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.

In the ordinary method of manufacturing such mantles, a cotton-net cylinder about eight inches long, more or less according to the size of mantle required, is made, one end being contracted by an asbestos thread. A loop of the same material, or in some cases a platinum wire, is fastened across the opening, to be used for suspending the mantle when in use. The cotton-thread cylinder is soaked in a solution of the nitrates of the metals thorium and cerium, and is then wrung out to remove the excess, stretched on a conical mold, and dried. The flame of an atmospheric burner being applied to the upper part at the constricted position, the burning extends downward, converting the nitrates into oxides, and removing the organic matter. Considerable skill is required in this part of the process, as the regular shape of the mantle is largely dependent

upon the regularity of the burning. As a finishing process a flame is applied to the inside of the mantle after it has cooled, to remove all traces of carbon that may remain.

The mantle is now ready for use, but is so fragile that it can scarcely be touched without breaking, and such handling as would be necessary for shipment would be out of the question. It is therefore strengthened temporarily by being dipped into a mixture of collodion and castor oil, which, when dry, forms a firm but elastic jacket surrounding all parts. It is this collodion jacket that is burned away when the new mantle is placed on the burner before the gas is turned on.

Quite recently the method of manufacturing mantles used by Clamond has been revived. In this method the cotton thread is dispensed with, the thread used being made from a paste containing the mantle material itself. The paste is placed in a proper receptacle the bottom of which is perforated with minute openings, and subjected to pressure, squeezing out the material in long filaments. When dry these are wound on bobbins, and, after being treated by certain chemical processes, are ready for weaving into mantles. It is claimed for mantles made on this principle that they last much longer and retain their light-emitting power more uniformly than mantles made by the older process.

THE INTRODUCTION OF ACETYLENE GAS

When the incandescent mantle had been perfected so as to be an economical as well an as efficient lightgiver, the position of coal gas as an illuminant seemed

again secured against the encroachments of its rivals, the arc and incandescent electric lights. But just at this time another rival appeared in the field that not only menaced the mantle lamp but the arc and incandescent light as well. Curiously enough, this new rival, acetylene gas, had been brought into existence commercially by the electric arc itself. For although it had been known as a possible illuminant for many years, the calcium carbide for producing it could not be manufactured economically until the advent of the electric furnace, itself the outcome of Davy's arc light.

Even as early as 1836 an English chemist had made the discovery that one of the by-products of the manufacture of metallic potassium would decompose water and evolve a gas containing acetylene; and this was later observed independently from time to time by several chemists in different countries. No importance was attached to these discoveries, however, and nothing was done with acetylene as an illuminant until the last decade of the nineteenth century. By this time electric furnaces had come into general use, and it was while working with one of these furnaces in 1892 that Mr. Thomas F. Wilson, in preparing metallic calcium from a mixture of lime and coal, produced a peculiar mass of dark-colored material, calcium carbide, which, when thrown into water, evolved a gas with an extremely disagreeable odor. When lighted, this gas burned with astonishing brilliancy, and, as its cost of production was extremely small, the idea of utilizing it for illuminating was at once conceived and put into practice. The secret of the cheap manufacture of the carbide

lies in the fact that the extremely high temperature required-about 4500° Fahrenheit-can be obtained economically in the electric furnace, but not otherwise. Thus electricity created its own greatest rival as an illuminant. It followed naturally that the ideal place for manufacturing the carbide would be at the source of the cheapest supply of electricity, and as the "harnessed" Niagara Falls represented the cheapest source of electric supply, this place soon became the centre of the carbide industry. Here the process of manufacture is carried out on an enormous scale. In practice, lime and ground coke are thoroughly mixed in the proportion of about fifty-six parts of lime to thirty-six parts of coke. When this mixture has been subjected to the heat of the electric furnace for a short time an ingot of pure calcium carbide is formed, surrounded by a crust of less pure material. The ingot and crust together represent sixty-four parts of the original ninety-two parts of lime and coke, the remaining twenty-eight parts being liberated as carbon-monoxide gas.

Calcium carbide as produced by this process is a dark-brown crystalline substance which may be heated to redness without danger or change. It will not burn except when heated in oxygen, and will keep indefinitely if sealed from the air. Chemically it consists of one atom of lime combined with two atoms of carbon (CaC2); and to produce acetylene gas, which is a combination of carbon and hydrogen (C2H2) it is only necessary to bring it into contact with water, acetylene gas and slaked lime being formed. One pound of pure carbide will produce five and one half cubic feet of gas

of greater illuminating power than any other known gas. The flame is absolutely white and of blinding brilliancy, giving a spectrum closely approximating that of sunlight. The light is so strongly actinic that it is excellent for photography.

Here was a gas that could be made in any desired quantities simply by adding water to a substance costing only about three cents a pound; its cost of production, therefore, representing only about one sixth of the dollar-per-thousand-feet rate usually charged for illuminating gas in our cities. It could be used in lamps and lanterns made with special burners and with the simple mechanism of a small water tank which allowed water to drip into a receptacle holding the carbide; or -reversing the process--an apparatus that dropped pieces of carbide into the water tanks. It was, in short, the cheapest illuminant known, generated by an apparatus that was simplicity itself.

There were, however, two defects in this gas: its odor was intolerable-the "smell of decayed garlic," it has been aptly called-and when mixed with air it was highly explosive. The first of these defects could be overcome easily; when the burner consumed all the gas there was no odor. The second, the explosive quality, presented greater difficulties. These were emphasized and magnified by the number of defective lamps that soon flooded the market, many of these being so badly constructed that explosions were inevitable. As a result a strong prejudice quickly arose against the gas, some countries passing laws prohibiting its use. But further inquiry into the cause of the frequent dis