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The Wonder Book of Chemistry by  Jean Henri Fabre

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N the damp walls of cellars, wine-vaults, and similar places, there is often to be seen a sort of fine white fluff resembling the most delicate down. One might imagine the stone to be covered with a soft fleece. We have already spoken of this curious coating, and Jules has told us how by brushing a damp wall with a feather he has collected some of this material, which, on being thrown on to live coals, has straightway caused a brilliant burst of flame. Its common name is saltpeter, which means salt of stone or salt of rock, because it is on the surface of stones in our buildings or of the rock in caves and underground vaults that this saline matter is found. Chemistry calls it ‘nitrate of potash,' a name indicating that it is composed of nitric acid and potash. It is a storehouse of easily obtainable oxygen, which explains why, on being thrown on to live coals, saltpeter makes the coals burst at once into flames. It is the oxygen set free from the saltpeter that produces this result.

"Man is able to make, by going the right way about it—that is, by bringing together the necessary elements—sulphuric acid, sulphurous acid, carbonic acid, phosphoric acid, and many other acids. Burn [376] sulphur, carbon, phosphorus, and there you have, immediately, the last three acids. Sulphuric acid is harder to make, calling for a far more elaborate process than simple combustion. Nevertheless it is made, and in great quantities, too. But nitric acid is quite a different matter: its formation is so hard to bring about that chemists have not yet succeeded in obtaining it by any direct combination of oxygen with nitrogen, the reason for this being the very slight propensity possessed by nitrogen for combining with other elements. It is an inert gas, and inactive element, rebelling against any sort of chemical combination. We have plain proof of this in what takes place every day in our furnaces and stoves and grates. Through the burning fuel, where a temperature is very high, there is constantly flowing a stream of atmospheric air, a mixture of oxygen and nitrogen; yet, despite the great heat, this latter gas does not burn, does not combine with its companion, oxygen, but comes out of the fire the same as when it went in. In short, it is incapable of combustion in the ordinary sense of that word.

"However, what neither chemical skill nor the heat of our furnaces can do, nature accomplishes slowly, noiselessly, without any use of fire, by subtle processes that elude our observation. In the porous substance of a damp stone, the combination of nitrogen with oxygen is brought about, resulting in nitric acid, which finds a little potassium in the stone wall, and so unites with it to make saltpeter. It is to this saltpeter on stone walls and elsewhere that we go for our nitric acid. The method is very [377] simple: all we have to do is to drive the nitric acid out of this compound with a stronger acid. Here the same brutal law prevails as with carbonic-acid gas. ‘Get out of here and make room for me,' say the stronger intruder. Sulphuric acid, the indispensable aid in most of these chemical changes, is made to accomplish this displacement. It is added to the saltpeter, and the whole is then heated. Dislodged from its place, the nitric acid escapes as a gas and is collected in a cold receiver, where it condenses as a liquid.

"Now we have obtained nitric acid, so that here we have, chemically united, the two gases (oxygen and nitrogen) that in a merely mechanical mixture constitute the air we breathe. If you were not already well aware of the enormous difference between mere mixture and chemical combination, you would here have a striking example of that difference. The air we breathe and the terrible acid furnished by saltpeter have as constituents the same two elements. I say ‘terrible acid,' and the adjective is deserved. Nitric acid is, in fact, so extremely violent in its effects that to denote its potent qualities we often speak of it as aqua fortis  (strong water). One drop of it on the skin instantly produces a yellow spot, and the result is that the affected skin is burned through and falls off as a dead scale. If the acid is kept in a corked bottle, it speedily corrodes the cork and reduces it to a yellow pulp.

"Metals themselves, even the hardest of the, are eaten by nitric acid. This liquid is a veritable [378] storehouse of oxygen, containing quantities of it and yielding it very freely. Hence, it corrodes or burns most substances that it touches. With its abundant oxygen it causes a sort of combustion, with results that are sometimes the same as those of ordinary combustion. No fire is seen, no flames burst forth, and yet there is really what amounts to combustion, since there is a combination of oxygen with the substance attacked, and this combination is attended by a marked rise in temperature.

