stagnant, such mine would very soon fill with gases, by which the accumulation of gases would at once take place.

Son In those mines, then, where much gas is produced, and there is not a proper quantity of air to dilute it, the accumulation will be very great.-Father: Yes; there is always an accumulation of gas going on in mines, especially if the quantity of air be not sufficient to dilute and render it harmless.

How Gases are Generated, Accumulated, and Produced in Mines.

Son: Is there any weight on the strata of coal and stone from where gas is produced, to compress the said gas therein? Father: There is the atmospheric weight or pressure always pressing against the strata from whence it generates.

Son: What is the pressure of the atmosphere against the strata?-Father: The pressure is nearly 15lbs. upon every square inch of the strata or surface.

Son: Is the amount of gas generating in mines then in proportion to its overbalancing pressure?-Father: Yes. The accumulation of gas in mines is like steam blowing away from a boiler. When steam is much compressed in a boiler the compressed steam will lift up the weight of the valve which presses it there, and it will continue to blow away therefrom until its compression is reduced to the weight of the valve; in like manner, gas blows out and generates from the strata because its compression therein is greater than the weight of the atmosphere.

The Cause, or, why it is that one Mine Generates and Produces more Gases than another.

Son: Why does one mine make or accumulate more gas than another?-Father: Because the compression of gas is greater in one mine than in another.

Son: Why do not all mines make explosive gas alike in quantity, when the atmospheric weight is equally alike reduced from the strata in all mines?-Father: The atmospheric weight is alike reduced from the strata, but the "compression" of gas is not alike in all mines; therefore, the accumulation of gas. is not equally alike..

Son: I cannot see why all mines do not accumulate gas alike, when the weight which presses in the gas is reduced alike in all. Father: Well, I will illustrate the case. Suppose three boilers to be full of steam, the weight on each valve 15lbs., the steam in one boiler so compressed, that a great quantity of steam blows off, another, not so compressed, blows off less steam, and a third blows off very little, because little compressed. Now, by taking 11b. weight from each valve, the

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greatest quantity of steam will blow off from the one most compressed; so, in like manner, will most gas accumulate in those mines where it is most compressed.

Son: Why is not the pressure of gas in mines alike ?Father: Because the gas has been blowing off longer in some than in others.

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Son: Will not the great pressure of gas in the strata all blow off in time, as steam will all blow off from a boiler, until the pressure of gas be the same as the pressure of the atmosphere?-Father: In time the two will become alike equal in

pressure.

Son: By this I understand the pressure of gas in the

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strata will become so reduced that it will press out with only a 15lb. pressure, like the 15lb. pressure of the atmosphere which presses it there, the two then being equal?-Father: Yes. After the great pressure of gas has been reduced there, the two will become equal.

Son. If the pressure of steam in a boiler was reduced to the same weight on the valve,-say if the two were alike equal to 15lbs., no steam then, I presume, father, would blow off, as it could not remove away that which was equal in power to itself. Father: No; in that case there would be no blowing away of steam, as it could not (as you say) lift away that which was equal in power to itself.

Son:

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Son: Then I presume no gas will accumulate in a mine after the pressure has become equal.-Father: I do not wish you to understand that no accumulation will take place; yet after the pressure has been so reduced very little will accumulate.

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Son: I cannot see how any gas can make its way out when its pressure is reduced to the atmospheric pressure. It will not be able to press away the atmospheric weight to make its escape.-Father: To look at it that way it would appear no accumulation could take place. But you know the atmosphere, as before stated, is always changing. To-day its pressure may be 15lbs.; to-morrow, little more than 14lbs. Therefore, when its pressure is reduced from 15lbs. to 14lbs., the accumulation of gas takes place. The weight of the valve is stationary-not like air, elastic; the weight of the valve cannot press into the boiler, and like air work in and out, backward and forward, at every change.

Son I see a little would accumulate, but the accumulation would only be until its pressure was reduced to the lowest atmospheric pressure.-Father: When gas is reduced in the strata to the lowest atmospheric pressure, say to near 14lbs., and the atmospheric weight returns to its former weight of 15lbs., the atmosphere itself is pressed into the strata to fill up the space of that gas which escaped when the weight of the atmosphere was reduced.

The Cause, or Why it is that some Mines Generate and Produce a Mixture of Carbonic Acid Gas (Black-Damp), and Explosive Gas.

Son: Do all mines accumulate alike explosive gas?— Father: No. One accumulates carbonic acid gas, which miners often call black-damp; another, explosive gas, which miners call fire-damp. But other mines accumulate a mixture of the two gases.

Son: You say all mines do not alike accumulate explosive gas. One accumulates explosive gas, another a mixture of gases, and others carbonic acid gas, or black-damp. I wish to know if all mines make explosive gas alike in quantity-that is, those which give off explosive gas ?-Father: No; one gives off a great quantity, another not so much, and a third very little.

Son: You said just now, father, that the weight of the valve is not elastic, like air. What am I to understand by the air being elastic?-Father: As air is elastic, it is capable of being pressed into the strata to occupy the same space in the strata as the gas which is pressed or rushed out.

Son: Is air pressed into the strata, then, to balance the atmospheric weight after the pressure of gas therein is reduced

below it?-Father: Yes. The air works in and out of the strata and mixes with the gas therein, as the two are elastic, and the same pressure which presses gas out will also press air in. Also, the atmospheric pressure is never long at a standstill, but works backwards and forwards, against and into the strata like a man breathing in and letting out air, backwards and forwards.

Son: Do you wish me to understand, then, by this that the strata afterwards gives off a mixture of air and explosive gas?-Father: I wish you to understand that when gas is so reduced the strata give off a mixture of explosive gas and black-damp.

Son: Is the confined air, then, which was pressed into the strata converted into black-damp ?-Father: I believe such is the case. You know a man, by breathing, lets air into his body, and that air, when let out, is converted into carbon; so, in like manner, the confined air is afterwards converted into carbonic acid gas, or black-damp.

Son: Then mines like this accumulate a mixture of explosive gas and black-damp?-Father: Yes. They accumulate the two gases until all the explosive gas is exhausted, after which the mine accumulates black-damp only.

Son : Is it not surprising? I see very clearly, now, how one mine accumulates explosive gas, another a mixture of gases, and a third black-damp, or carbonic acid gas. But have you seen air pressed into the strata the way you show?— Father: I have seen air enter into open spaces in coal, stone, and rock, which drew in a candle-flame, and the next day the same open space which drew in air gave off a mixture of gases, and, by observing them I found the open spaces alternately gave off and let in air as the weather changed.