Fluids, from their very nature and constitution, contain a greater quantity of caloric in a latent state than solid bodies: indeed it is now universally admitted, and may be easily proved, that the fluidity of all bodies is altogether owing to the quantity of fire which they retain in this latent or combined state, the elasticity of which keeps their particles remote from each other, and prevents their fixing into a solid mass. All bodies, therefore, in passing from a fluid to a solid state, emit a quantity of fire or heat. When water is thrown upon quick lime, it is absorbed by the lime, and in this state it is capable of retaining a much smaller quantity of caloric than in its natural state; on the slacking of lime, therefore, a very intense heat is produced, the matter of fire which preserved the water fluid being disengaged and detached. If spirit of vitriol is added to strong oil of turpentine, they will condense into a solid mass, and a great quantity of heat will be sensibly emitted. Upon the same principle it will be found, on the other hand, that when any body passes from a solid to a fluid state, the adjacent bodies will be deprived of a quantity of their natural heat. [This theory of what is called _burning lime_, is not sufficiently clear. Fire does not enter into the pores of the lime by burning. The mineral commonly used for procuring _quick lime_ is the _carbonate_ of lime, or common _limestone_, which is composed of carbonic acid with a small quantity of water, 43, and lime 57, in 100 parts. By submiting it to a strong heat, the carbonic acid is driven off, and the quick or pure lime remains, which is an _oxide of calcium_. The loss in weight is owing to the expulsion of the carbonic acid, with the small portion of water. By adding water to the quick lime, it is dissolved, and falls into a powder. This process is called _slacking lime_, and the product, _slack-lime_. During the process a large quantity of heat is disengaged; and if the slacking be done in the dark, _light_ is also observed to be thrown out. This heat is given out by the _water_, not the lime. The lime having a _greater affinity_ for the _hydrogen_ of the water than exists between the hydrogen and oxygen in water, seizes upon it, and the _oxygen passes off_, together with the _latent caloric_ of the water, and thus the heat is produced which is observed in slacking lime. The hydrogen of the water combines with the lime and becomes solid, forming an _hydrate of lime_, which is the common slacked lime used in mortar.]