"Let us take some examples of this corrosion of metals. I pour a little nitric acid on some iron filings. A dense red vapour immediately rises, while there is a very audible sound and the mixture becomes heated. In a few moments the iron is completely burnt up, turned to rust. I apply the same treatment to this tinfoil, which was wrapped around a cake of chololate, and the same red vapour rises, the same noise is heard, the same increase of temperature is felt. The tin is turned to a white pulp. It is now burnt tin, rusted tin, oxid of tin. I repeat the experiment with copper, and the same results follow, except that the copper-rust is dissolved in the acid as fast as it forms, and produces a greenish-blue liquid. But there are some metals that remain unaffected by nitric acid, and of this number is gold, which never rusts. Here is a piece of gold-leaf such as is used for gilding, and so thin that the slightest breath of air will waft it away. Well, this delicate gold-leaf stays in the acid without showing any effect whatever. It keeps its luster, and will always keep it. Gold does not corrode [379] even it the acid is heated to the boiling-point. I will ay in passing that this is the test applied by gold-smiths to distinguish the precious metal from copper, which it so nearly resembles in appearance. Copper is eaten by nitric acid, whereas gold remains unaffected.

"Metal-engravers turn this property of nitric acid to account. When they wish to engrave a copper plate, for example, they first overlay it with an impermeable coating, using melted wax for this purpose. On this coating they then trace the design to be reproduced, and with a fine-pointed instrument remove the wax so as to lay bare the metal wherever they wish it to be eaten away. After this they pour weak nitric acid on the plate thus prepared. Wherever the copper is protected by the wax coating, no effect is produced, but where it is exposed the acid plows a furrow. As soon as the acid is thought to have done its work, the layer of wax is removed and the design is found reproduced in the lines cut into the metal by the corrosive acid.

"So much of nitric acid. Now let us briefly consider its compound called saltpeter or nitrate of potassium. This is used chiefly in the manufacture of gunpowder, which is made by mixing well together, in the right proportions, sulphur, carbon, and saltpeter. So you see there are in gunpowder two highly inflammable substances, sulphur and carbon, together with a third substance, saltpeter, which furnishes an abundant supply of oxygen when it decomposes. Consequently, as so, on as gunpowder is set fire to, the saltpeter gives off oxygen [380] freely, and this sulphur and the carbon, which thus become suddenly converted into gas. The amount of gas so generated is enormous. If left free to expand to its full volume, it would occupy one hundred and fifty times the space filled by the gunpowder producing it. Confined, then to a space much too small for it, this gas makes a vigorous effect to free itself, pushing out its way with great violence the bullet or ball or anything else that blocks its path, just as a spring forcibly pressed down exerts a powerful thrust on whatever holds it in that position.

"We must now make the acquaintance of another nitrogen compound, one of the utmost importance, especially in agriculture. In this bottle is a liquid that looks exactly like water. I should not, however, advise you to put the open mouth of the bottle to your nose, as your sense of smell would be too painfully affected; but take the slightly moistened cork and give it a cautious sniff. Now what can you tell me about it?"

"Pfui!"  cried Emile after smelling of the stopper with the utmost circumspection, so suspicious was he of all chemical odors since his experience with the chlorin. "My, how that smarts! It gets up your nose and makes it feel as if it were pricked with a lot of sharp little needles." And he rubbed his eyes, in which the tears were gathering, though he felt not the slightest inclination to cry. He passed the cork to Jules, who at once recognized the liquid by its smell.

"Why, that must be ammo?ia," he declared. [381] "It's what the tailor was using the other day to take out a grease-spot and clean the collar of an old coat. I knew it as soon as I smelt it. Besides, this stuff makes the tears start, and that's just what the tailor's ammonia did when I got too near it. It was in a cup mixed with water. For a minute or two my eyes were all red and full of tears."

"You are quite right," replied his uncle. "It is indeed ammonia I am showing you in this bottle. It is also called ‘volatile alkali' and ‘spirits of hartshorn,' but ‘ammonia' is the usual name. A useful property it possesses is that of uniting with grease and making a soluble combination that can be removed by washing. That is why we use it in cleaning garments spotted with grease. With a small stiff brush we first rub diluted ammonia into the soiled places, after which a simple washing with ordinary water will take out the grease. That is what you saw the tailor doing.

"In its composition this cleansing liquid is water containing in solution a large amount of a peculiar gas called ammonia gas. This solution is liquid ammonia or volatile alkali or spirits of hartshorn, its active ingredient being the gas I have just referred to."

"Then liquid ammonia and ammonia gas are tow different things?" asked Jules.

