Abstract
IT is usually stated that the carat weight of jewellers and diamond merchants is derived from the hard seeds the locust tree, Ceratonia siliqua, which were anciently used as weights. Having had occasion to obtain some of the beans, I weighed several of the seeds to see what scrt of error would be incurred if they were used as weights. Out of forty-four seeds, four were shrivelled and obviously abnormal, weighing from 0.037 to 0.064 grm. each; the remaining forty seeds varied from 0.120 to 0.268 grm. The average weight of seed was 0.2004 grm., whh a probable variation of ± 0.0235. The median was 0.207, and the modal average 0.204. The variations were not well distributed. The old diamond carat, of which 151½ made 1 oz. troy, would weigh 0.205 grm.; the decimal carat now in use is 0.200 grm. It would appear, therefore, that the carat weight could be recovered with some approach to accuracy by weighing a number of seeds of the locust bean. It is also evident that the use of such seeds as weights must have given opportunities for fraudulent dealing in the precious commodities gauged by means of them, since deviations of from 30 to 40 per cent. from the average may occur. The variations in weight due to varying humidity of the air are not great; twenty-five seeds exposed to the air of a room for twenty-three hours in rainy weather gained 0.06 per cent. in weight, and after thirty-six hours over sulphuric acid lost 1.71 pr cent. in weight.
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COSTE, J. Ceratonia Siliqua and the Carat Weight. Nature 99, 185 (1917). https://doi.org/10.1038/099185c0
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DOI: https://doi.org/10.1038/099185c0
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