The use of the oil is to soften the acrimony of the alkali, and give it a slipperiness, by which more mechanical force can be employed in washing.

To this class of combustibles we refer all the fossil combustible substances, some of which are fluid, others solid. The fluids differ in degree of fluidity and subtilty. Among the principal of these we reckon naphtha, asphaltum, amber, jet, and pit-coal. The grand constituent of all these is carbon, but so infinitely varied, that, though the detail of their properties might be entertaining, it would greatly exceed our limits.

3. Tan is, however, a production of a different nature; but its constituent principles have not yet been ascertained. It differs according to the vegetables from which it is produced. Nutgalls are supposed to contain the largest proportion. From its propensity to unite with the gelatinous parts of animal matter, and to form with them a hard substance, arises its great use in tanning leather. It is also of extensive use in dyeing, and in the making of ink.

LECTURE XXXII.

CHEMISTRY.

METALS.

METALLIC substances are valuable for their durability in resisting the effects of humidity and of air: for the closeness of their texture, which renders them capable of polish; for the strong cohesion of their particles, which renders them highly proper for utensils where strength is to be combined with moderate bulk.

The first and most obvious property of metals is their remarkable weight in general; but whilst in some their specific gravity is very great, it being in platinum 21,5, in potassium it is so low as 0,865.

2. They have a great degree of opacity, throwing off all the rays of light without receiving any into their substances; and the thinnest film of metal is as perfectly opake as any solid mass. It has been thought that gold leaf, which transmits some rays of light, is an exception; but there is reason to think that this depends not on any pores in the metal, but on cracks and flaws produced by beating, which always reduces metals to a degree of rigidity and brittleness. To re

medy this, the metals are generally heated redhot, and cooled again slowly; in this case their malleability will be increased; and this is called annealing them: but this is not done in beating out gold leaf; it may therefore be very probably broken in several places, and have minute flaws which transmit the light. Krammer says, if the gold is annealed, the leaf will be quite opake.

3. Metals are likewise distinguished by their properties with regard to electricity, which rank them among the most powerful of conductors, and render them more easily affected by lightning than any other matter.

4. Another property of metals is fusion by heat. In this state they always retain their opacity; and there is always a sort of repulsion between them and any vessels into which they can be poured, except metallic ones. Their particles, when melted, have an attraction for one another, in consequence of which they form into spheres. This is evident in quicksilver when thrown upon a table, the globules reflect all the objects round. In general, melted metals have a repulsion for other fluids; hence they cannot be mixed with oils, melted salts, or earths; but some of the combustible bodies unite with them in their melted state.

5. The most common and best known metals possess malleability; and metals are the only bodies in which this property is found.

As to the effects of heat upon metals, they be

come fluid, like other bodies, in different degrees of it. Many are also volatile when exposed to an intense heat. There is one phænomenon of melted metals, which, to prevent mistake, I will mention; the violent explosion they produce when they touch any thing moist. From what has been previously stated, it is obvious that this effect depends on the great degree of heat the metal is capable of receiving, which converts the moisture into vapour; yet when poured into moulds of sand there is no explosion, though humid, because the vapour easily finds a way through the pores. In general, however, the workmen pierce a number of holes through the sand. Copper and brass produce this effect most, because they receive the greatest quantity of heat when exposed to any given degree. Thus if we expose equal bulks of copper, iron, lead, tin, and silver to the heat of boiling water, and throw them into equal quantities of cold water, the copper will heat the water most, the iron next, and so on in the order recited.

When metals are heated to a very high degree, and the heat continued in the open air, they will be reduced to a cala, or cinder-like appearance. This is occasioned by their attracting the oxygen from the atmosphere, which then uniting with them, forms them into oxides; and in this state they are soluble in some degree in water. Gold, silver, and platinum are exceptions to this, for their attraction for oxygen is so weak, that they

remain unchanged by heat, except by that of the voltaic flame. These have therefore been called perfect metals. Some become oxidized at the ordinary temperature of the atmosphere, as sodium, potassium, &c.

Metals are also calcined, or more properly, according to the new chemistry, oxidized, by being immersed in the acids, which part with their oxygen to the metals, and reduce them to a calx or oxide. The perfect metals, however, require a peculiar acid to oxidize them, which is called aqua regia, and is in fact a super-oxygenated acid. But of this I shall treat more in speaking of the action of acids on them.

All the metals are capable of uniting with chlorine, and even in preference to oxygen. Iodine also acts upon many of the metals.

The calcination of volatile metals, such as zinc, &c. is similar to the burning of combustibles. Their vapour, when it mixes with the air, is oxidized and has the appearance of white smoke, like that of phosphorus. It attaches itself to the surface of other bodies, and, when collected, proves to be an oxide of the metal. In some cases their dissipation is manifest, as in zinc, where the heat and light are evident; but in most others there is no light, though perhaps if inspected in a very dark room, by a person who had been long in the dark, light might be perceived in many of them.

Copper oxidizes in a heat below that neces