GAS LIGHTING
While the rivalry between the candle and the new forms of lamps was at its height, and just as the lamp was gaining complete supremacy, a new method of artificial illumination was discovered that was destined to eclipse all others for half a century, and then finally to succumb to a still better form. As early as the beginning of the eighteenth century the Rev. Joseph Clayton, in England, had made experiments in the distillation of coal, producing a gas that was inflammable. A little later Dr. Stephen Hales published his work on Vegetable Staticks, in which he described the process of distilling coal in which a definite amount of gas could be obtained from a given quantity of coal.
No practical use was made of this discovery, however, until over half a century later. But just at the close of the century a Scot, William Murdoch, became interested in the possibilities of gases as illuminants, and finally demonstrated that coal gas could be put to practical use. In 1798, being employed in the workshops of Boulton and Watt in Birmingham, he fitted up an apparatus in which he manufactured gas, lighting the workshops by means of jets connected by tubes with this primitive plant. Shortly after this, a Frenchman, M. Lebon, lighted his house in Paris with gas distilled from wood, and the Parisians soon became interested in the new illuminant. England seems to have been the first country to use it extensively in public buildings, however, the London Lyceum Theatre being lighted with gas in 1803. By 1810 the great Gas-Light and Coke Company was formed, and within the next five years gas street-lamps had become familiar objects in the streets of London, and house illumination by this means a common thing among the wealthier classes.
In the early days of gas-lighting the results were frequently disappointing, because no suitable and efficient type of burner had been devised; but in 1820 Neilson of Glasgow discovered the principle of the now familiar flat burner, of which more examples still remain in use the world over than of all other kinds combined. Indeed, this simple, but as we now regard it, inefficient burner, would probably have remained the best-known type for many years longer than it did had not the possibilities of lighting by electricity aroused persons interested in the great gas-plants to the fact that the new illuminant was jeopardizing their enormous investments; making it clear that they must bestir themselves and improve their flat burners if they would arrest disaster. To be sure, several modifications of the round Argand burner had been introduced from time to time, some of them being a distinct improvement over the flat burner, but these did not by any means seriously compete with electric light. And it was not until the incandescent mantle was perfected that gas as a brilliant illuminant was able to make a stand against its new competitor.
THE INCANDESCENT GAS MANTLE
It has been known almost since the beginnings of civilization that all solids can be made to emit light when heated to certain temperatures. Some substances were known to be peculiarly adapted to this purpose, such as lumps of lime, and for many years the calcium light or "lime-light" as it is popularly called, had been in use for special purposes, and was the most intense light known. This light is made by heating a block of lime to the highest practicable temperature by means of a blast of oxygen and coal gas; but such lights were too complicated and expensive for general purposes. It had been determined even as early as the beginning of the nineteenth century, however, that the high temperature necessary for producing this light was due in part at least to the fact that such a large amount of material had to be raised to incandescence. It was evident, therefore, that if a small amount of some such substance as lime and magnesia could be spread out so as to present a large surface in a small space, such as is represented by basket-work, sufficient heat for making it incandescent might be obtained from an ordinary gas-and-air blowpipe.
Here then was the germ of the "mantle" idea; and such an apparatus, known as the Clamond mantle, which was made of threads of calcined magnesia, was shown at the Crystal Palace Exhibition, in London, in 1882. Curiously enough, this mantle and burner worked in an inverted position, the mantle being suspended bottom upwards below the burner through which the blast of gas was forced. The light given by this mantle was most brilliant—little short of the older calcium light, in fact—but the device itself was too complicated to be of service for ordinary lighting purposes. The principle was correct, but the construction of the mantle was defective.
Meanwhile a German scientist, Dr. Auer von Welsbach, who had become famous in the scientific world for his researches on rare metals, was experimenting with certain oxides of different metals, and developing a method of handling them that finally resulted in the perfected incandescent burner in use at present. His process, which in theory at least was not entirely original with him, was to dip an open fabric of cotton into a solution of the nitrates of the metals to be used, drying it, and converting the nitrates into oxides by burning; the cotton fabric disappearing but leaving the skeleton of the oxide, which retained its original shape.
At the same time corresponding improvements were made in the type of burner, which is quite as essential to success as the mantle itself. It had been found that it was absolutely essential for such a burner to give a practically non-luminous flame, as otherwise the deposit of carbon particles will ruin the mantle. Two ways of obtaining this are possible; one by mixing a certain quantity of air with the gas before combustion, the other to burn the gas in so thin a flame that the air permeates it freely. Several burners of both types were used at first, but gradually the burners in which the air is mixed with the gas became the more popular, and most of the incandescent burners now on the market are of this type.
In the construction of mantles at the present time, while the principle of their use remains the same as that of the lime-light, lime itself is not used, the oxides of certain other metals having proved better adapted for the purpose. Thus the Welsbach patent of 1886 covered the use of thoria, either alone or mixed with other substances such as zirconia, alumina, magnesia, etc.; thoria being considered as having a very high power of light emission. Later it was discovered that pure thoria emits very little light by itself, although it possesses a refractory nature that gives a stability to the mantle unequalled by any other material as yet discovered. When combined with a small trace of the oxides of certain rare metals, however, such as uranium, terbium, or cerium, thoria mantles have a very high power of light emission, most modern mantles being composed of about ninety-nine per cent. thoria with one per cent. cerium.