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The Wonder Book of Chemistry by  Jean Henri Fabre


 

 

CHLORIN

[360]

"W
E have more than once had occasion to speak of salt, common cooking-salt and I have told you that it is composed of sodium and chlorin, a metal and a metalloid. In the language of chemistry, salt is chlorid of sodium."

"Are you going to show us some sodium and let us see what it looks like?" asked Emile, his curiosity aroused by the little he had already heard about his metal.

"No, my young friend. Sodium, though not so very rare in drug stores, is too expensive a luxury for our little village laboratory; and so we must content ourselves with a mere description of it. Imagine something that shines like freshly cut lead, and so soft as to yield to the pressure of one's fingers. In fact, it can be molded like wax. Put a piece of it on water and, floating there, it will catch fire and spin around all wrapped in flames. Potassium, the metal contained in ashes, does the same, only more violently. We are now in a position to understand why these two metals have this strange peculiarity of catching fire as soon as they come into contact with water.

"What is water really made of? Oxygen and [361] hydrogen. Ever since our visit to the blacksmith's shop we have known that red-hot iron decomposes water, appropriating its oxygen and releasing its hydrogen. Unheated iron and also zinc will decompose water in the same way, with no use whatever of fire, if sulphuric acid is added to assist the process. Well, potassium and sodium, as well as some other substances, notably the metal of lime, or calcium, are more active than iron and zinc in the presence of water. Left to play their part unassisted, without heat and without the help of sulphuric acid or anything else, they decompose the water, take to themselves its oxygen, thus becoming oxids, and release the hydrogen. Now, it is this uniting of the oxygen with the metal, this combustion, that develops heat sufficient to set the freed hydrogen on fire; and that explains the flames enwrapping the metal as it spins around on the surface of the water. When the flames die down, the potassium or sodium has disappeared, without leaving the slightest visible trace behind; but the water in which the oxid of the metal was dissolved has now a burning taste and an odor of lye; moreover, it will restore the blue color to litmus previously turned red by an acid.

"If I have no sodium to show you, I can at least let you see the element with which it is joined in partnership in our common salt; I can show you chlorin, which is of more importance that sodium. To obtain chlorin from salt, use is made of sulphuric acid, that powerful agent employed in so many operations in the domain of chemistry. In the present instance the acid's part is to appropriate the [362] metal and thus set the chlorin free. But in order to combine with the acid, the sodium must first be converted into an oxid, into soda; and for this, oxygen is required. The duty of supplying this oxygen is assigned to dioxid of manganese, that same black powder we used in obtaining oxygen from a salt. Its function then was to regulate the heat throughout the mass of salt and thus make easier the decomposition of the latter. Now, however, its purpose if quite different: itself very rich in oxygen, its office is to give up some of this to the sodium, which will thus become soda; and this in turn will then combine with the sulphuric acid to form sulphate of soda. These combinations being effected, the chlorin will be left in a free state, no longer fettered by the bonds hitherto uniting it with the metal sodium.

"The apparatus for this operation is the same as that used in obtaining oxygen. Into our glass balloon I put a handful of common salt and the same amount of dioxide of manganese. I mix them well and sprinkle freely with sulphuric acid. Then, with its tube attached, the balloon is set in place over the brazier so as to receive the heat from the few live coals the brazier contains. A very gentle heat is enough, the release of chlorin beginning even before the temperature has risen at all. Chlorin is a gas heavier than air, and so we will collect it as we collected our carbonic-acid gas; that is, we will arrange the tube from the balloon in such a manner that it will reach to the bottom of our wide-mouthed bottle or jar in which we propose to store the chlorin.

"Up to this point we have had to do only with in- [363] visible gases. Air, nitrogen, oxygen, hydrogen, carbonic-acid gas, and carbon monoxid are gases that not even the sharpest eyes can see; and most other gases are of like character in this respect, so that gases as a class are thought of by us as invisible. Now, however, we have a gas that is as subtle and impalpable as the others and yet can be seen very well. It owes its visibility to its pale greenish-yellow tinge. Its name, chlorin, which comes from a Greek word meaning green, takes note of this quality.

"Because of this slight color possessed by chlorine we can watch the gas as it accumulates in the bottom of the jar, where it is held by its weight, and where, also, it displaces the air previously occupying the same space but now driven out by reason of its lesser weight. See there! In the bottom of the jar there appears a kind of fine yellowish vapor, the layer increasing in thickness little by little and filling more and more of the jar from the bottom upward. That yellowish vapor is chlorin; what is above it, colorless and invisible, is air. Wait a few minutes and the visible layer will reach the neck; the jar will then be full of chlorin."

