8. In order to remove the oxygen, some inflammable substance-such as sulphur, alcohol, or phosphorus-is allowed to burn in a portion of air confined in an inverted jar, till it go out. The substance used

must be one which, when united as an oxide with the oxygen, is soluble in water, and this those named are. Alcohol and phosphorus are most convenient to use. Put some of either in a small raised metallic cup, placed in a basin containing some water, or you may set it afloat on the water in a small dish. Then set fire to it, and put over it a bell-glass, which will of course be full of air. If you have used alcohol, the carbon and hydrogen in it will now rapidly unite with the oxygen of the air within the glass, and form carbonic acid and water, which are immediately absorbed or dissolved in the water within the bottom of the glass. As soon as the alcohol ceases to burn the water will begin to rise in the jar, and it will gradually fill a fifth part of it, taking the place of the oxygen which has united with the alcohol to form water and carbonic acid. The gas which remains is nitrogen. The nitrogen thus obtained by burning alcohol is not, however, always quite pure, though it is colourless, for the spirit sometimes ceases to burn before the oxygen is exhausted. Phosphorus, on the other hand, entirely removes the oxygen, but if it be used, the jar must remain undisturbed till the white fumes caused by the combustion have been absorbed by the water.

Another way of obtaining nitrogen is by passing a slow stream of air over copper shavings heated to redness in a tube of hard glass. The copper absorbs the oxygen, and sets the nitrogen free.

Nitrogen may also be very readily made by heating 16 grammes of potassic nitrite with 10 grammes of ammonic chloride, dissolved in about 50 cubic centimetres of water.

Nitrogen has neither smell, colour, nor taste; it neither supports combustion, nor does it burn; it cannot sustain respiration, and it has no effect on colouring matter. A lighted candle dipped into nitrogen at once goes out.

Yet, though it will not burn when by itself, it is slightly inflammable when mixed with hydrogen. Nitrogen is one thirty-sixth part lighter than common air.

CARBONIC ACID GAS.

1. This gas, like oxygen and hydrogen, has no colour, and is consequently invisible; but, unlike them, it has a perceptibly sharp sour taste, and a slight smell.

Its smell may be observed by holding the nose over soda-water, or a seidlitz powder, when they are effervescing, and thus allowing the gas to escape. Its taste is most readily felt by drinking some cold water which has been saturated with it by shaking briskly a corked bottle, filled half with water and half with the gas. The gas will be immediately absorbed by the water.

2. Carbonic acid gas extinguishes flame, and animals die if made to breathe it. It is about half as heavy again as air, and it may therefore be poured from one vessel to another, or poured out of a jar on a lighted candle, which it will at once put out. Its weight also causes it to collect below common air, and hence, when formed naturally, it is always found in a layer extending from the ground upwards, so that a man lying on the ground may perish where one standing erect would be unharmed.

3. Water dissolves its own bulk of this gas, and gains a sparkling appearance and a slightly stimulating taste, from its presence. The effervescence of soda water is caused by the escape of carbonic acid, which has been forced into it under heavy pressure. A quantity of the gas, say a gallon, is compressed into half a gallon, and, to this, half a gallon of water is added, which at once absorbs the whole of the gas; for water absorbs its own volume of the gas, however compressed. The saturated water is now "soda-water," and is strongly corked and wired to prevent the gas forcing its way out. When the cork is withdrawn, and the extra pressure thus removed, the compressed half-gallon of gas rushes out in bubbles, and causes the effervescence we have all seen.

Wines which effervesce do so from carbonic acid formed in them during fermentation.

4. Carbonic acid, when passed through lime-water—that is, through water in which lime has been slaked, and well stirred, but which has afterwards settled and grown clear again-makes it milky or cloudy. This is owing to the union of the gas with particles of lime held in solution in the water, which now form carbonate of lime, or chalk.

Carbonic acid becomes a liquid when put under a pressure of sixty atmospheres, or about 850 lbs. on a square inch. It is usually condensed by a powerful pump into a strong wrought-iron bottle, on which cold water is kept running. When the liquid is permitted to escape through a jet, part of it instantly passes into gas again, and the cold caused by this rapid evaporation freezes the rest of the liquid, which then forms a snowwhite, flaky solid, which evaporates much less quickly. By the evaporation of this substance, mixed with ether, in a vacuum, the very low temperature of -110 has been produced.

5. This gas, though death follows immersion in it, does not act, like nitrogen, by simply keeping away oxygen, and thus causing suffocation; it actually poisons. Air containing so little of it that flame burns in it will nevertheless extinguish life, if breathed for a sufficiently long time. How necessary, then, to avoid the vitiation of the air we breathe by imperfect ventilation! Many deaths occur in France from the habit of using braziers with red-hot charcoal to heat rooms in winter. The carbonic acid given off gradually accumulates, till insensibility and death result. All fires ought to be connected with chimneys, that the gases formed during combustion may be carried out to the open air.

6. The difference between the action of carbonic gas and nitrogen is also marked. If a lighted candle with a long wick be dipped into the former, even the wick will be at once totally extinguished; whereas, if it had been dipped into nitrogen, the wick would have remained red-hot, and would have broken into flame again if immersed in oxygen. Nitrogen puts out a flame simply by cutting off the necessary supply of oxygen, but a candle will go out if put into a mixture of four parts of carbonic acid and one of oxygen, though this fifth part of the latter gas is as large a proportion as there is in the atmosphere.

7. The softening of hard water by boiling is caused by the heat setting free carbonic acid gas, which holds lime in solution in the water. This lime, falling to the bottom, forms the cakes of stony matter we so often see in kettles and boilers.

Fermentation produces carbonic acid in many cases, and hence a candle lowered towards the surface of beer fermenting in a brewer's vat, will go out, and lime-water will become cloudy by the action of the gas on it.

