Abstract If all the experiments had been recorded, which at different times must undoubtedly have been made on the subject of Pyrometry, by those engaged in operations requiring the accurate management of fire, the catalogue would consist principally of abortive attempts, if not of decided failures. The efforts to obtain exact measurements of high temperatures have probably been abandoned, partly from the occurrence of unforeseen difficulties, partly from the uncertainty of the results obtained: such, at least, appears to be the only way of accounting for the blank presented in this interesting and practically important branch of chemical knowledge. In the admeasurement of the lower portions of the scale of temperature, and the determination of the proper methods of graduation, and the laws of expansion, gaseous tension, &c. a great degree of accuracy has been introduced. To the extent of the boiling point of mercury, indeed, we have tolerably exact values of the dilatation of metals and fluids; and by Messrs. Dulong and Petit’s experiments, the table has been extended to the irregularities of the thermometric indications of several substances, compared with the supposed uniform expansion of air, or of any other gas in a dry state.