The intense hardness of the diamond can be illustrated by the following experiment. On the flattened apex of a conical block of steel place a diamond, and upon it bring down a second cone of steel. On forcing together the two steel cones by hydraulic pressure the stone is squeezed into the steel blocks without injuring it in the slightest degree.
In an experiment I made at Kimberley the pressure gauge showed 60 atmospheres, and the piston being 3·2 inches diameter, the absolute pressure was 3·16 tons, equivalent on a diamond of 12 square mm. surface to 170 tons per square inch of diamond.
The use of diamond in glass-cutting I need not dwell on. So hard is diamond in comparison to glass, that a suitable splinter of diamond will plane curls off a glass plate as a carpenter’s tool will plane shavings off a deal board. The illustration (Fig. 17) shows a few diamond-cut glass shavings.
The specific gravity of the diamond varies ordinarily from 3·514 to 3·518. For comparison, I give in tabular form the specific gravities of the different varieties of carbon and of the minerals found on the sorting tables:
SPECIFIC GRAVITY. Amorphous carbon 1·45-1·70 Hard gas coke 2·356 Hard graphite 2·5 Quartzite and granite 2·6 Beryl 2·7 Mica 2·8 Hornblende 3·0 Boart 3·47-3·49 Carbonado 3·50 Diamond 3·514-3·518 Garnet 3·7 Corundum 3·8 Zircon 4·4 Barytes 4·5 Chrome and titanic iron ore 4·7 Magnetite 5·0
There is a substance, the double nitrate of silver and thallium, which, while solid at ordinary temperatures, liquefies at 75° C. and then has a specific gravity of 4·5. Admixture with water lowers the density to any desired point.
If a glass cell is taken containing this liquid diluted to a density of about 3·6, and in it is thrown pieces of the above-named minerals, all those whose density is lower than 3·6 will rise to the surface, while the denser minerals will sink. If now a little water is carefully added with constantly stirring until the density of the liquid is reduced to that of the diamond, the heterogeneous collection sorts itself into three parts. The graphite, quartz, beryl, mica, and hornblende rise to the surface; the garnet, corundum, zircons, etc., sink to the bottom, while the diamonds float in the middle of the liquid. With a platinum landing-net I can skim off the swimmers and put them into one dish; with the same net I can fish out the diamonds and put them in a second dish, while by raising a sieve at the bottom I can remove the heavy minerals and put them into a third. The accurate separation of diamonds from the heterogeneous mixture can be effected in less time than is taken to describe the experiment.