(355.) The dilatation of bodies by heat forms the subject of that branch of science called pyrometry. There is no body but is capable of being penetrated by heat, though some with greater, others with less rapidity; and being so penetrated, all bodies (with a very few exceptions, and those depending on very peculiar circumstances,) are dilated by it in bulk, though with a great diversity in the amount of dilatation produced by the same degree of heat. Of the several forms of natural bodies, gases and vapours are observed to be most dilatable; liquids next, and solids least of all. The dilatation of solids has been made a subject of repeated and careful measurement by several experimenters; among whom, Smeaton, Lavoisier, and Laplace, are the principal. The remarkable discovery of the unequal dilatation of crystallised bodies by Mitscherlich has already been spoken of. (266.) That of gases and vapours was examined about the same time by Dalton and Gay-Lussac, who both arrived independently at the conclusion of an equal dilatability subsisting in them all, which constitutes one of the most remarkable points in their history. (356.) The dilatation of air by heat affords, perhaps, the most unexceptionable means known of measuring degrees of heat. The thermometer, as originally constructed by Cornelius Drebell, was an air thermometer. Those now in common use measure accessions of heat not by the degree of dilatation of air but of mercury. It has been shown, by the researches of Dulong and Petit, that its indications coincide exactly with that of the air-thermometer in moderate temperatures; though at very elevated ones they exhibit a sensible, and even considerable, deviation. By this instrument, which owes its present convenience and utility to the happy idea of Newton, who first thought of fixing determinate points on its scale, we are enabled to estimate, or at least identify, the degrees of heat; and thereby to investigate with accuracy the laws of its communication and its other properties. Were we sure that equal additions of heat produced equal increments of dimension in any substance, the indications of a thermometer would afford a true and secure _measure_ of the quantity present; but this is so far from being the case, that we are nearly in total ignorance on this important point; a circumstance which throws the greatest difficulty in the way of all theoretical reasoning, and even of experimental enquiry. The laws of the dilatation of liquids, in consequence of this deficiency of necessary preliminary knowledge, are still involved in great obscurity, notwithstanding the pains which have been bestowed on them by the elaborate experiments and calculations of Gilpin, Blagden, Deluc, Dalton, Gay-Lussac, and Biot.