Moissan's method is based upon the capacity of iron of absorbing carbon at a high temperature and of giving it back in the shape of grains and crystals while the iron mass is cooling. When iron has been saturated with carbon at a temperature of about 2,000° Fahr., a mixture of amorphous carbon and graphite is discovered in the iron mass. At higher temperatures the fused iron dissolves more and more carbon, and the cast iron of our blasting furnaces, after having been heated to about 3,000° and slowly cooled down, contains, as known, an abundance of graphite crystals. It was thus natural to see whether a still higher temperature, and cooling under high pressure, might not give the still denser form of carbon—that is, the diamonds.
In order to thoroughly saturate iron with carbon at a high temperature, and to cool it under a high pressure, Moissan resorted to a very simple and effective means. He took a hollow cylinder of soft iron, filled it with some purified sugar charcoal, and corked the cylinder with an iron screw. Then about half a pound of soft iron was molten in a crucible in Moissan's new electric furnace, which readily gives a temperature of about 3,000° C. (5,400° Fahr,), and the cylinder was plunged into the molten metal; iron was thus thoroughly saturated with carbon. The crucible was then taken out of the furnace and plunged into a pail of cold water until the surface of the iron mass was cooled to a dull red temperature, whereupon it was taken out and left to cool in the air. This was the ingenious means of obtaining a high pressure. It is known that water when it becomes ice increases in volume, and that if it freezes in a strong shell the Interior pressure of the crystallizing water often bursts the shell; but if it can not burst the shell it necessarily solidifies under an immense pressure, due to the molecular forces. The same was done by Moissan with the liquid iron, which also has the property of increasing in volume while it solidifies. An outer solid crust having been formed by a sudden immersion into cold water, the crust prevents the further expansion of the iron mass, which is thus bound to solidify under an immense pressure, like the water in the shell.
The next step was to separate the iron from the carbon crystals which it might contain. This was done by dissolving the iron in hydrochloric acid, and three different varieties of carbon crystals (which are not attacked by the acid) were received as a residue. Some graphite, some chestnut-colored, curved needles of carbon, and diamond dust could be seen; and they were separated from each other by several complex operations indicated by Berthelot in one of his previous works. A few grains of diamond dust were finally obtained—most of them belonging to the carbonado variety, while a few of them proved to be real diamonds; they were