As soon as it was full, the jar was covered with a piece of glass and a second jar filled in the same manner. But during the operation a little of the gas had escaped into the room, perhaps allowed to do so by Uncle Paul in order to teach his pupils how disagreeable chlorin is to breathe. Emile in particular learned the lesson so as never to forget it. Happening to be near the apparatus just when the empty [364] jar was substituted for the full one, he received a whiff of the offensive gas in his nostrils, whereupon he was seized with a fit of coughing such as no cold or whooping-cough had ever caused him. And so our giddy-pate coughed and spat and spat and coughed, but all in vain, in his effort to get rid of what was choking him. It needed his uncle's reassurance to calm his fears as to the consequences of this accident.

"It is nothing, my young friend; you'll get over your coughing in a few minutes. It was the chlorin that started it, but luckily only a very little of it reached you, and that little was mixed with a good deal of air, so that you smelt the terrible gas more than you swallowed it. Drink a glass of cold water, and that will help to clear your throat."

The cough did in fact soon subside, and the misadventure had no other consequence than to make its victim rather more cautious thereafter about venturing near jars of chlorin.

"Now you are over-cautious, my child," said his uncle. "A slight smell of chlorin is nothing to be afraid of; it may even be rather wholesome, especially when the air is foul with the products of decay. What is to be feared is breathing this gas pure and admitting it into the lungs in some considerable quantity. Whoever should fill his chest with it, as we fill our chest with atmospheric air, would die in fearful agony after a few breaths."

"I should think as much," declared Emile, "after the fit of coughing I got from just a whiff of it. But how queer it is that common salt should be [365] made of chlorin that suffocates us and sodium that would burn out our mouths if we took the least bite of it! It's lucky those two fearful substances change so much when they come together, or I should never again dare to salt a radish before eating it."

"It is lucky, too," continued his uncle, "that as soon as it is separated from sodium chlorin regains its energetic properties, for manufacturers profit greatly by these in certain branches of their industry. We will, however, confine ourselves to the one chief use that is made of chlorin, which is in bleaching. Into this jar filled with chlorin I pour some writing-ink from your inkstand there. I shake the jar to make the gas act on the liquid, and in a trice the thing is done. The ink, which was at first of a deep black, has turned to a pale yellow and now looks like slightly muddy water. The liquid so strongly colored at the start is left with hardly any color, the chlorin having destroyed the black of the ink.

"Another way to show the same thing will interest you even more. Here is a sheet of paper written on with ordinary ink, a sheet taken from one of your old copy-books. I moisten it with water to hasten the chemical action of the gas, and put it into our second jar of chlorin. Is it not marvellous to see what now follows? The written characters fade away rapidly, and the paper is left as white as it ever was. I take the paper out of the jar so that you may examine it more closely. Look at it well and tell me whether you can see any trace of the writing that was on it."

The boys gave the sheet of paper the closest [366] scrutiny, but could not distinguish a single letter. The paper looked as if it had never been used, and the utmost they could make out was an occasional scratch of the pen where it had pressed very hard.

"The writing has all disappeared," declared Jules, "and the paper is as good as new. Would sulphur gas do that? It turns violets and roses white."

"No, it would have no effect. Sulphur is too weak a bleacher. In many cases it is without effect, and here too it would be powerless. Chlorin, on the contrary, has so great a bleaching power that few dyestuffs can withstand it. In this respect it is the most useful agent known to the industrial arts. Nevertheless, not all colors fade away when subjected to the action of chlorin, as a third experiment will prove to you. I take a leaf of an old book of no value and write on it with ordinary ink, even smearing it with blots like a child that does not know how to handle a pen. When the ink is dry, I moisten the page a little and put it into the chlorin. My writing and my ink-blots vanish as by magic, but the print remains as black as ever. The page is now as clean as when first leaving the hands of the printer. The marks I made with writing-ink have been removed, but the printed words have lost nothing of their clearness: they stand out very black against the white background of the paper. The ink-stained and illegible page has been so transformed by the chlorin that it now looks like a leaf from a new book."

"But why is it," asked Jules, "that the printing- [367] ink stays as it was while the writing-ink is all bleached out?"

