This beautiful property of spongy platinum was happily applied to the construction of lamps for producing an instantaneous light. The form given to the lamp by Mr. Garden of London is shown in the annexed figure, where AB is a globe of glass, fitting tightly into another glass globe CD by a ground shoulder m n. The globe AB terminates in a hollow tapering neck m n o p, on the lower end of which is placed a small cylinder of zinc o p. A brass tube a b c, is fitted at a into the neck of the globe CD, and through this tube, which is furnished with a stop-cock d, the gas can escape at the small aperture c. A brass pin c f, carrying a brass box P, is made to slide through a hole h, so that the brass box P, in which the spongy platinum is placed, can be set at any required distance from the aperture c. If sulphuric acid, diluted with an equal quantity of water, is now poured into the vessel AB by its mouth at S, now closed with a stopper, the fluid will descend through the tube m n o p, and if the cock d is shut, it will compress the air contained in CD. The dilute acid thus introduced into CD will act upon the ring of zinc o p, and generate hydrogen gas, which, after the atmospheric air in CD is let off, will gradually fill the vessel CD, the diluted acid being forced up the tube o p m n, into the glass globe AB. The ring of zinc o p floats on a piece of cork, so that when CD is full of hydrogen, the diluted acid does not touch the zinc, and consequently is prevented from producing any more gas. The instant, however, that any gas is let off at c, the pressure of the fluid in the globe AB, and tube m n o p, overcomes the elasticity of the remaining gas in CD, and forces the diluted acid up to the zinc o p, so as to enable it to produce more gas to supply what has been used.
The lamp being supplied with hydrogen in the manner now described, it is used in the following manner. The spongy platinum in P being brought near c, the cock d is turned, and the gas is thrown upon the platinum. An intense heat is immediately produced, the platinum becomes red-hot, and the hydrogen inflames. A taper is then lighted at the flame, and the cock d is shut. Professor Cumming, of Cambridge, found it necessary to cover up the platinum with a cap after every experiment. This ingenious chemist likewise found, that, with platinum foil the 9,000th part of an inch thick kept in a close tube, the hydrogen was inflamed; but when the foil was only the 6,000th of an inch thick, it was necessary to raise it previously to a red heat.
Spontaneous combustion is a phenomenon which occurs very frequently and often to a great extent within the bowels of the earth. The heat by which it is occasioned is produced by the decomposition of mineral bodies and other causes. This heat increases in intensity till it is capable of melting the solid materials which are exposed to it. Gases and aqueous vapours of powerful elasticity are generated, new fluids of expansive energy imprisoned in cavities under great pressure are set free, and these tremendous agents, acting under the repressing forces of the superincumbent strata, exhibit their power in desolating earthquakes; or, forcing their way through the superficial crust of the globe, they waste their fury in volcanic eruptions.
When the phenomena of spontaneous combustion take place near the surface of the earth, its effects are of a less dangerous character, though they frequently give birth to permanent conflagrations, which no power can extinguish. An example of this milder species of spontaneous combustion has been recently exhibited in the burning cliff at Weymouth; and a still more interesting one exists at this moment near the village of Bradley, in Staffordshire. The earth is here on fire, and this fire has continued for nearly sixty years, and has resisted every attempt that has been made to extinguish it. This fire, which has reduced many acres of land to a mere calx, arises from a burning stratum of coal about four feet thick and eight or ten yards deep, to which the air has free access, in consequence of the main coal having been dug from beneath it. The surface of the ground is sometimes covered for many yards with such quantities of sulphur that it can be easily gathered. The calx has been found to be an excellent material for the roads, and the workmen who collect it often find large beds of alum of an excellent quality.
A singular species of invisible combustion, or of combustion without flame, has been frequently noticed. I have observed this phenomenon in the small green wax tapers in common use. When the flame is blown out, the wick will continue red-hot for many hours; and if the taper were regularly and carefully uncoiled, and the room kept free from currents of air, the wick would burn on in this way till the whole of the taper is consumed. The same effects are not produced when the colour of the wax is red. In this experiment the wick, after the flame is blown out, has sufficient heat to convert the wax into vapour, and this vapour being consumed without flame, keeps the wick at its red heat. A very disagreeable vapour is produced during this imperfect combustion of the wax.
Prof. Dobereiner, of Jena, observed that, when the alcohol in a spirit of wine lamp was nearly exhausted, the wick became carbonized, and though the flame disappeared, the carbonized part of the wick became red-hot, and continued so while a drop of alcohol remained, provided the air in the room was undisturbed. On one occasion the wick continued red-hot for twenty-four hours, and a very disagreeable acid vapour was formed.
Fig. 79.
On these principles depend the lamp without flame which was originally constructed by Mr. Ellis. It is shown in the annexed figure, where AB is the lamp, and h a cylindrical coil of platinum wire, the hundredth part of an inch in diameter. This spiral is so placed that four or five of the twelve coils of which the cylinder consists are upon the wick, and the other seven or eight above it. If the lamp is lighted, and continues burning till the cylindrical coil is red-hot, then if the flame is blown out, the vapour which arises from the alcohol will by its combustion keep the coils above the wick red-hot, and this red heat will in its turn keep up the vaporization of the alcohol till the whole of the alcohol is consumed. The heat of the wire is always sufficient to kindle a piece of German fungus or saltpetre paper, so that a sulphur match may at any time be lighted. Mr. Gill found that a wick composed of twelve threads of the cotton yarn commonly used for lamps will require half an ounce of alcohol to keep the wire red-hot for eight hours. This lamp has been kept burning for sixty hours; but it can scarcely be recommended for a bed-room, as an acid vapour is disengaged during the burning of the alcohol. When perfumes are dissolved in the alcohol, they are diffused through the apartment during the slow combustion of the vapour.