tubes, as in the figure for exhibiting the theory of sulphuric acid (or fig. 55.). On the application of heat to the side flasks, ammonia and hydrochloric acid are evolved, and unite in the middle flask, where they form dense white clouds of sal ammoniac. Ammonia may also be detected by the brown colour which it communicates to turmeric paper moistened. Paper moistened with an alcoholic solution of parietin (a colouring matter extracted from the common wall lichen by alcohol) is coloured pink by ammonia. Salts of NH3. All the oxygen acid salts of ammonia contain an atom of water; the formula for nitrate of ammonia is NH, NO, HO; and of sulphate of ammonia NH, SO, HO. The carbonate of the shops is a sesquicarbonate, consisting of 2NH, 3CO2 2HO. Ammonia unites with many salts which have been deprived of water, and seems to perform the functions of water. Sulphate of magnesia absorbs 2 atoms, sulphate of nickel 3.

-

Uses. Ammonia is a most important body in chemical analysis, from its action upon oxides. (See Tables on Tests.) Ammonia is used as a blistering agent in medicine. As a stimulant it is administered in water by the stomach. In sore throats it is mixed with olive oil (1 ammonia, 2 oil), and applied externally, under the name of volatile liniment; when held to the nostrils it acts as an antidote to prussic acid, and to recover from fainting. It enters into the composition of ammoniacal alum, and is of great value as a manure in agriculture, as plants readily assimilate nitrogen in this form. Guano owes its value to the The urine of animals

nitrogen and ammonia which it contains. is another fertile source of ammonia, the urea which that fluid contains being converted by standing into 2 atoms of carbonate of ammonia by the assumption of 2 atoms of water; this last salt being volatile, the ammonia must be united with sulphuric acid. This is effected by collecting the urine in a tank, and adding to it common oil of vitrol. Liebig considers that the nitrogen of plants reaches them from the air in the form of ammonia, which falls to the earth with the rain, and gains access by the roots.

3

3

Theories of N H.-1. Ammonia Theory. By this view ammonia is considered as a gas, and combines directly with acids. With SO, it forms NH, SO3 + HO. With Cl it forms sal ammoniac, NH, Cl H. 2. Ammonium Theory. If we rub in a mortar a small portion of sodium, with some mercury (filtered through leather or paper), until an amalgam is formed, and then drop it into a strong solution of sal ammoniac in a tube, the amalgam swells and rises up in the tube to an extraordinary height. This increase of volume is supposed to be owing to the union of ammonium (NH4) with the mercury. Na + Hg, and NH,+Cl becoming Na Cl and Hg NH4. According to this view, the ammoniacal salts are no longer compounds of the gas with an acid, but of acids with the aid of a metal; sulphate of ammonia being NH2OSO. 3. Amide Theory. According to

this theory ammonia is an amide of hydrogen (H+NH2). This is proved by passing dry ammonia over potassium in a tube at a red heat; a green substance is formed called potassamide (KNH), and hydrogen is liberated. Sulphate of ammonia is, upon this view, HNH, HO, SO3.

CARBON.-Charcoal.

C. 75. The most familiar form of carbon is charcoal, which has been known from a very early period of history. The term coals, mentioned in Isaiah (liv. 16.) 712 years before Christ, obviously refers to charcoal. Carbon occurs in various forms. 1. The diamond is perfectly pure carbon crystallized as an octa

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hedron, or an eight-sided figure, consisting of two pyramids applied base to base, the eight faces of which are equal-sided (equilateral) triangles; sp. grav. 3.4 to 36, hardness 10, and is not scratched by any substance in nature. Diamonds are found in the debris or decayed portions of the rocks which were deposited before the existence of animals on the earth in Brazil and India. The Pitt diamond, in possession of the late King of France, weighs 419 grains, and its estimated value is half a million of pounds sterling. When the diamond is heated in a stream of oxygen playing on a lamp, intense light is produced, the diamond unites with oxygen, and is converted into carbonic acid (CO2). 2. Plumbago, Graphite, or Black Lead, familiarly known in black lead pencils, is another form of carbon, sp. grav. 1.9 to 2.32, occurring in pieces about the size of the fist in the Graywacke rocks of Keswick. The purest black lead consists of 95 per cent. of carbon, 3 of water, and 2 of iron and manganese. It is sawn into slices for use, and is fitted into the grooves of cedar pencils, or rounded for ever-pointed pencils. Plumbago is much used to prevent friction, and its power in this way may be exhibited by rubbing a button smartly on the table where powdered plumbago has been strewed, and another button on the bare table. In the former case little or no heat is generated, while in the latter case the heat is sufficient to inflame phosphorus. 3. Wood Charcoal is often formed in this country in the distillation of wood for the preparation of vinegar; the charcoal remains in the iron still. It is formed on the continent by piling up branches of trees, covering them with turf, and setting them on fire. They are converted into carbon by slow combustion. The purest charcoal may be obtained by burning sugar, gum, or glue. Black crayons are made from willows for drawing purposes. Sp. grav. 3.5. It absorbs fluids and gases with great facility. 4. Animal Charcoal or Ivory Black, is prepared by burning bones deprived of their fat and

grease in cast-iron cylinders; the bones give out a quantity of gas, and fluid containing ammonia, which distils off, while the carbon and bone earth remain. It is used to remove colours from fluids, as in sugar refining, and disagreeable odours from water and putrid fluids. The first experiment may be easily shown by digesting brazil or barwood in hot water, filtering the liquid, and shaking it with animal charcoal, and then filtering; the fluid passes through colourless. 5. Lamp black is made by burning fir, rosin, or tar, and collecting the smoke; it is used for printers' ink. 6. Anthracite, Welsh culm, is a kind of hard coal, found in Wales and other places, which contains upwards of 90 per cent. of carbon. 7. Coal or Pit coal is well known under the names of cannel, parrot, or gas coal, caking or Newcastle coal, cherry and splint coals. These are all supposed to have been originally converted from wood into their present forms by heat and pressure; they contain from 65 to 85 per cent. of carbon.