"That is because the two inks are made of different materials. Printers' ink is made of lampblack and linseed oil. Lampblack is a form of carbon and hence a simple substance, an element, and not something that can be decomposed. Now, chlorin acts on dyestuffs by decomposing them and then combining with one of their elements, hydrogen. Lampblack, which is carbon and nothing more, cannot be decomposed; it cannot give to the chlorin any hydrogen because it had none to give; hence it remains lampblack and thus keeps its black color. Not so with writing-ink, however: that contains various ingredients, being usually made of sulphate of iron, gallnuts, and logwood. The two latter belong to the vegetable kingdom and contain hydrogen, which the chlorin seizes; and, one of the elements being thus removed, the color disppears.

"The principal use of chlorin as a bleacher is in the manufacture of woven fabrics and of paper. The perfect whiteness of our linen and our writing-paper we owe to chlorin: and so it comes about that before we can have any white paper for writing or printing, or any white cotton or linen for making shirts, handkerchiefs, curtains, and other articles, as well as for making gaily colored calico, we have to call the services of salt,—or, rather, of that element in salt that we obtain with the help of sulphuric acid and then use as a bleaching agent. There you have an example, to add to the many [368] other examples, of the important part sulphuric acid plays in our manufactures.

"The natural color of hemp and linen is a light reddish, and so fixed is it that it disappears only after repeated washings; hence, the longer a piece of linen has been used and the more times it has passed through the laundry, the whiter and softer it is. To give to linen the utmost possible whiteness, we spread it out on a closely mown meadow and leave it there for weeks at a time, exposed to the sunlight by day and to dampness by night. This prolonged exposure to sun and air, with alternate moistening and drying, finally weakens the hold of the reddish color so that subsequent washings gradually remove it entirely.

"But this method of bleaching is very slow, and when it is applied to great quantities of cloth and over long periods of time, it is also very costly, because it withholds a considerable area of land from productive use. Consequently, in factories where linen and hemp and especially cotton goods are made, a more powerful and less dilatory bleaching agent than sun and dew is called into service; and this agent is chlorin, whose speedy effect on ink you have just seen. Evidently a gas that can take out so black a color as that of ink, and do it so quickly, can easily remove the faint reddish tinge marring the whiteness of hemp, linen, and cotton."

"Wool and silk, too," suggested Jules, "could be bleached with chlorin, and that would be a much quicker way than with burning sulphur."

"But no one except a bungler would for a moment [369] think of trying it," replied his uncle. "This gas would attack wool and silk and soon reduce them to a mere pulp."

"But you say cotton, linen, and hemp can stand it."

"Yes, but their resistance to the action of drugs has no parallel,—a fact that give them inestimable value. Just think of the many uses we make of linen and cotton and hemp fabrics, and what rough handling they receive,—repeated washing with corrosive lye and the strongest soap, rubbing and beating, and exposure to sun, air, and rain. What sort of material, then, is this that withstands the harsh treatment of washing with soap and of exposure to sun and weather, that remains intact even when all around it is decay, and that defies the manufacturers' powerful drugs and emerges from all these tests whiter and more supple than before? This almost indestructible material is the vegetable fiber of the plants known to us as hemp, flax, and cotton, a fiber unrivalled in its kind. Chlorin, which leaves textiles made of this fiber uninjured, white at the same time bleaching them to an exquisite whiteness, destroys all fabrics made of animal fiber, such as woollen cloth from the sheep's fleece and silk goods from the silkworm's cocoon.

"So common is the use made of chlorin as a bleaching agent that there are many factories devoted solely to its preparation. To make it portable and convenient for use we store it up, as we might say, in lime, which absorbs it freely. This compound is a white powder like lime itself, with a very [370] strong and penetrating odor. It is called chlorid of lime and is a veritable storehouse of chlorin; and it is this that is commonly used whenever a powerful bleaching agent is needed.

"Now I must tell you about the part played by chlorin in paper-making. We commit our thoughts to writing without reflection on the many processes necessary to produce the white paper on which we write. Thousands of years ago the Assyrians of Babylon and Nineveh wrote with a sharp-pointed style on an unhardened clay brick, which was then baked in an oven to fix the writing for all time. If any one wished to send a letter to a friend, it took the form of a brick similar in weight and size to those we use to-day for building."

"With a load of letters like that," said Emile, "postmen to-day would be so weighed down they could n't stagger along."

"If it was desired to write a book," resumed his uncle, "for after ages to read,—a history of the memorable events of the time, for instance,—the work occupied whole shelves of a library, each page of the history being represented by one of these baked bricks. A single one of our printed volumes would have required in the writing enough bricks to build a house. You can judge from that how comparatively small must have been the amount of reading-matter in even a large library of those remote times, when each leaf was so bulky and cumbersome. A few remains of those ancient brick books have come down to us, having been dug up on the sites of Nineveh and Babylon; and these [371] literary remains have been deciphered and translated.

