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considered as warm; but they have no heat in themselves, for they keep in fact any body cool better than other substances, as ice in ice-houses is kept under straw. These bodies hinder the caloric from escaping, their interstices being filled with air, which is the rarest of fluids. Snow keeps the ground warm also, from its being of a soft and spongy texture. It is true it cannot keep the ground warmer than the freezing point, but this is warm when compared with the intense cold felt in several climates, frequently 32° below frost; the freezing point being as much warmer than this, as our summer weather is hotter than frost.

Fluids convey heat very readily. Air cools bodies extremely fast. This, however, may depend upon the expansibility of air when heated, which will produce the effects of a constant change. Thus, if I expose a hot body to the air to cool, the air that is in contact with it expands and becomes lighter, consequently it is driven upwards; and thus there is a constant succession of cold air applied to the body: and if we place the heated body between the sunshine and a wall, we shall see the rarefied vapour rising like an undulating smoke upon the wall. The reason why we see it is, because the rays of light which pass through these steams are turned aside; by this means the wall is less illuminated in this part than the rest, and therefore we see the shadow, for the same reason that we see the

shadow of smoke. This circumstance makes objects seem to change place and tremble, when viewed through the rarefied atmosphere of a heated field when the sun shines upon it; the fluid contained in the earth and plants constantly evaporating, and the vapours having a tendency upwards. It is owing to this tendency upwards that iron or any other body will heat a body held over it sooner than under it. Hence the cold observed at sea upon approaching mountains of ice; the cold air being condensed falls down the sides of the ice and floats along the surface of the sea. A vessel of water being placed over the fire, the heat expands and renders lighter the bottom of the water, which of course ascends to the top, while another part, denser and colder, falls to the bottom; and thus there is a constant circulation from the under to the upper part of the vessel. When we have occasion to heat glass all over it is very apt to break, if the fire is not equally applied. For this end we immerse the vessel in water, using the operation of the balneum mariæ, as it is improperly called.

From the nature of fluids, deep lakes of water do not freeze in the winter: the cold air rushing over the surface, a portion of the water at the top being thus rendered heavier will sink, and its place be supplied with a warmer portion from below, which in its turn must also be cooled. Thus the cold air has the whole heat of the water to carry away, which is frequently not

done during a winter. Hence the equal temperature upon the ocean and upon islands, when compared with continents in the same degree of latitude.

But the equal distribution of caloric or heat does not seem to agree with some phænomena which frequently occur. The highest parts of the air are the coldest, and the contrary. On the Alps, Pyrenean mountains,' &c., the ice and snow are higher than the clouds, and seem to increase, and we are further convinced of the cold of the superior parts of the atmosphere by showers of hail which fall in the summer.

This distribution of heat has been explained in different ways; but the real principle is as follows: It appears that the sun is the source of heat to this globe, and we must observe that the rays of the sun do not heat a body that is perfectly transparent. When the body is not perfectly transparent, and reflects some few of the rays, it is somewhat heated, though nothing in comparison with opake bodies. Hence black bodies are soonest heated. If a burning glass is so placed that the focus falls a little below the surface of a transparent water, it does not heat the water if then you plunge a stick into that part of the water, it will be immediately burnt to a coal in its interior parts, the surrounding water preserving the exterior ones. The rays of the sun, therefore, not heating transparent bodies, have little effect upon the air; but the upper

part is more transparent than the under, and the lower parts receive almost all their heat in a secondary manner by reflection from the earth. But it still remains to show why the lowest part continues warmest, and does not rise as in the other fluids. This fact was partly explained in a former lecture. Air is a fluid very different from water: it is very compressible. The atmosphere may be supposed to be composed of very eccentric layers, the lowest layer being the densest. The lower strata, then, are rarefied a little by heat, but are compressed by the superior; and the heat is preserved by this compression of the superior air on the surface of the earth, where it is intended to act.

The coldness of the air seems to condense the vapours, and causes them to fall in showers, upon which the life of vegetables depends. This shows the use of planting the superior parts of country. As green-houses made of glass receive the heat transmitted through it, but confine the air when heated; so, on the contrary, the ground that is quite exposed and naked has its heat so quickly carried away by scorching winds that it is rendered barren.

3dly. FLUIDITY, as was formerly mentioned, is one of the most general effects of heat. Not only solid bodies, we find, may be rendered fluid by heat alone, but even those bodies which generally appear fluid owe their fluidity to the caloric they contain, which being sufficiently

diminished, they become solid. It is somewhat remarkable that mixtures of bodies generally melt more easily than the bodies by themselves: of this we have an instance in metals; but this is only an increase of the disposition to fluidity which depends upon caloric at last. Some bodies require the most intense cold for their congelation, the smallest is sufficient to freeze others. Before Professor Braun, it was thought that mercury was not to be frozen.

The substances which have shown the greatest repugnancy to become fluid are some earths and stones, but these are few in number, and the number has still been diminished since the employment of the voltaic apparatus and the present highly improved blowpipes. Those bodies which cannot melt by themselves, melt in mixing with others; we may therefore reasonably conclude they would become fluid, if we could apply a sufficient degree of heat, and in a proper

manner.

There is this difference between expansion and fluidity, that in expansion there is a regular increase or contraction of bulk, according to the degree of heat; whereas in fluidity the transition from a fluid to a solid state, or the contrary, is sudden, and below or above a particular point of heat a body always remains fluid or solid. There are, however, some bodies which appear in an intermediate state of fluidity, as wax, sulphur, pitch, tallow, &c.; yet every particular degree of softness depends upon a particular degree