when exposed to the air, will also absorb a considerable portion of the moisture that the air always contains. It appears that the purifying power of charcoal depends, in some measure, upon this property of absorbing the different gases, and possibly somewhat upon that of absorbing moisture; for it is found that a piece of charcoal that has been exposed to the atmosphere, does not possess this antiseptic property in nearly so great a degree as a piece newly burnt. The reason is, that the charcoal that has been exposed, has absorbed a certain quantity of the gases of which the air is composed, and hence loses in some measure, the power of absorbing more of the gases that may arise from the substance which is to be preserved.

Carbon has a strong attraction for oxygen, and it unites with it to form a gas: this gas has the properties of an acid and is called carbonic acid gas. To produce this combination of charcoal, or carbon and oxygen, it is only necessary to set it on fire. I explained to you, in a previous lecture, that the atmosphere is composed of oxygen and nitrogen gases: and when we set fire to a piece of coal, almost the whole of it gradually unites with the oxygen of the atmosphere; and while this union is going on, light and heat are given out by the coal, and probably by the oxygen also. In other words, the coal burns, so that we lose sight of the whole of it, with the exception of the small quantity of ashes that remains. The coal, therefore, by being burnt, is not destroyed, as we have not the power of destroying any kind of matter: it merely changes its form, and from a solid substance it becomes a gaseous or aeriform one. You have probably observed that when salt is put into a tumbler, and water poured upon it, the salt will gradually disappear. In this case, the salt is not lost, but is dissolved in the water. It is in a manner somewhat like this, that a piece of coal, after it is set on the fire, becomes dissolved in the oxygen gas of the atmosphere, and unites strongly with it. If a piece of coal be set on fire, and introduced into a jar of oxygen gas, it will burn with much greater brilliancy than in the atmosphere. The experiment by which it is proved that the diamond is nothing but pure carbon, is its combustion. When the diamond is heated to a red heat, and introduced into a jar of oxygen gas, it burns; that is, it unites with the oxygen; and by their union there is produced nothing but pure carbonic acid gas. The diamond, a piece of charcoal, and lamp black, are, then, the same substance, excepting that there is added to the two latter an exceedingly small quantity of other substances. The principal difference between them is, that the carbon in the two latter is in the form of a black opaque substance, while in the diamond it is crystallized so as to form the hardest substance known, and the most brilliant of the precious stones.

Carbonic acid is colourless like our atmosphere; and when contained in a bottle, we cannot see it any more than if the bottle were filled with common air. It is one of the weakest of the acids; but it is at the same time one of the most important. You were informed in a previous lecture, that one of the properties, by which many of the acids may be known, is that of converting the blue colour that may be obtained from some vegetables, to a red; this property is possessed by carbonic acid. We can obtain a blue liquid by boiling in water some leaves of the blue cabbage; if a small quantity of this blue liquid be poured into a glass vessel of this gas, and then agitated, the liquid

will become red. Although carbonic acid exists in a gaseous form, when uncombined, it may be made to assume a solid form by uniting it with some other substance. Chalk, marble, and limestone are all composed of carbonic acid united to lime. A hundred parts by weight of these substances, contain usually about forty four parts of carbonic acid; and the remainining fifty six parts are principally lime with small portions of other ingredients. It may be shown by an experiment that carbonic acid becomes solid, while uniting with lime. Let a small piece of quick lime be put into a glass vessel, containg water: the water will dissolve a small portion of the lime. We must now take a bladder containing carbonic acid gas, with a pipe stem inserted into its neck, and dip the end of the pipe stem into the solution of lime in the water. By squeezing the bladder, the gas will bubble from the end of the pipe stem, through the solution; and as it passes through it, it will combine with a portion of the lime, that was dissolved in the the water. The combination of lime and carbonic acid thus formed, is almost insoluble in the water; and it therefore falls to the bottom of the vessel. As it is in the form of very light powder, however, it will remain suspended for some time in the water, giving it a cloudy ap

pearance.

There are other acids that have a stronger attraction for lime than carbonic acid has; the attraction between lime and sulphurous acid is stronger than that between carbonic acid and lime. If, therefore, we pour upon a piece of chalk some sulphuric acid, it will unite with the Îime which is one ingredient of the chalk; and the carbonic acid will be set free from its combination with the lime; and being left alone, it assumes the state of a gas, and passes off in that state. It is well to put a little water on the chalk, before pouring on the acid; and then the gas may be perceived bubbling up through the water.

The following is the best method of procuring carbonic acid gas for chemical experiments. A piece of chalk or marble, half the size of a small apple, is to be broken into small pieces, and put into a retort; and half a wine glass of sulphuric acid, (oil of vitriol,) diluted with its bulk of water, is to be poured upon it. The carbonic acid gas will be copiously disengaged, and may be collected in receivers over water. The chemical name for marble, chalk, and limestone, is carbonate of lime.

