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Sir, The experiments of Mr. Buffon upon ignited bodies seem to prove, that, when heated to the degree he mentions, they are more ponderous than when cold. The experiments which I have made on heated metals, suggest a different idea, and contradict the fact he relates; so that I am induced to believe, that some circumstance, not attended to, has introduced a mistake in the relation this learned philosopher has published as the result of his inquiry. His experiment stands thus recorded (Suppl. Nat. Hist. vol. 2. p. 11.): a mass of iron, after receiving a white heat, weighed 49 lbs. 9 oz.; when restored again to the temperature of the atmosphere 49 lbs. 7 oz. Hence he concluded, that the igneous particles, contained in the heated iron, increased its absolute weight 2 ounces. My experiments are as follows: 1st, One penny-weight of gold, made red-hot, became apparently lighter; but, when restored again to the temperature of the atmosphere, its former weight was perfectly restored. 2d, One penny-weight of iron, heated as above, was also apparently lighter; but, when it became cool again, its weight was visibly augmented.

XXXVIII. Experiments on ignited substances. By Mr. John Whitehurst, in a letter to James Stuart, Esquire, F.R.S