"Much later, another method of writing, no less strange in our eyes, came into use in those same regions of the East. A reed cut to a pen-point was the writing-instrument, and a black liquid made of soot stirred up in vinegar was the ink, while the paper was a bone, the broad flat shoulder-blade of a sheep, bleached by long exposure to the weather. A packet of writing on the subject—a book, in short—was made up of a number of these bones all tied together with a string.

"In the Europe of long ago, particularly in Greece and Rome, where civilization was most advanced, it was customary to use wooden tablets coated with a thin layer of wax, on which one wrote with a pointed instrument, a style, sharp at one end and having a wide flat head at the other. The pointed end used for tracing the characters in the wax, the flat end for erasing them and for smoothing the soft surface for fresh writing.

"Of all ancient peoples, the Egyptians came the nearest to inventing something like the paper of modern times. On the banks of the Nile there grows in abundance a kind of reed called papyrus, whose outer covering peels off in long strips, thin and white. These strips were soaked in the muddy water of the river, which served as glue, and were then arranged side by side, with a similar layer over them, but running crosswise. Pressed flat and beaten with a hammer, the whole made a sheet suitable to write on. Here again the pen was a pointed [372] reed and the ink the same liquid prepared from soot. From the word ‘papyrus' we get our word ‘paper.'

"Papyrus sheets were not cut into small oblong pieces with square corners, such as we are so familiar with, but were made each in one continuous strip, its length varying with the amount of writing to be received. Hence, a papyrus book was all in one sheet or strip, which, for convenience in handling, was rolled around a little wooden cylinder to which the end of it was fastened. When we read a book we turn the leaves one by one, and these leaves have printing on both sides. The ancients followed a different method: they unrolled little by little the long strip of papyrus containing writing on only one side.

"The invention of paper is attributed to the Chinese. In the ninth century the Arabs introduced its manufacture in the nearer East, but its use did not become general in Europe until the thirteenth century. About the year thirteen hundred and forty the first paper factories were established in France. Paper such as you have in the fair white leaves of your copy-books, and such as is used in making our more costly printed books, comes from the despised contents of the ragbag. Shreds and tatters are collected, some being taken out of the mud in the street, and some bearing the marks of unspeakable filth. They are sorted out, the better ones for fine paper, the inferior for coarse. After receiving a vigorous and much-needed washing, they are shredded until the woven fabric is reduced to lint, this process being the work of a cylinder [373] equipped with sharp blades and revolving in a trough containing the rags soaking in water. Thus torn to bits, the rags are at last reduced to a sort of pulp or semi-liquid paste, which is gray in color and has to be thoroughly bleached before it can become the perfectly white paper so familiar to us. This bleaching is done by adding to the pulp, while the cylinder is still in motion, a weak solution of chlorin furnished by the chlorid of lime already mentioned. That is the office of chlorin in paper-manufacture, the bleaching of the rag-pulp to a spotless whiteness.

"But before paper can serve for writing on, it must be prepared in such a way that ink will not soak into it and spread in all directions, thus making the written characters illegible. To this end the pulp or paste receives a certain amount of what is known as size, made of resin and starch. If, however, the paper is intended for use in printing, this preparation is unnecessary; and that is why the paper of our books absorbs ink so freely if we try to write on it.

"The rag-pulp, bleached by chlorin and treated with resin and starch, is now ready for the final operation. With its bits of thread crisscrossing in all directions, it will presently come forth in a thin sheet that will be paper. A machine too complicated to be described here accomplishes this final part of the process. The paste runs in a continuous film over a fine wire netting, which retains the coarser particles and lets the finer pass through. A second and still finer wire netting, moving on rollers, [374] receives what falls from the first, retains the pulp, and drains off the water, the drainage being hastened by a slight side-to side movement of the netting. In this way the rag-pulp is spread out in a uniform thin layer. Carried onward by the netting on which it is spread, this layer, this undried sheet, is brought into contact with a broad woollen belt, to which it clings, and by which it is conveyed over a hollow cylinder heated within by steam. On this cylinder the paper becomes dry and firm, after which it is rolled up on a second cylinder in a continuous broad strip of indefinite length. A few minutes only are needed to transform the semi-liquid to pulp in the trough into paper ready for use. All that has to be done after this is to cut the strip rolled up on the last cylinder into sheets of the desired size.

"In future, whenever you read a printed page or write in your copy-book, remember that we owe the beautiful whiteness of the paper to chlorin, the gas made from our common salt."


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