oily matter is boiled a considerable time in water, as is sometimes practised with olives, and more especially with the animal oils. The water in this case dissolves the matter of which the cells are composed, and sets the oil at liberty.

"It deserves attention," Dr. Thomson remarks, "that the only part of vegetables in which fixed oils are found is the seeds of bicotyledinous plants. In animals they are usually deposited in the liver, though they are found also in the eggs of fowls."-This, however, must be received with some allowance with respect to some animals, since in the most copious quantity they are found in the adipose membrane, as in the lard of hogs, the tallow of cattle, and the blubber of the whale.

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Fixed oils are at present considered as composed only of hydrogen and carbon. By the analysis of Lavoisier 100 parts of olive oil consisted of about seventy-nine parts carbon, and twentyone hydrogen.

Some of these oils have the property of drying easily without losing their transparency, such as linseed oil, nut oil, poppy oil, and hempseed oil. This property is increased by the addition of litharge (an oxide of lead) which is partly reduced by the process, whence it follows that this property is increased by the absorption of oxygen, or by taking away part of the hydrogen.

The mild alkalies, carbonat of potass, soda, &c., have little effect upon these oils; but the

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caustic, or pure alkali, has a considerable attraction for them, and unites with them into a compound called soap. The caustic alkali gives the soap an attraction for water, and penetrates the oily matter, which we want to wash out. 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, bitumen, 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 tauming 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 which they exceed all other bodies. By this they are distinguished from earths, which in some states they resemble: none of the earths, however, are much more than four times their bulk of water in weight; whereas tin, the lightest of metals, is seven times as heavy.

2. They have a great degree of opacity, beating 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 me

tals to a degree of rigidity and brittleness. To remedy 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.

Their

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. 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 calx, 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