Carbonic acid is formed in caverns, mines, and in deep wells: it is called by the miners, choke-damp: the common name for this gas is fixed air.

If a candle or any other common combustible substance be lighted, and introduced into a jar of carbonic acid gas, it will be extinguished. Carbonic acid gas is much heavier than common air. If, therefore, a jar of it be held with its mouth downward for a few seconds, the gas will all fall out of the jar, because it is heavier than the common air; while if the jar be held with its mouth upward, the gas will remain in it for some time. It will, however, gradually mix with the common air, and at last all escape. This gas, from its weight, may be poured from one vessel into another, if a piece of lighted candle be placed at the bottom of a jar or tumbler, and some of the gas be poured from another jar the flame of the candle will be extinguished. When this gas is found in vats or deep wells, there is usually the greatest quantity of it near the bottom of the vat or well, and persons are sometimes in

danger of losing their lives by descending into them. As a candle will not burn where this gas has taken the place of the common air, and we can test its presence by lowering a candle into the well, if there be so much of the gas present, that the candle is extinguished, it would not be safe for any one to venture into the well or vat.

It is well known that persons have frequently lost their lives by remaining in a room where charcoal was burning. The reason of this is, that almost the whole of the oxygen of the atmosphere unites with the charcoal or carbon, and forms by this union, carbonic acid gas. You were informed in a previous lecture that the only gas upon which we could live for any length of time was oxygen gas, diluted as it is in our atmosphere, with a large bulk of nitrogen gas. There is to be sure, oxygen gas in the carbonic acid gas; but the oxygen gas in this state is strongly united with a portion of carbon; and in this state, it will not yield any more support to our breathing, than water would, if we were immersed in it. I have found that in some places, there is an opinion quite prevalent, that there is not any danger in remaining in a room where there is a pan of coals burning that have been taken from a common wood fire. This is a mistake: the coals from a common wood fire are exactly the same substance as the charcoal; and equal quantities by weight of these coals and of charcoal, will produce exactly equal quantities of carbonic acid gas. It is also by some persons supposed, that after charcoal has been burning for some time in a fire-place, there is a deleterious quality that gets burnt out of it, and that then it may be burned in a close room without danger. It is scarcely necessary for me to say that this is also an error.

The Lehigh and Schuylkill coals are composed of carbon united to a portion of some earthy substances; and in the burning of these coals, that is in the uniting of the carbon which they contain, with the oxygen of the atmosphere, there is carbonic acid produced, and of course there would be danger in burning these in a close room, as well as in burning charcoal. This would be the case also, in burning cannel and other bituminous coals; as their principal ingredient is carbon.

Carbonic acid gas, unlike the gases of our atmosphere, has a pungent and agreeable taste. If a quantity of this gas be condensed upon the surface of some water, the water will absorb it, and will acquire a pungent taste, which is derived wholly from the carbonic acid. The pungency of soda water is derived from this acid. To prepare this beverage, a very small quantity of carbonate of soda is put into water contained in a strong cask, and there is then crowded into the cask a large quantity of carbonic acid gas, which becomes absorbed by the water, and which then constitutes our common soda water. The object in dissolving in the water the small quantity of soda, is, to induce the water to absorb much more of the carbonic acid, than it otherwise would. As soon as any of this soda water is drawn off from the cask, there escapes from it a considerable quantity of air; this air or gas is carbonic acid; and it is evident that the pungency of the soda water is derived from the acid, because if the liquid be exposed to the air for some time, almost all the carbonic acid will escape from the water, which will lose its sharp taste. Beer, cider, and Champagne wine, that have been kept in bottles, contain a quantity of this acid, which escapes when the liquid is poured into a tumbler. The pungent taste of these liquids also depends upon this acid; as agitating them favours the

escape of the carbonic acid, they should be poured carefully from the bottle, without raising its neck from the tumbler.

Carbon has also a strong attraction for hydrogen, and it unites with it in two different proportions to form two gases.

