III. The Table of Degrees. Observe the different intensities and vibrations of different kinds of light. The flame of wood emits a strong light; the flame of spirit of wine a weaker; the flame of coals thoroughly ignited one very dusky and hardly visible. Other instances.
IV. Colours of Light. Observe concerning the colours of light, what kinds there are, and what not. Some of the stars are white, some bright, some reddish, and some lead-coloured.