"Yes, they are different from each other. Ammonia gas is an invisible, colorless gas that stings the nose smartly and draws tears; but when we speak simply of ammonia we commonly mean the [382] liquid preparation, made by dissolving a great quantity of this same gas in water, to which it imparts its peculiar properties. What I am showing you in this bottle is water in which is stored up an enormous volume of ammonia gas. I say an enormous volume, for in one liter of water there are more than six hundred liters of ammonia gas, the two making together about one liter of liquid. From this well-filled storehouse there is a gradual escape of gas, and that is what has so strong a smell and makes tears come into the eyes. If the gas escaped rapidly, as it would if we heated the liquid, it would overpower us with its pungent odor."

"And it would make us all cry our eyes out, even though we might wish to laugh instead," added Emile. "Chloin is the gas that makes us cough, ammonia the gas that makes us cry. Each one has a trick of its own."

"That is well said," his uncle agreed. "Ammonia acts strongly on the eyes, making them red and filling them with tears. This peculiarity, together with the pungent odor, enables us easily to detect the presence of this gaseous compound.

"To obtain ammonia gas, we heat to redness certain animal substances of little value, such as old woollen rags, hair, bones, and scraps of leather; and among the gaseous products of the resulting decomposition is the gas we are after. It is collected by simply dissolving it in water. The process of getting illuminating-gas from coal also gives ammonia. The water through which the crude coal gas is passed in order to purify it arrest this other [383] gas in its passage, and finally becomes abundantly charged with it.

"Ammonia gas is composed of nitrogen and hydrogen. On account of nitrogen's disinclination to combine with other elements, any direct union of the two gases so as to produce this compound is a difficult as it the direct production of nitric acid. Chemical science is still unable to make ammonia gas in this direct way, and it is doubtful if it will ever be able to make it thus in any large quantities. This inability is much to be regretted from the farmer's point of view, for ammonia, which to you is merely a good cleaner of soiled clothing, plays a most important part in our fields and gardens, contributing greatly, in the crops it helps to produce, to our daily bread. All forms of life, vegetable as well as animal, contain nitrogen. When they die they give back their elements to the inanimate world by decaying. Their carbon is dispersed in carbonic-acid gas, their hydrogen in water, and their nitrogen in ammonia. But all these products of decay are taken up again by vegetation, the carbonic-acid gas giving carbon, water yielding hydrogen, and ammonia gas supplying nitrogen, while oxygen is everywhere present. Out of these four elements thus assembled by the plant, is built up the substance of our bread, our vegetables, our fruit of all kinds. Refashioned by the animal, which finds it in the plant, this same material becomes flesh, milk, fleece, or some other useful product. In short, nitrogen, in order to reach the animal, must pass through the plant; and in order to reach [384] the plant, the world of lifeless matter must supply it in the combination known as ammonia. We see now why barnyard manure, which is so rich a source of ammonia, is so valuable a fertilizer in agriculture.

"A few word more on ammonia gas dissolved in water: This solution, this liquid ammonia or volatile alkali, is a colorless fluid of the same penetrating odor as the gas itself. It has a burning taste like that of lime and potash; and the resemblance goes even farther, for ammonia has the peculiarity of restoring its original blue color to litmus reddened by an acid. Potash, soda, or lime could not bring back the blue color better or more promptly. We have seen lime turn violets and other blue flowers green. Ammonia, too, turns them green, whether from their natural color or after they have been reddened by an acid.

"The uses of ammonia are numerous. We have spoken of it as a cleanser where grease-spots are to be removed; but it should be added that it will also act on the coloring matter in our garments, changing delicate shades. Hence, it should be used only on material of dark and fast colors that resist the action of this potent cleanser. And here let me tell you something that may be useful to you some day. Those who are engaged in chemical experiments often spill acid on their clothing. Dark-colored cloth is usually turned red by an acid; but a drop of ammonia on the red spot will make it disappear, restoring very nearly the original color.

"Ammonia is also used to counteract the effects [385] of a venomous sting, such as that of a scorpion, wasp, or bee, or even to prevent the more serious consequences of a viper's bite. Into the little would a drop of ammonia is poured, and if this is done promptly enough it usually forestall the action of the venom.

"Finally, ammonia, as it is found in various salts, is a most important food for all plants and vegetables, giving them nitrogen as it does so abundantly. Hence its great value in agriculture; and since manure in the process of decay gives out ammonia gas very freely, it is plain that this dressing must be very beneficial to land under cultivation. But nowadays there is a great demand for artificial fertilizers containing potash and phosphoric acid as well as ammonia."

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