Below are shown the Fire-fly of warm climates, and the Glow-worm, which, in our comparatively cool country, cheers the summer evenings with its pale lamp. As to the source of this mysterious light, which burns without producing heat sufficient to be recognised by our most delicate instruments, we know but little.
There are instruments so infinitely more sensitive than the best thermometer, that they will record instantaneously an increase of heat if a human being passes in front of them, though at several yards’ distance. Yet no effect is produced on them by any of the Fire-flies or the Glow-worm. The spectroscope itself gives little or no information, the spectrum of the light being without bands or bars, and being what is technically called a “continuous” spectrum.
Last year I tried numbers of Glow-worms with the spectroscope, and always with the same result. I never saw the Fire-flies alive, but, no matter what may be the colour of the light, the spectrum, whether of the Glow-worm or any of the Fire-flies, seems to be always continuous, and so to give but little information as to its source.
There appears, however, to be little doubt that animal electricity is the real cause of this curious phenomenon, and that the force which is expended in the torpedo and electric eel, in giving shocks accompanied by slight electric sparks, may develop itself in these insects by producing a continuous light. And just as the electric fishes can emit or withhold the shock as they please, so can the Fire-flies and Glow-worms give out or retain the light by which they are so well known.
Then we come to the multitudinous luminous inhabitants of the sea, which, as many of my readers have probably seen, convert the waves into rolling masses of living fire.
Magnetism.
Now we come to another condition of electrical force, called Magnetism.
One form of it is strongly developed in the Loadstone, an ore of iron. This ore has the property of turning east and west when suspended freely, it attracts any object made of iron, and can communicate its powers to iron by merely stroking it. There is in the Museum at Oxford a splendid specimen of the Loadstone, which has imparted its virtues to thousands of iron magnets, and has lost none of its virtues by so doing.
All bodies are now known to be magnetic in some way or other. Several, such as iron, nickel, and one or two other metals, turn north and south when suspended on a pivot, but the great bulk of other bodies turn east and west, and are called Diamagnetics.
As we all know, the property of turning north and south has been utilised in the Compass, without which modern science would be paralyzed, and travel rendered impossible.