Carbonic Acid.-CO, 2.75. Fixed air (Black, 1755.) Chalk acid (1777). Specific gravity 1.5277. Weight of 100 cubic inches 47.376. Refracting power 1.526

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1. Carbonic acid may be prepared from marble, chalk, or limestone, or any carbonate, by introducing them in fragments into a retort or gas bottle (fig. 67.), supplied with a bent tube, and pouring in hydrochloric or sulphuric acids. The gas passes over in torrents, and may be collected over water, although it is very The action in this experiment is as

soluble in that fluid. follows:

The production of carbonic acid may be familiarly illustrated by placing chalk in the bottom of a wine or ale glass, and pouring upon it strong vinegar or hydrochloric acid; the glass will soon be filled with the gas. 2. Carbonic acid is produced in the most

familiar cases of combustion, as when wood and coal are burned; the carbon of the combustible body uniting with the oxygen of the air to form this gas.

Characters. It is colourless and transparent like common air. When brought in contact with the nostrils, a pungent sensation

is produced. Carbonic acid may be proved to contain carbon by heating slices of sodium with chalk in a German glass tube over a gas flame, a detonation takes place, and the mixture becomes black by the deposition of carbon (Ca O CO, and 2 Na becoming CaO, C and 2 NaO).

Experiments.-1. Its taste is distinctly sour. 2. It colours infusion of litmus and moistened litmus paper red; the colour being restored by exposure to the air. 3. Carbonic acid is neither a combustible nor a supporter of combustion. When a

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jarful of it is poured upon a lighted taper the flame is extinguished (fig. 68.). This property may be compared with that of oxygen by having a jar filled with oxygen alongside of the carbonic acid jar. 4. If the upper half of a tall jar open at both ends be filled with oxygen, and then the lower part be filled with carbonic acid from a retort, a taper, when introduced into the jar, burns brilliantly in the upper half, and is extinguished in the lower part of the jar. The extinction and relighting of the taper, repeated frequently, has a fine effect. 5. The great weight of carbonic acid is shown by experiment 3. It may be further illustrated by passing the gas from a flask and bent tube into a jar filled with common air. The carbonic acid, from its great weight, falls to the bottom, and presses up the common air by replacement (fig. 69.). In consequence of the weight of the gas, it remains for some time in contact with the earth when it is emitted in nature, as in the Dog's Grotto near Naples, and in the Upas valley in Java. The presence of 5 or 6 per cent. of CO, in air extinguishes a candle. Hence it is usual to test the salubriousness of air in mines by lowering a candle or lamp into the tainted atmosphere. If the candle goes out the conclusion is drawn that human life would also be extinguished; but if the candle burns it is inferred that the miner may proceed to his work in safety. 6. If we place some water in a bottle containing CO, and then agitate it, closing the mouth tight with the finger, we shall find that the finger is sucked in, and that if we remove the finger under water, a new portion of fluid will forcibly enter. This is owing to the absorption of the gas by the water. It has been found that a bottle full of gas will be taken up nearly by the same bottle full of water. Hence it is said that water absorbs its own bulk of it, or 100 volumes absorb about 107 volumes of

gas.

It is upon this principle that soda water is made, which consists simply of water into which CO2 is forced by pressure. The presence of carbonic acid may be proved in soda-water after it has stood for some time by throwing into it pounded sugar and stirring it well. The CO2 is pushed out by the sugar which takes its place. 7. Its presence may also be shown by adding lime water (prepared by mixing quicklime previously slaked with water, and pouring off the clear liquid) when carbonate of lime falls. 8. If a current of gas be now passed through the water containing this precipitate, the latter will dissolve, and assume the condition which lime possesses in well water, as explained by this diagram :-

CaO CO2 Carbonate of lime, S Bicarbonate, soluble

CO2

insoluble in water in water.

We can again succeed in precipitating the lime by adding some more lime water. 9. Carbonic acid is capable of being condensed into a liquid, and also into a solid form resembling snow, by pressure, in a strong iron apparatus contrived for the purpose; as the performance of this experiment requires great precaution, and is attended with much risk, it seems unnecessary to describe it here. 10. That carbonic acid is produced in cases of combustion can be proved by placing a lighted candle under a jar along with some lime water. Carbonate of lime is formed. 11. The same gas is produced in the lungs; for if we breathe through lime water by means of a glass tube or the stalk of a tobacco pipe, carbonate of lime is thrown down in a milky form, the precipitate effervescing in acids as muriatic acid or vinegar. 12. That CO, is produced in germination, and that the oxygen of the air disappears, may be shown by putting a few beans into a small quantity of water in a phial, which is then corked and sealed with wax. In a week or two the air of the phial will be found to extinguish a taper. A sprig of mint placed in this air will remove the CO2 and restore the oxygen. 13. What is usually termed effervescence (from effervescens, boiling) is produced by the rapid escape of this gas, as in champaigne and fermented liquors (ale, porter, beer, &c.), when they rise in bubbles when poured out; if carbonic acid is absent they are said to be flat.

Weighing CO2 in Carbonates.-The simplest method, and perhaps the most delicate, is to introduce into a small flask, weighing about 300 grs, a quarter ounce measure of sulphuric acid, and half an ounce of water. Weigh out in a small tube weighing 30 or 40 grs., the weight being accurately noted, 50 or 100 grs. of the carbonate to be examined; suspend it by a thread in the