The compound of carbon and hydrogen in which there is the smallest quantity of carbon, is called subcarburetted hydrogen. This gas is combustible, and it burns with a yellow flame, like that of a candle; it is destitute of colour, taste, and odour. This gas may be procured from the muddy bottoms of ditches and pools; to collect this gas we must hold a wide mouthed vessel with its mouth downward, in the ditch water, being careful that the vessel is filled with the water, before it is inverted. If the bottom of the ditch below the vessel be stirred with a stick, a quantity of air bubbles will arise into the vessel and displace the water. The air thus collected will be found to be carburetted hydrogen, mixed with a portion of some other kinds of gas; and if a lighted taper be applied to it, it will take fire. If from six to twelve parts of common air be mixed with one of this gas, and the mixture be set on fire, it will explode. Subcarburetted hydrogen is frequently generated in coal mines, and becoming mixed with the common air of the mine, explosive mixtures are formed. In the extensive coal mines in England, very large quantities of these explosive mixtures are frequently produced. A few years ago, these mixtures would take fire by the lamps used by the miners, producing the most tremendous explosions, and destroying the lives of many of the workmen. There was an explosion in one of the Newcastle collieries, by which one hundred and one persons perished in an instant. There has been discovered, however, an expedient that prevents these dreadful calamities. The remedy consists in nothing more than a cylinder of wire gauze, fastened upon the upper part of the lamp, which the workmen are obliged to use. This round cage or cylinder of wire gauze, is about one inch and a half in diameter, and six in length, and is fastened by means of a screw to the upper part of the lamp, so as to surround the flame; and all the air which feeds the flame of the lamp must pass through the apertures. The lamp thus guarded, may be used with perfect safety, in the most dangerous explosive mixtures, where the naked flame of a lamp would produce an instant explosion. The explosive mixture passes through the apertures to the inside of the cage; it burns in the cage; and the flame will sometimes come in contact with the inside of the gauze ; yet there is not any danger of its passing through the apertures, so as to set fire to the gas on the outside, and produce an explosion. This lamp was the discovery of Sir Humphrey Davy; and it was the result of a long investigation of the laws and nature of flame.

The other gas resulting from the combination of carbon and hydrogen, contains twice as much carbon as the one first mentioned, and is called carburetted hydrogen. This gas is colourless like our atmosphere, and it burns with a brilliant white flame. The gas that has been so much used for several years past, for lighting large towns and cities, is a mixture of the two gases just described; they are procured from coal or oil. To prepare these gases, an iron retort is heated to redness, and then the coal or oil is put into it. If oil be used, it is suffered to run into the red hot retort only in very small streams. The gas from oil contains a larger proportion of the carburetted hydrogen, than the coal gas; and as carburetted hydrogen burns with a much

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more brilliant flame, than the subcarburetted hydrogen, the oil gas will burn with a brighter flame, than the same quantity of coal gas. The flames from burning wood, oil, or coal, are all of them the flames of one of these gases, or more probably in most, the flame of the two gases, the carburetted and subcarburetted mixed in different proportions.

Account of a Visit to an Elementary School.

[Theory is most clearly seen, and is best understood, when embodied in practice. It thus becomes also most instructive and useful. Occasional articles, therefore, of the kind now presented, will, it is thought, prove serviceable to those who are engaged in the office of instruction.]

When we entered the school room, the children were eating their luncheon, and the teacher was standing in the midst.

'Have you any apples to divide?' he was asking; upon which three or four little hands were extended with an apple in each. He took them, and while he was cutting them, asked if there was any other word which they could think of besides dividing' that meant the same thing. Some said 'separating,' some distributing,' and some 'giving away.' * After he had dispensed all the apples but one, he asked them how many were willing that he should have all that one himself.

Every voice shouted, simultaneously. Then he went up to one of the children, and asked him whether he wanted all the apple he held in his hand, or had rather it should be divided among the rest 'I want it all,'-replied the little boy. 'Why?' 'Because I do. But this is no reason at all.' 'Are you generous, when you want it all?' 'No-stingy! selfish!' exclaimed several. Well,' said the little boy, seeing the tide against him, give me half, and divide the other half among the rest.' 'Oh, but why should you have so much more than the rest?' asked the teacher. 'Because - Because, he is selfish,' said one of the rest.

The little claimant at last consented that all, and he among the rest, should have an equal share. After they had finished eating, the teacher told them he would read them a story, if they would like to hear one. They all held up their hands in token of assent, and entreated him to read about Peter Parley. So he took up the book, and began, first telling them what the name, Parley, meant, and asking the meaning of the difficult words as he went on. He asked them, in one instance, what 'miserable' meant? One said, 'not comfortable,' another 'poor,' and another pointed to the fire, (which was quite out,) and said, 'That is a miserable fire.' The teacher asked what 'violently' meant? 'Hard,' 'quick,' 'strong,' were some of the answers. But one of the children got down off his seat, and stamping with his foot, shouted in a loud voice, That is violently!' 6 After a while, the teacher shut up the book, and they all exclaimed. 'Oh! do finish the story.' 'If you don't, you are a naughty man,' said one. 'Am I?' said the teacher, looking

*He made them spell these words.