congelation, which is in its turn moderated by the heat developed in consequence of the diminution of capacity, which takes place in the water during its transition to a solid state. The reason why boiling water in the open air never reaches a higher temperature than 212 degrees is evident, if we consider, that the capacity of those portions of liquid, which are successively resolved into a vapour, becomes thereby sufficiently augmented to enable them to absorb the superabundant caloric as fast as it is communicated. The most obvious effect of caloric on bodies, is the change, which they undergo when exposed to its action. That it acts constantly in opposition to the attraction of cohesion or of aggregation, by which bodies pass from a solid to a fluid, and from a fluid to an aeriform state, and produces also different changes in bodies,--are facts that come under our daily observation. It occasions changes in the bulk of bodies; hence solids, liquids, and gases are expanded. The expansion, and subsequent contraction of atmospheric air, give rise to various winds, which are currents of air rushing from one point of the compass to another to maintain an equilibrium. The theory of the winds is predicated on this fact, although some have asserted, that they depend greatly on the diurnal motion of the earth. The air thermometer of Sanctorius, and the differential thermometer of Leslie, are founded on this principle, of the expansion of air. Fluids expand until they arrive at the boiling point, as is the case with water, alcohol, &c. The expansion of mercury, in a glass tube, furnished with a graduated scale, forms the mercurial thermometer, by the rise and fall of which, the different variations of temperature are marked. Notwithstanding caloric has the property of expanding bodies, there are some exceptions to this law, which may be proper to notice. Water, for instance, at the temperature below 40° contracts at every increment of temperature until it reaches 40°, which is its maximum of density. Above 40° it expands, until it arrives at the boiling point. Alumina, or pure argillaceous earth, also contracts by heat; hence it is used in the pyrometer of Wedgwood, to measure by its contraction intense degrees of heat. Various saline substances, in the act of crystallization, also expand. Several of the metals, when previously melted, on cooling exhibit the same character; and water, in the act of freezing, exerts a powerful force by its expansion, competent to the bursting of shells, and the splitting of rocks.