8. Carbonic acid is largely formed in the body of animals by the action of the oxygen they breathe on the waste particles of the blood, and is given off in respiration. In proof of this, if you breathe through a tube into lime-water for a few minutes, the carbonic acid will cause the lime in the water to fall in cloudy white chalk.

Carbon is one of the main sources of the nourishment of plants, but as it cannot be dissolved in water in its simple form, it cannot be absorbed in that form by plants, since the cells absorb only dissolved substances. All the carbon found in plants must, consequently, have entered them in a form soluble in water, and this we find in carbonic acid, which consists of carbon and oxygen.

9. A part of this carbonic acid is derived from decaying animal and vegetable matter in the soil. But plants of some kinds thrive on mere sand, and cresses and hyacinths grow in pure water. It is, besides, to be noted, that the ground cannot yield all the carbon consumed by plants, since, notwithstanding the vast amount absorbed by the grain of each harvest, it is never necessary to make up this loss to the soil again. The air is the great magazine from which plants derive carbon, for though there be only two parts of carbonic acid gas in 5,000 of the atmosphere, the extent of the air is so boundless, that it contains at least 8,440 billions of pounds of it, a quantity more than enough to nourish all the vegetation spread over the whole surface of the earth.

10. This gas is absorbed by plants from the air, partly through their stomata or mouths, but more largely through their roots, in solution in water, which absorbs it readily. That the leaves absorb a large quantity is certain, for green leaves shut in with air containing carbonic acid will drink it all into their structure. But the leaves after all absorb much less of it than the roots.

11. It might be feared that the constant abstraction of so much of this gas from the air by plants would, in the end, make a sensible difference in the quantity left. But if we remember that the breath of all living creatures, the processes of combustion and decay, and the perpetual outbursts of volcanic action, produce great quantities of it constantly, it will be seen how the wonderful fact may be explained that the quantity in the atmosphere is always the same. There is an unceasing process of absorption of carbonic acid by plants on the one hand, all over the world, and on the other a production of it to meet their wants. That they should thus absorb it is necessary for the preservation of the atmosphere in the condition required for human and animal life.

12. Carbonic acid is composed of two parts of oxygen and one of carbon, or charcoal. It constitutes, as we have seen, a 2,500th part of the atmosphere, and abounds in limestone, marble, shells, corals, &c. In many parts it issues from the earth, in large quantities, and it often rushes from the face of workings in mines, destroying all life which cannot escape from it. It is to be found in all water, and it is given off by plants during darkness, and by animals in breathing. Fermentation and animal decay produce it abundantly, and it is a product of all combustion.

13. Carbonic acid may be prepared by putting pieces of broken marble in a glass retort, with sufficient water to cover them, and then adding a little muriatic acid. Another way is to suspend a well-kindled piece of charcoal, by a wire, in a jar of oxygen, which is thus changed into an equal quantity of carbonic acid gas, for a gallon of oxygen thus transformed makes exactly a gallon of carbonic acid. Its weight is increased, however, by three-fourths, as you may readily understand, from the absorption of so much carbon.

Carbonic acid is also called Carbonic Dioxide-or double oxide-from its consisting of two parts of oxygen to one of carbon.

DISTINCTIONS BETWEEN METALS AND NON-METALS.

1. Of the sixty-three elementary or simple bodies known, fifty are commonly reckoned metallic, and it is very probable that other metals will yet be discovered. Many are, however, rare, some very rare.

2. The distinctions between a metal and a non-metallic body are entirely artificial. They are four in number, but are seen, as a rule, only in masses of metal, and only in a faint degree, if at all, if the metals be reduced to powder.

3. One distinction is that metals are excellent conductors of electricity, as is seen in their use in lightning conductors, and electric telegraph wires and cables, through which the spark flashes with the speed of lightning. 4. A second distinction is that they are admirable conductors of heat. Indeed they are the best of all; no non-metallic substances approaching their power in this respect.

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5. A third distinction is, that metals are deposited at the negative pole of an electric battery when a compound of any of them is decomposed by electricity. Yet while all metals have this characteristic, there are several substances, such as hydrogen, which have it also.

6. The fourth distinction is, that metals are powerful reflectors of light, as we know by constant experience. Our mirrors are reflectors formed by a thin coating of metal (mercury) on glass, and every one is familiar with the lustre of gold, silver, burnished brass, &c. There are certain other minerals, however, non-metallic, which are almost as brilliant as metals.

COMBINATION BY WEIGHT AND VOLUME.

I.-WEIGHTS AND MEASURES, &c.

1. The weights and measures used in chemistry are those of the metric system, which are wonderfully simple compared to any others. They have the great additional advantage of being used by scientific men all over the world, so that there is no need to waste time and labour in changing those of one country to the equivalents used in another.

2. In this system, the METRE, from which the MEASURES OF LENGTH are calculated up and down, is equal to 39.37 English inches. The shorter measures are a 10th of a metre, a 100th, or 1,000th; the longer measures, are 10 metres, 100 metres and 1,000 metres. For the measures shorter than a metre, that is its subdivisions, LATIN prefixes are used-deci (for decem) meaning a 10th, centi (for centum) a 100th, and milli (for mille) a 1,000th; so that the tenth of a METRE is called a deci-metre, the hundredth of a metre a centi-metre, and the thousandth of a metre, a milli-metre.

3. The measures above a metre in length have GREEK prefixes—deca, ten; hecto (for hecaton), 100; and kilo for (kilioi, or chilioi), 1,000. The whole table of measures of length is, hence

4. THE MEASURES OF VOLUME are based on subdivisions of the LITRE

into tenths, and multiples of it by tens, thus:—

Kilolitre, 2200 gallons, equal to

1000

litres