among simple substances. The third, ammonia, has been long ascertained to be a compound of nitrogen and hydrogen; an account of it will therefore come properly under the class of Alkaline Salts, when some further observations will be introduced on the fixed alkalies.

X. XI. The earths and metals will require distinct lectures; and being substances of a peculiar character, and not entering so copiously into the composition of those bodies which are found on the surface of the earth as those I have been describing, any observations on their rature or combinations would be foreign to the object of this lecture.

I have thus endeavoured to lay before you a general outline of the doctrines of modern chemistry. I fear this lecture will appear uninteresting to many of you.-It is, however, of so much importance that, if it is well understood, it will greatly facilitate the study of all the remaining branches of this science. What follows will be rather a comment or elucidation of the contents of this lecture, and an application of its principles to practical purposes. I must therefore recommend your particular attention to it, and particularly to those leading principles, which I have ventured to term the canons of modern chemistry.

LECTURE XXVI.

CHEMISTRY.

THE CHEMICAL EFFECTS OF CALORIC; EXPANSION, FUSION, FLUIDITY, VAPOUR, AND COMBUSTION.

Ir was necessary in the last lecture to give a general view of the properties of caloric. I fear I shall be thought in the present to be guilty of something like repetition; but it is necessary to the understanding of the effects of caloric as an agent in chemistry, to call your attention at present to the subject rather more in detail.

1st.The presence of caloric EXPANDS all bodies. There are three forms under which matter can appear.

The first comprehends solid bodies, as iron, &c.; the second, unelastic or incompressible fluid bodies, as water, oil, spirit of wine, &c.; the third, elastic compressible fluids, as air. I shall now show the effects of caloric in expanding all these. If an iron or brass wire of about a quarter of an inch in thickness is cut so as when cold to fall just withinside, or short of two rings made to receive it, by rubbing it very hard with your hands or a piece of cloth it will be made warm, it will consequently be expanded, and lengthened so as to reach both rings, and be suspended in them while it remains warm.

If spirit of wine is contained in an oblong vessel, and immersed in hot water, the spirit will be observed to rise like the mercury in a thermometer.

A bladder, with a little air in it, when heated will expand so as almost to burst; but on removing it into the cold, it condenses and shrinks to its former state.

Rarer and lighter bodies expand more than the heavier and denser; but this proportion does not always exactly correspond to the respective density and rarity of matters; for metals expand more than glass.

As an exception to this doctrine it has been observed, that water suddenly swells in passing from a fluid to a solid state: but this is to be explained upon different principles. Boyle took a brass tube, three inches in diameter, and put some water into it; he then brought down into the tube a plug with a weight placed at the head of it of 74lb., exposing the tube to the cold, and the water freezing and expanding itself raised the 74lb. The Florentine academicians filled a brass globe with water, closing the orifice by a well-fitted screw, and immersed it in freezing water; but as the sides were too thick it did not burst. They then pared off such a quantity of the brass as left the sides of the globe unable to resist the expansion of the water, the force which was required to burst the globe in this state was computed at upwards of 27,000lbs.

Boyle says, the expansion of water in freezing is about an 8th or 9th of its bulk.

The burst

ing of leaden pipes placed near or within the earth's surface proceeds from the same cause. The pavement even suffers from the frost, which swells the earth and loosens the stones; nay, rocks have been known to burst in frosty weather. Frost is by some supposed to fertilize, by loosening the cohesion of the particles of earth.

As ice is never clear or transparent, and as we find several cavities in it, some have thought that the air insinuates itself, and in this way have endeavoured to account for the expansion. But this has been refuted by water being frozen under an exhausted receiver, and the same cavities being found in the ice; the ice, indeed, instead of being heavier, was lighter, and floated on the water. M. D. Mairan at length solved the difficulty. He says, the particles of water in freezing assume a different arrangement, are not in so close contact, and cut each other at angles of 60°. If this is the case, then, as indeed is now generally agreed, we cannot say with propriety that the solid particles of water expand, but that from their crystallization into the form' of ice they require more room, or occupy a greater space.

As salts are observed in crystallization to put on regular figures, it was thought that the starlike appearance of snow was owing to a salt mixed with the water; but Margraff proved

that snow is in fact composed of the purest

water.

Reaumur observes that cast iron melted in passing from a hot to a cold state expands. This effect is more sensible in this than in any other of the metals, on account of its platey texture. He found also that cast iron thrown among some of the same metal in a melted state swims upon the top. In this case of immediate expansion upon congealing, the iron seems to agree with water. But they differ in this, that the iron never expands by cold afterwards, whereas the ice being exposed to greater degrees of cold becomes still more bulky; the solid parts not being so closely connected form a parti cular arrangement, which renders the whole mass specifically lighter than before.

Denser bodies for the most part expand less than rarer; but this I observed is not an invariable rule, for metals expand more than glass or stones. The expansion of metals was found a great obstruction to the regular going of clocks, but is obviated now by the ingenious contrivance of making use of two different metals, which do not expand equally, in constructing the pendulums.

Of the three classes of bodies which compose the universe, solid bodies suffer the least expansion from the presence of caloric. Liquids. are more expansible than solids; and aëriform fluids are most expansible of all. By the accurate experiments of Mr. Dalton of Manchester,