(there may be other substances mixed with it which are useful).

Again, the mode of separating the metal from the different ores-in some cases breaking it into small pieces and roasting it-thus driving off volatile substances, which become vapour, at a comparatively low temperature-why breaking it before this process-smelting-that when a mass of any particular ore is heated to the point at which the metal fuses, it sinks down in this fluid state to the bottom of the furnace ;-to point out how certain other substances are sometimes used, called fluxes, to assist in the fusion of minerals ;-that when a sufficient quantity has accumulated in a fluid state, and sunk down from the earthy and other matter in the ore, the furnace is tapped, and it runs off into moulds-called pigs, sows, &c., by the workmen.

Swansea, in Wales, a place where a good deal of ore is carried for this purpose-from Ireland, and also foreign ores are taken there.

One mode of separating silver from the other substances in the ore is by pouring in quicksilver, which unites with the silver, and is afterwards pressed out.

The metals themselves, pointing out those which are called precious metals, those which are most useful-the particular properties which make them so useful, such as being fusible, ductile, malleable, and the different degrees in which they are so; their melting point, and the temperature at which they do melt, showing a very wide range (by calling their attention to these extremes, the instruction becomes more striking, and is more attended to) their specific gravities, which may be pointed out from a table, making them handle the substancesplatina and gold, how heavier than any of the others-twice, three times, &c. heavier than some- -the property of welding only belonging to iron and platina-how much this increases the usefulness of the former.

It is easy to see the rougher and more every-day purposes of life for which the metals are used, but it will be also useful, more particularly in the schools in our large towns, to call their attention to their uses in the arts; why one metal oxidizing rapidly in the atmosphere or in water, and another not, would, in certain cases, make the latter preferable, as in the copper sheathing of ships, &c.

Again, a union of metals is called an alloy-when one is quicksilver, an amalgam ;-an instance of the former, bronze, consisting of copper, with a small proportion of tin, and sometimes other metals, and used for casting statues, cannon, bells, &c.; of the latter, an amalgam of tin, with which lookingglasses are covered on the back surface; mercury very readily combines with gold, silver, lead, tin, bismuth, and zinc, but more difficultly with copper, arsenic, and antimony, and scarcely at all with platina and iron. Mercury, from the circumstance of its dissolving completely many of the less valuable metals, is very often adulterated.

Some metals have so little of affinity for each other that they have never yet been known to form an alloy, and even many whose fusing point is nearly the same will not unite; the density of an alloy is sometimes greater than the mean density of the two metals of which it is made up, which shows that a decrease of volume has taken place, as bronze ;-others again are lighter, showing an increase of bulk.

Alloys which consist of metals that fuse at different temperatures will often be decomposed by heating them to a temperature at which one of them melts; this is practised in extracting silver from copper. The copper containing silver in it is melted with three and a half times its weight of lead, and this alloy of three metals is exposed to a sufficient heatthe lead carries off the silver in its fusion, and leaves the copper in a spongy lump-the silver is afterwards got from the lead by another operation.

Alloys containing a volatile metal may be decomposed at a strong heat, driving off the metal which is volatile, as water is driven off at a less temperature from any salt it may contain.

The specific gravity of an alloy is a means of finding out the proportion of two metals in a given substance.

The substances used for soldering are instances of alloys; they are mixed metals for the purpose of uniting metallic bodies, but it will be necessary that the solder should melt at a lower temperature than the bodies to be soldered.

Those which are called hard solders will bear hammering, and are generally made of the same metal with the one to be soldered, mixed with some other which makes it more fusible.

Soft solder, such as tin and lead in equal parts, used by the glaziers, melts easily, and cannot be hammered: tin, lead, and bismuth, in equal parts, melt still more easily. In the opera

tion of soldering, the surfaces should be made clean, otherwise they would not unite so well. The glaziers use resin with the solder, to prevent the metals rusting, uniting with the oxygen of the air.

A bar (whose length at 32° is taken as unity) of the following substances will, when heated to 212°, the boiling point of water at the ordinary pressure of the atmosphere, expand: glass, of its length; steel, about; iron, 1; copper,

; silver, ; tin, 4 ; lead, 1; or a rod of iron whose length (temperature 32°) is 846 inches, will, at the heat of boiling water, expand one inch, and become 847; tin, length 462 inches, would become 463.*

In consequence of this expansion of iron by heat, it is necessary to make allowance for it in building bridges, otherwise the difference between winter and summer heat might cause an expansion which would bring the bridge down.

