The third case of change of capacity, by the action of heat itself; is, perhaps, productive of more important effects in nature than either of the other two. The capacities of all bodies are increased in some proportion to the dilatation of their bulk, and the disaggregation of their constituent particles, as well by the agency of caloric as by any other 'cause. Hence, when a solid is fused, or a liquid resolved into vapocir, cold is produced by the tim.mentation of its capacity ; and, e converoo, When steam is condensed, or congelation takes place, heat is developed by its diminution. Thus, if equal quan tities of pounded ice and water, each at S2° of Fahrenheit, be exposed to heat in two similar vessels in a water-bath, the water will be heated in 178° before the ice is all dissolved, the water produced from which will, of course, still remain at 22°, so that the increase of capacity in the ice, during its solution, is sufficient to enable it to absorb, without any eleva tion of its temperature, as much caloric as has raised the temperature of an equal quantity of water 146°; and the like quan tity is also again emitted on its becoming again congealed. If a quantity of water be exposed without agitation to a degree of cold equal to 24° or 25°, it will fre quently acquire this temperature without freezing; hut as soon as congelation be gins, the thermometer will immediately rise to 32°. and the whole will remain at
that temperature till all the water is con verted into ice.
This latter change of capacity appears to be absolutely essential to the well-be ing of the universe, as affording a con stant modification of the action of heat and cold, whose effects would otherwise be inordinate. If this did not take place, the whole of a mass of water which was exposed to a temperature above the boil ing point would be instantly dissipated in vapour with explosion. The fact, how ever, is, that the capacity of those por tions of the 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 : and it is for this reason that boiling water in an open ves sel never reaches a higher temperature than 212°. The polar ices would all in stantaneously dissolve, whenever the temperature of the circumambient air was above 32°, if it were not that each particle absorbs a quantity of caloric in its solution, and thereby generates a de. gree of cold, which arrests and regulates the progress of the thaw ; and the con verse of this takes place in congelation, which is in its turn moderated by the heat developed in consequence of the dimi nution of capacity, which takes place in the water during its transition to a solid state.