caloric suddenly emerges. These experiments not only prove the existence of latent caloric before the water freezes, but likewise that the freezing does not depend on the diminution of sensible heat; since we find it may retain its fluidity, though cooled eight degrees below the freezing point: nor does it depend merely on its tranquillity; for, if it did, we should expect, upon disturbing it, that the whole would be congealed. This does not happen; and the reason is, that a quantity of latent caloric is impelled inwards, which raises the temperature above thirty-two degrees, below which it must fall before it can be frozen. In the ordinary freezing of water something happens like this, though by insensible degrees; when water is exposed to freeze, before the whole of it is converted into ice, it must all the time be imparting heat to the air.

Several facts and phænomena are explained upon this principle, as the production of cold by a mixture of ice with the nitric acid or saline substances. If the nitric acid is mixed with ice or snow, there will be a sudden liquefaction, consequently a sudden absorption of caloric, which will produce a degree of cold in the surrounding region.

Mixture in general promotes liquefaction. In this instance a most intense cold is produced, and the cause is as follows: If I can liquefy ice, I know I must throw in a prodigious quantity of obvious heat or caloric, and which must imme

VOL. II.

E

diately become latent in the liquid, and hence it is not discoverable by a thermometer. By producing a sudden liquefaction of ice, the cold is so great that all the neighbouring bodies must supply or communicate from their obvious heat the caloric that is to become latent in this liquefaction, and to restore the equilibrium; and thus it proves in this experiment. If I pour a quantity of nitric acid upon ice, it immediately liquefies, and produces a great increase of cold. This is no other than the obvious heat turned into the latent, to supply the newly produced fluid. This experiment is just the contrary of that where you have water cooled some degress below the freezing point, and if it is at rest it does not freeze; but if you shake the glass you make a portion of it freeze; this, on becoming solid, parts with its latent caloric, which becomes obvious, and by its means the water which has not frozen has its temperature raised.

The fourth general effect of heat is the formation of VAPOUR. By vapour I mean a transparent fluid, which, like air, is compressible and very elastic, and which suffers great alteration in its bulk from considerable accessions of heat. If a drop of water is placed on the bottom of an exhausted receiver, it will suddenly disappear, and be converted into a subtile vapour which will fill the vessel: and its pressure against the internal surface will be so strong when heated to a certain degree, that it

will be almost impossible to confine it, and it will often burst the vessel with a loud explosion. This effect of the force of vapour is sufficiently exemplified by the small pieces of glass called candle-balls, which burst by the expansion of the vapour. This great expansion of the vapour is the true cause of the danger of throwing water into boiling oils or melted metals, especially brass or copper. The water being a heavier fluid than the oil falls to the bottom, where it is immediately converted into vapour, and causes a violent ebullition. A small quantity of humidity when mixed with hot metals, will be converted into vapour with such rapidity as to produce a more violent explosion than gunpowder. Hence the danger of casting copper or iron vessels; for, if the fluid metal meets with the least moisture in its passage from the furnace to the mould, the watery particles are instantly expanded, and throw the burning metal to a considerable distance. Vapour, when condensed, resumes its former state; and upon these principles, viz. the elasticity and condensibility of vapour, depends the construction of the steamengine, as was formerly explained.

To compare the weight of steam or vapour with that of water, place a flask with a small quantity of water in it before the fire; the moment the whole of the water is converted into vapour, close the vessel, and it will then be filled with steam. Weigh the vessel thus filled with steam,

weigh it again filled with air, and lastly filled with water. This will give you the weight of steam compared with that of water, which is in the proportion of about 1664 to 1.

The point at which a fluid is converted into vapour depends' upon a certain degree of heat, and this we call the vaporific or boiling point, which differs widely in different bodies. The terms volatility and fixedness are only terms of comparison. Thus, when we say a body is volatile, we only mean that it requires less heat to convert it into vapour than most other bodies; and by a contrary reason we define bodies fixed. The vaporific point is always the same in the same bodies. It agrees in this with the effect of heat in producing fluidity, but the vaporific point is remarkably influenced by mechanical pressure. The greater the pressure, the greater the degree of heat necessary to convert the fluid into vapour. Fahrenheit, by the barometer, observed that according to the greater or less pressure of the atmosphere, a fluid required a greater or less degree of heat to convert it into vapour; he marked the vaporific point of water on his thermometer at the mean height 212°. If water, however, can be prevented from going off in steam, as it may by means of a particular contrivance, it will acquire a degree of heat equal to that of metals when red-hot. The machine for this purpose is called Papin's Digester, and is a copper vessel half filled with water, the head screwed in

and well luted: when the water is so hot as to send off vapour, its escape is prevented by means of a weight and lever across the lid, which confine it in proportion to the increased pressure of the vapour. The water by these means acquires a degree of heat, which Muschenbroek says he found sufficient to melt lead or tin. Bones have by these means been totally dissolved in water, and reduced to their constituent parts, nothing remaining but earth or ashes, which may be easily crumbled between the fingers.

As the vaporific point is always the same under the same pressure, we may consider it as always the same in the open air; at least it does not vary more than three degrees. It is a matter of curiosity to examine the difference of the vaporific point of any liquor under the exhausted receiver, and when exposed to the pressure of the atmosphere. It is found that water boils in vacuo at ninety degrees.

When a single drop of water is heated to its vaporific point it immediately becomes vapour; but if the quantity is more considerable, the phænomenon will be varied. For, if a quantity of water is thrown into an iron vessel heated red-hot, it will seem to run about the vessel like quicksilver, but without touching the bottom or sides of the vessel. The reason is, the water nearest the bottom and sides is converted into vapour, which prevents the water coming into contact with the vessel. This is the reason, too,