that a lump of iron dropped into water continues red-hot for some little time, for the water nearest the iron being instantly converted into elastic vapour, keeps off the rest of the fluid.

Water does not become hotter by being boiled long. This phænomenon has been variously accounted for: some supposed that water will not contain above a certain quantity of heat; the additional heat, they say, bursts through the fluid, and gives it a bubbling motion. Others have supposed that the air contained in the water becomes elastic by the heat, and forces its way through the fluid. Neither of these theories is, however, satisfactory; but the true explanation

is

easy. In the common way of boiling water, the bottom of the fluid arriving at that point of heat, beyond which it cannot continue without being converted into vapour, is thrown up in vapour to the furface, and this occasions the violent ebullition observed there. Thus the quantity of water is diminished, while its heat is not, and cannot be, increased; for, after the fluid has been raised to its boiling point, the continued application of heat converts it into vapour, but does not make it hotter. In this conversion of water into vapour, it will be found by some experiments that the caloric applied to the water disappears, and becomes latent in the

vapour.

The degree of sensible heat is only necessary as a condition, but is not the immediate cause of

the conversion of a fluid into vapour. If we observe the heating of water in a furnace, we shall find that the heat flows into it very fast till it arrives at the boiling or vaporific point. Suppose in the last five minutes its heat is increased ten degrees, in the next five we should imagine it would be at least six or seven degrees more, and that this would be sufficient to convert the whole into vapour; and it would be so if nothing more was requisite for this effect than sensible heat: but this is not the case; for in reality very little of the water is evaporated, and the remainder is not sensibly hotter.

Water more easily or with less heat boils in vacuo than under the pressure of the atmosphere (viz. at ninety or ninety-five degrees); but to convert it all suddenly into vapour requires as much, or more fuel, than in the common way, over an open fire can be applied to it.

Boyle placed some warm water, which he had previously boiled, to extract its air under an exhausted receiver. On applying heat, the water boiled violently at a degree of heat not much above that of the human blood. Dr. Cullen also placed ether under the receiver of an airpump, into which he would let down or raise a thermometer at pleasure, without admitting the external air. He no sooner began to extract the air than the ether was thrown into a violent agitation or boiling, at the same time its temperature sunk to a surprising degree. When the

ether was first put in, it was about fifty-eight degrees, but it became so cold when boiling, that a quantity of water in a surrounding vessel was suddenly frozen. The manner in which the phænomenon may be accounted for is this: the weight of the atmosphere being removed, the caloric the ether contained was sufficient to make it boil. The heat that disappeared, or, in other words, the cold that was produced, while the ether was boiling, is easily accounted for. The boiling of the ether, like the boiling of water, arose from the conversion of the fluid into a vapour more subtile than itself; but the conversion could not take place without the absorption of a considerable quantity of caloric, that is, much of the sensible heat of the fluid became latent in vapour. Now it is well known that water and spirit of wine boil in vacuo many degrees below their boiling point under the pressure of the atmosphere. It is natural, therefore, for ether, which boils in the open air, when heated to about the heat of the human blood, to boil in vacuo twenty-four degrees below 0, which is a degree of cold sufficient to freeze any water that be in contact with the vessel which contains the ether.

may

This experiment ought to be attended to, because it is a clear proof that the sudden diminution of heat is not always occasioned by its being taken away by the surrounding matter.

This evaporation at low temperature is not

confined to liquids; many solids are also susceptible of it without previously passing through the fluid state, of which camphor and carbonate of ammonia are familiar examples. These very substances, however, as well as others which have been supposed incapable of liquefaction from their volatility, are, in the common processes for purifying them, reduced to the liquid state, and even made to boil in vessels freely communicating with the external air.

Hence may be understood the nature of several chemical operations, as evaporation, distillation, and sublimation. These processes are performed on bodies composed of different ingredients, some of which are more easily volatilized than others. The most volatile will be first converted into vapour. If we take a mass of clay, water, and quicksilver, and expose this mass to heat, the water will first rise in vapour, and be entirely expelled before the mercury begins to rise, and this vapour may be condensed into pure water. By increasing the heat, the quicksilver will rise also, and leave the clay by itself. The process is called evaporation, and is used when we wish to obtain the more fixed parts, and are not solicitous about the volatile as in the making of salt. When the volatile parts are the object, the process is called either distillation or sublimation; the former when the vapours condense into fluids, the latter into solids. The product of sublimation is called sublimate,

when it is close and compact; it is called flowers, when its texture is loose like soot.

Animal and vegetable substances are composed of various ingredients, which may be separated by the application of heat. But heat operates upon them in a different manner from what it does on other substances. If a piece of flesh is exposed to heat, it is not like iron expanded, but on the contrary, is contracted; for the humid parts fly off, and leave only the fixed or denser parts behind.

Heat applied to animal substances does not convert them into a vapour that may be condensed into any substance of the same kind, but into oil, water, charcoal, and some saline substances. When the volatile parts are evaporated, the remainder has a different arrangement from what it had before.

It may seem rash to assert that heat has a power of bringing into fusion and volatilization all bodies without exception, as we have not sufficient data or experiments to render this opinion absolutely certain; for all those earthy substances that have been rendered fluid have not as yet been converted into vapour, and there are some earthy substances that have not even been brought into fusion.

Some metals, especially gold and silver, were once thought to be absolutely fixed. Boyle took a small quantity of each, and exposed them for two months to a glass-house furnace, and at