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being very cold in frosty weather, and much colder than your hand, it carries away the heat much more rapidly than wood, and it has very little to give back in return; this rapid loss of heat causes a very unpleasant sensation; if you hold the iron long enough, it will get the same degree of warmth as the hand, and the unpleasant effect will cease; the stream of heat from the iron to the hand, and the hand to the iron, will exactly balance each other; that is, the two substances, your hand the one, and the iron the other, will then impart equal heat to each other.
They may also be told to touch the different substances, marble, wood, stone, iron, etc., with their lips, which, as they are much more sensible to cold, will point out to them more strikingly how much the sense of touch deceives them.
Experiment. The teacher taking a polished cylindrical piece of iron, with a piece of white paper held tightly over it, holds it in the flame of a candle, and observes it does not char- a piece of wood, exposed in the same way immediately turns black ; the iron being so good a conductor does not allow the heat to rest with the paper, but immediately takes it away, etc. ; the wood not conducting it so rapidly causes the paper to burn.
On this principle, water may be made to boil in a paper kettle, or in an egg-shell-when boiled away, both substances would immediately burn.
Experiment. Metallic rods of equal lengths and substance, one end of each smeared with beeswax, and immersed in a heated fluid, the heat travels along each rod, from particle to particle, and the one on which the wax melts first is the best conductor, the one on which it next melts the second best, and so on-the order in which the wax melts being the order in which, the rods conduct the heat.
The following experiment, which is easily tried, shows the way in which a fluid, as water, is heated by a flame placed under it: take a glass tube, open at one end, and about an inch or so in diameter ; pour water into it, so that there may be a column of several inches in length, and place it over a spirit lamp. As the flame heats the water, drop sand into it, and a double current will be observed, one downwards along the sides of the vessel, the other upwards through the centre of the fluid: apply the heat to the surface on the sides of the vessel, and the currents will be reversed. The reason of all this to be pointed out.
Glass - a solid, can be softened by heat, so as to be drawn out into a fine thread-allows light to pass through it; in what way does man turn this property to his use? Windows, lanterns, spectacles, telescopes, etc.; does not allow the heat of the fire to pass through it—the heat of the sun does-What other substances allow light to pass through them? Water, horn, air, etc.
Why will a glass sometimes break by pouring hot water in it? · Answer. Solids convey heat from particle to particle,* and some solids do this more slowly than others; glass conveys it very slowly, and the hot water in contact with the inner surface causes the inside surface of the glass to expand, but the outer one, not being so hot, will not follow it, and so snaps, being very brittle. Thin glass will not break so readily as thick, the distance between the two surfaces being smaller, the heat gets through sooner, and the inner and outer surface are almost instantaneously raised to the same temperature -- hence chemists use thin retorts.
On the snbject of metals used for the various purposes of social life, the class of teachers for whom these pages are intended may give a great deal of useful instruction.
They might draw attention to the different ores, showing specimens of them, and mentioning the kinds of earths and other substances with which they are generally mixed where found in our own and other countries—the per centage of metal found in an ore, in one case making it
* The following question is very suggestive, as an application of a domestic kind: one joint of meat is roasted, another is baked on a hot metallic plate heated by connection with the bars of the kitchen fire, and a third is boiled:-in what way is the heat transmitted in each case by which these joints are cooked?
wbat is called a rich one, in another so small as scarcely to make it worth working--anything peculiar in the way in which metallic veins run, not being stratified, etc.- depth of mines—the number of workmen employed in the mining of any particular ore, the method and necessity of transporting it from the place were it is found for the purpose of smelting, either from the people not knowing how or for want of coal, etc.,-great inconvenience of this in a commercial point of view, from having to transport so large a proportion of the ore which is useless (there may be other substances mixed with it which are useful).
“When a mass of matter is to be removed, a certain force must be expended; and upon the proper economy of this force the price of transport will depend. A country must, however, have reached a high degree of civilization before it will have approached the limit of this economy. The cotton of Java is conveyed in junks to the coast of China, but from the seed not being previously separated, three quarters of the weight thus carried is not cotton. This, perhaps, might be justified in Java by the want of machinery to separate the seed, or by the relative cost of the operation in the two countries. But the cotton itself, as packed by the Chinese, occupies three times the bulk of an equal quantity shipped by Europeans for their own markets. Thus the freight of a given quantity of cotton costs the Chinese nearly twelve times the price to which, by a proper attention to mechanical methods, it might be reduced” (BABBAGE on the Economy of Machinery).
Again, the mode of separating the metal from the different ores in some cases breaking it into small pieces and roast. ing 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, etc., 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 substances - platina and gold, how heavier than any of the others - twice, three times, etc., 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 the uses in the arts; why one metal oxidising rapidly in the atmosphere or in water, and another uot, would, in certain cases, make the latter preferable, as in the copper sheathing of ships, etc.
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, etc.; of the latter, and amalgam of tin, with which looking-glasses 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 temperature 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 heat—the 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 operation 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.
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