Again, on the absorption and radiation of heat by different substances a few useful lessons may be given, and the simple and well-known experiments of Leslie, which are easily tried, may be made very instructive.

From these it is shown that smooth polished surfaces of metal reflect heat, and absorb comparatively little; that scratching or in any way roughening the surface of a metallic body increases its power of absorption, and blackening it with anything increases it still more.

Experiment. Take, for instance, three circular pieces of metal, as tin, nine inches in diameter, and raised on a stand of a few inches high-one smooth, another scratched and roughened, the third blackened-the back of each being smeared with tallow, or some substance which melts at a low temperature; then placing a red-hot ball of iron at equal distances from any two of them, it will be found that the tallow on the blackened one will very soon melt, that on the roughened surface next, while the smooth surface would remain nearly at the temperature of the room; of course this experiment might be tried with different substances, and metals scratched and blackened in different degrees.

Another of Leslie's experiments. Take a cubical vessel, made of tin, one surface blackened, a second scratched, the third more roughened, and the last smooth; fill it with boiling

* See Appendix (B)..

water, and place his differential thermometer near it, and, turning each side in succession towards it, it will be found that the quantity of heat radiated, or thrown off from the different surfaces, will be in the order mentioned above. Professor Leslie covered the surface of the vessel with thin plates or layers of different substances of different colours, and noted the number of degrees which the thermometer rose, and thus ascertained the radiating power of each particular covering.

He then, instead of blackening or otherwise meddling with the faces of the tin vessel, made it perfectly smooth, and covered the bulb of the thermometer with the different substances, and found by the way in which it was affected that they absorbed heat much in the same way as they had before radiated it when on the tin vessel.

His experiment of heat reflected from parabolic reflectors is a very curious one, and they are well worth the expense of purchasing, in order to try the experiment, from the instruction it gives. A pair of these reflectors is a useful apparatus in a school.

As an instance of roughened bodies absorbing heat and then radiating it again, and of polished surfaces reflecting it-take the case of a blackened rough fender and polished fire-irons→ the latter are generally nearer the fire than the fender, touch them and they will be found much the coolest; the fender having absorbed the heat, the irons reflected it.

The different degrees in which bodies absorb heat depends also on colour.

Dr. Franklin observed that when he laid pieces of differently coloured cloth upon snow, it melted more rapidly under the dark colours than the light. And black and red inks, for example, when exposed to the sun, become heated in different degrees from their absorbing the light which falls upon them, and consequently the heat in different degrees; while pure water seems to transmit all the rays equally, and is not sensibly heated by the passing light of the sun.

The teacher should also note the difference between the radiation of heat from the sun and that from any other bodies -that from the sun passing through air and glass, water, &c., the other not, or if so, in a very slight degree.

The following experiment, attended with no expense, affords a good practical hint-two old teapots will serve, one of white metal, the other of black earthenware.

Fill them with boiling water, or with hot water from the same kettle-after standing a given time, place a thermometer in them, and it will be found that it will stand much higher in the metal one than in the other; showing that for the purposes of making tea the metal one is the better, not radiating the heat so rapidly; but if placed before the fire the black one will absorb heat better than the other. A black earthen teapot loses heat by radiation, in the proportion of 100; while one of silver or other polished metal loses only as 12.

Thus hot water running in a blackened pipe or rough one, will give out its heat more rapidly than in a polished smooth

one.

A solid, when changed into a fluid state, absorbs heatsome solids soften in melting, as wax, tallow, butter, and then become fluid; others, as ice, change at once.

In changing from a fluid to a state of vapour, heat also is absorbed; on the contrary, bodies in passing from vapour to fluid, and from fluid to solid, give out heat.

Water in freezing gives out heat, while in the melting of snow and ice heat is absorbed; hence the chilling cold felt in a thaw, after there has been a great fall of snow; also the gradual melting, in consequence of the latent heat in changing from snow into water.

Fluids become vapour also at different temperatures, their boiling point depending upon the pressure of the atmosphere, which varies with the altitude above the level of the sea, as well as from other causes; they may also be heated beyond their boiling point in the atmosphere, by subjecting them to artificial pressure.

The following questions will suggest a few important things, on which the teacher who wishes to understand this subject may inform himself.

Why, as water in boiling becomes vapour, and as it were boils away, does its temperature not rise above 212°? When