or closed crucible or tube containing the substance heated. This is the simplest form of laboratory furnace. Various modifications are' used, but in all the size is small and the arrangement simple. A powerful arc plays in the smallest possible cavity with the object of attaining the maximum of temperature, expense and duration of material being secondary considerations. Lime and magnesia are the best materials, because they are at the same time the most refractory substances available and are poor conductors of heat. A furnace top one and one half inches thick may be heated by so powerful an arc that the melted quick lime drips from the inner surface, while the outer surface is scarcely warm to the touch of the hand. Moissan has utilized in these little furnaces currents of electricity of varied strength, the lowest being that given by a four horse power dynamo, the highest that generated by three hundred horse power. The highest temperatures obtained were about 3,500° centigrade (6,300° Fahrenheit), with the heat constantly increasing; the limit to the obtainable temperature—as far as the experimental evidence showed—was merely the lack of any known substance refractory enough to bear the heat; for at the temperature mentioned quick lime and magnesia not only melt but are changed into gases, so that the furnace was filled with the vapors of its own material.
The effect of the heat on single substances is very interesting. Refractory metals, such as iron, manganese, uranium, platinum, melt rapidly and then become gaseous; the most refractory non-metallic elements, silicon, boron, carbon, are also changed into the gaseous form. Very refractory compounds are broken down into simpler ones. Magnesium pyrophosphate yields phosphorus, magnesium oxide and oxygen. Asbestos—a magnesium silicate—gives as chief product magnesium silicide; the other substances formed being silicon, silicon dioxide and a little magnesium oxide.
Such are the astounding changes wrought by simple heat upon those substances which we are accustomed to regard as infusible. It must be remembered that the range of temperature which chemists employ in ordinary laboratory work is not very great and that the conditions of work in the laboratory and of nature's work on the earth's surface at the present day favor the formation of two classes of compounds—the oxides and their hydrates. Although air is a mixture consisting mainly of four parts of nitrogen and one of oxygen, atmospheric nitrogen is generally inert at ordinary temperatures, and it is the oxygen of the air which is the more important factor in the growth of living things and in changes in lifeless matter. Water, a compound of oxygen and hydrogen, is present everywhere, either in the liquid form or as vapor in the air; even in the flame of the hottest fires there is water vapor in abundance, since water is one of the chief products of combustion of most forms of fuel. Is it a