Artificial Diamonds

Abstract

THE fate of the Glasgow diamonds, as recorded in NATURE, vol. xxi. p. 203, reminds me of an adventure of my own that happened about ten years ago, and is likely to be repeated by others. For showing the popular class-room experiment of burning phosphorus in oxygen, I was in the habit of using a little cup of chalk made deeper and with smaller rim than the brass cups usually made for the purpose. The object of this was to limit the too rapid outburst of combustion. I noticed that a cup which had been used several times was coated on the inside with a hard, glassy enamel, which I supposed to be phosphate of lime. To test this, the cup was thrown into some hydrochloric acid and dissolved bodily, but I found at the bottom of the beaker an insoluble residue of crystalline particles. What were these? Could it be possible that the carbonic acid driven off by heating the chalk had, on reaching the heated phosphorus, become dissociated, its oxygen combining with the phosphorus, and its carbon thrown down as veritable diamond? To test this startling theory, I collected the particles and rubbed them between a glass pestle and mortar. They appeared hard enough to scratch the glass, but were too small for further examination. To obtain a better supply, I dissolved some phosphorus in bisulphide of carbon, pounded some chalk and made it into a paste with the solution, then filled a porcelain crucible with this and fired the mass by heating it over a Bunsen burner. It blazed magnificently, throwing out eruptive jets of flame. Here, in the absence of surrounding oxygen, the carbonic acid had every opportunity of becoming dissociated or reduced by the heated phosphorus. The residue was treated with hydrochloric acid, and this time I found at the bottom of the beaker quite a respectable quantity of crystalline grains. These left unmistakable scratches on the glass pestle and mortar, and seemed to make some fine scratches on an agate pestle and mortar. I next examined them under a microscope, and found that they were more like pebbles than crystals, so much so as to suggest another theory of their composition and origin, viz., that they were miniature chalk flints formed by the fusion and aggregation of the siliceous cuticles of fossil diatoms, or such-like organisms of which chalk appears to be in some degree made up.