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The Wonder Book of Chemistry by  Jean Henri Fabre





HE boys had just found an old rusty knife-blade in the garden. A few weeks earlier they would not have paid the slightest attention to this useless scrap of iron: it was not worth looking at, much less picking up; but since their uncle had told them about the combustion of metals they were looking at things in a different light, and so this scrap of rusty iron was considered worthy of examination. There is nothing like knowledge to give food for thought. What the ignoramus disdains to look at, the well-informed person picks up, examines, and often finds deserving of serious study. So the old knife-blade was picked up by Jules, who at once noticed the close resemblance between the reddish iron-rust and the fine powder with which the burning of iron in oxygen had coated the inside of the bottle. He called his brother's attention to this resemblance.

"Here is a scrap of iron," they said to each other, "that no one ever thought of burning in a bottle full of oxygen, and yet it has turned to rust just as the watchspring did after being set fire to with a bit of tinder. How did it happen? Let's go and ask Uncle Paul."

[225] In reply to the question, their uncle said to them at lesson-time:

"Most metals, if left to themselves after being polished, will gradually become tarnished, taking on a sort of coating very different from the original luster. If you cut a piece of lead with a knife, the section shows bright and shiny; but his brightness soon fades, and the cut tarnishes until after a while it looks as dull as the rest of the lead. Something of an ashy-gray color has overspread the shining surface. So with iron or steel: how it shines when it comes freshly polished from the manufacturer's hands, and how little of that shine is left after it has been for some time exposed to the air! At first of a brilliant luster almost rivalling that of silver, it gradually becomes covered with reddish spots that grow in size every day, until finally they spread over the whole surface and eat into the metal. This process we call rusting, and sooner or later the metal will be completely changed to a red earthy substance. That is what happened to the knife-blade you found in the garden.

"Lead also rusts, in its own way. Instead of turning into a red earthy substance, it turns into a grayish earthy substance. The dull coating that so quickly tarnishes the fresh cut in lead is the beginning of rust. In time this coating would become a thick earthy layer. Zinc rusts in like manner: on the surface it is of a dull gray; within, or a shiny white. Copper keeps its polish no better, becoming overlaid after a while with a green coating due to the rusting of the metal. So it is that our com- [226] monest metals all have a serious defect: they lose the brightness so pleasing to the eye and turn into a crumbling earthy substance. In a word, they rust.

"Such is the fact. What is the cause? We need not look far to find it. You have seen iron burning in oxygen and powdering the inside of the bottle with a fine reddish dust having all the appearance of rust. In fact it is rust, nothing else. When we put a few pieces of zinc into an iron spoon, the other day, and thrust the spoon into a bed of glowing coals, you saw the zinc melt, catch fire, and turn into a white flaky substance. That substance is zinc-rust. Lead, if kept melted long enough in a furnace supplied with a current of air, changes into a yellowish earthy substance, which is lead-rust. A sheet of copper held in the fire loses its red color and turns black, at the same time giving the flame a beautiful green tinge. The black material thus formed is copper-rust. In short, these different rusts are all burnt metal: they come from the combination of these various metals with oxygen; in other words, they are oxids.

"So far we have been on familiar ground, thanks to our experiments in the burning of metals; but here we come to something new. These oxids that come into being amid such dazzling splendors of light and such generation of heat—these rusts that have their birth amid gorgeous displays of fire-works—are in no wise different from those common rusts that slowly tarnish the surface of metals. When a piece of iron, buried in the damp earth, [227] slowly becomes encrusted with reddish matter, and when another piece of iron burns with a brilliant light in a bottle filled with oxygen, it is the same chemical action going on in both instance. When one bit of zinc takes on a grayish coating and another, melted in an iron spoon, burns with a beautifully colored flame, leaving white flakes as the result of the combustion, the process is essentially the same in the two cases. In both, the oxygen of the air combines with the metal. Common rust is an oxid, a burnt metal, and whenever and wherever it forms there is real combustion, whether heat be sensibly present or not. A few further examples will not be out of place here.

"Exposed to the air for a long time, a piece of wood is gradually consumed: it turns dark, and finally crumbles to a brown dust. This crumbling of the wood is really slow combustion, differing only in its gradual accomplishment from the combustion attended by fire. The rotting wood combines with the oxygen of the air as burning wood does in a fireplace, and, like burning wood, it even gives out heat. We all know something about the heat that is thus produced. This inside of a dung-heap is decidedly warm, and a damp haystack may become so heated as to take fire. In both cases there is a slow burning of vegetable matter under the action of the oxygen in the air. So, too, with decaying wood: it is in slow combustion, and it gives out heat.

"Why this heat is not felt can be easily explained. Suppose a log takes ten years to be consumed by the slow process of decay, and that a similar log would [228] burn to ashes in an hour. In both cases heat would be generated, but with the rotting wood this heat would be given out very slowly, as it must spread itself over ten years. Naturally, then, it would be too slight to be felt. With the log that is burned in the fireplace, however, heat would be rapid and abundant, for it would all be crowded into one hour of time, and could not fail to make itself felt. It is clear therefore that, although at bottom the chemical action is always the same, there are many different degrees of rapidity with which things may burn. An old decaying tree trunk, a heap of steaming dung, a damp haystack smoldering within, a stick of wood blazing on the hearth—all these are so many examples of slow or rapid combustion; in them all, the oxygen of the air combines with solid matter that can burn, and the only point of difference lies in the rate of combustion. There is rapid and there is slow combustion,—rapid when the substance is ablaze and burns with much heat and light, slow when it is consumed gradually and without any flame or light, and often with no perceptible heat. The first kind of combustion is brilliant but short; the second makes no show and is much longer.

"Rust, then, is to metals what decay is to a heap of vegetable matter: it is the result of slow combustion. Exposed to the air, especially to damp air, metal combines with oxygen, becomes oxidized, as we say; that is, it turns into the compound we call an oxid. This explains why the old knife-blade took on a reddish crust, why freshly cut lead tar- [229] nishes almost immediately, why zinc, lustrous inside, wears a grayish coating on the outside. That red crust is oxid of iron, that dull film on lead is oxid of lead, and that grayish coating is oxid of zinc. A longer or shorter contact with moist air has sufficed to burn the metal, at least on the outside.

"Nearly all metals act thus. Eaten by the oxygen of the air, they turn to rust, yellow or red for iron, green for copper, garish white for lead and zinc. Not all turn with the same readiness. Among common metals iron rusts the quickest; then come zinc and lead; next in order are copper and tin, and after them we have silver, which can be kept untarnished a long time. Gold is an exception: it never rusts, and it is precisely this quality of always retaining its luster that makes it so valuable. Gold coins and ornaments have come down to us from the earliest times as clean and bright as if made only yesterday, despite the long centuries they have lain in a damp soil that would have reduced other metals to nothing but rust."

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