The greatest heat that has yet been produced artificially is that of the electric arc. The exact temperature of the electric arc is not known with certainty. It is known, however, that the temperature of the hottest part of the arc is not less than 6500 degrees Fahrenheit. When we compare this with the temperature of the hottest coal furnace, which is about 4000 degrees, we can very easily understand that something is likely to happen at the temperature of the electric arc that could not happen in an ordinary furnace. If an electric arc is enclosed by something that will hold the heat in we have an electric furnace, and any substance placed in the furnace may be made nearly as hot as the arc itself. In the electric furnace any substance, whether found in nature or prepared artificially, may be melted or vaporized. It was Henri Moissan, Professor of Chemistry at the Sorbonne in Paris, who made the first great discoveries in the use of the electric furnace and produced the first artificial diamonds. The study of diamonds led Moissan to believe that in nature they are formed by the cooling of a melted mixture of iron and carbon. He could prepare such a mixture with his electric furnace, he thought, and so make diamonds like those of the diamond mines. So, with an electric furnace having electrodes as large as a man's wrist, a mixture of iron and charcoal in a carbon crucible, and a glass tank filled with water, Moissan set out to change the charcoal to diamonds. At a temperature of more than six thousand degrees the iron and charcoal were melted together. For a time of from three to six minutes the mixture was in the intense heat. Then the covering of the furnace was removed and the crucible with the melted mixture dropped into the tank of water. With some fear this was done for the first time, for it was not known what would happen when such a hot object was dropped into cold water. But no explosion occurred, only a violent boiling of the water, a fierce blazing of the molten mass, and then a gradual change of color from white to red and red to black. With boiling acids and other chemicals the refuse was removed, and the fragments that remained were found to be diamonds, small, it is true, so small that they could be seen only with the aid of a microscope, but giving promise of greater things to come. The outer crust of iron held the melted charcoal under enormous pressure while it slowly cooled and formed the diamond crystals. The process of manufacturing diamonds is illustrated in Figs. 105, 106, and 107. [Illustration: FIG. 105--MANUFACTURING DIAMONDS--FIRST OPERATION Preparing the furnace. Charcoal and iron ore placed in a crucible and subjected to enormous heat electrically.] [Illustration: FIG. 106--MANUFACTURING DIAMONDS--SECOND OPERATION The furnace at work.] [Illustration: FIG. 107--MANUFACTURING DIAMONDS--THIRD OPERATION Plunging the crucible into cold water. Observe the white-hot carbon just removed from the furnace.]