4. For many years it was the opinion of chemists that mercury is essentially liquid, and that no degree of cold is capable of congealing it. Professor Braun’s accidental discovery that it may be frozen by cold, like other liquids, was at first doubted; and when it was finally established by the most conclusive experiments, it was inferred from the observations of Braun that the freezing point of mercury is several hundred degrees below zero on Fahrenheit’s scale. It became an object of great importance to determine the exact point of the congelation of this metal by accurate experiments. This was done at Hudson’s Bay, by Mr. Hutchins, who followed a set of directions given him by Mr. Cavendish, and from his experiments Mr. Cavendish, in a paper inserted in the Philosophical Transactions for 1783, deduced that the freezing point of mercury is 38·66 degrees below the zero of Fahrenheit’s thermometer. 5. These experiments naturally drew the attention of Mr. Cavendish to the phenomena of freezing, to the action of freezing mixtures, and the congelation of acids. He employed Mr. M’Nab, who was settled in the neighbourhood of Hudson’s Bay, to make the requisite experiments; and he published two very curious and important papers on these subjects in the Philosophical Transactions for 1786 and 1788. He explained the phenomena of congelation exactly according to the theory of Dr. Black, but rejecting the hypothesis that heat is a _substance_ sui generis, and thinking it more probable, with Sir Isaac Newton, that it is owing to the rapid internal motion of the particles of the hot body. The latent heat of water, he found to be 150°. The observations on the congelation of nitric and sulphuric acids are highly interesting: he showed that their freezing points vary considerably, according to the strength of each; and drew up tables indicating the freezing points of acids, of various degrees of strength. 6. But the most splendid and valuable of Mr. Cavendish’s chemical experiments were published in two papers, entitled, “Experiments on Air,” in the Transactions of the Royal Society for 1784 and 1785. The object of these experiments was to determine what happened during the _phlogistication of air_, as it was at that time termed; that is, the change which air underwent when metals were calcined in contact with it, when sulphur or phosphorus was burnt in it, and in several similar processes. He showed, in the first place, that there was no reason for supposing that carbonic acid was formed, except when some animal or vegetable substance was present; that when _hydrogen gas_ was burnt in contact with air or oxygen gas, it _combined_ with that gas, and formed _water_; that _nitrous gas_, by combining with the oxygen of the atmosphere, formed _nitrous acid_; and that when _oxygen_ and _azotic_ gas are mixed in the requisite proportions, and electric sparks passed through the mixture, they _combine_, and form _nitric_ acid.