and one atom of hydrogen: its specific gravity is 1.1805, and the weight of 100 cubic inches is 36.008 grains. It is soluble in water, which is capable of absorbing three times its volume of this gas; alkaline solutions absorb it rapidly: chlorine decomposes it by combining with the hydrogen, forming a deposit of sulphur. Mixed with air, it takes fire at the approach of flame, the products of the combustion being water and sulphurous acid.

When present even in small quantity in a gaseous mixture it blackens the white oxide of lead and bismuth, which enables us easily to detect its existence. It is sufficient to expose to the mixture in which it is contained, slips of paper which have been dipped in a solution of acetate of lead and allowed to dry.

It exercises upon the animal economy an influence deleterious in the highest degree: a bird perishes in air containing 1-1500th part of its volume of this gas: the 1800th part is sufficient to kill a moderate sized dog (Thenard). The later researches of M. Parent Duchatelet would, however, seem to show that the poisonous effects of this gas have been somewhat exaggerated, at least in the application of these results to man. He observed that workmen breathed with impunity an atmosphere containing 1-100th part of sulphuretted hydrogen, and he states that he himself respired without serious symptoms ensuing, air which contained 3 per cent.

This gas is formed whenever sulphur in a very comminuted form is brought into contact with hydrogen in a nascent state. Thus it may form in mines where there is a decomposition of iron pyrites. It has been met with in old colliery workings, but its occurrence is rare.

4. OXIDE OF CARBON.-This gas consists of one atom of oxygen and one atom of carbon. Its specific gravity is 0.972; weight of 100 cubic inches 29.64 grains.

According to the recent work of M. Leblanc, oxide of carbon produces upon the animal economy an action more deleterious than that caused by carbonic acid. It burns with a beautiful blue flame, and gives out but little light; when mixed with common air it does not explode like fire-damp, but burns brilliantly: and from this circumstance it appears that a portion of this gas contained in any atmosphere might produce a compound in which a candle might burn brightly, but in which human life must be immediately extinguished; and I am very strongly of opinion that

there exist instances of this nature, some fatal accidents having occurred, almost unaccountable excepting under this supposition.

From the properties of the gases above described, we may (excepting oxide of carbon) penetrate without danger into any atmosphere which we find to possess no disagreeable odor, which will not blacken acetate of lead, and in which a safety lamp will burn with facility; but as even under all these circumstances the atmosphere may, from the presence of oxide of carbon, be unfit for respiration, we are led to the one only practically safe conclusion, that we should, under all circumstances, be accompanied by a sufficient circulation of fresh air, the means of obtaining which I propose to treat of in the following order :

1st. Natural ventilation. 2d. Artificial ventilation, a by waterfall, b by furnace, c by steam, d by machinery.

1. NATURAL VENTILATION.

When we have two shafts of unequal depth to the same level underground, we have a circulation of air established, the character of which will be as follows:-It will be remembered that there is a point where the temperature of the earth corresponds with the mean temperature at the surface, and that below this point the temperature increases with the depth. The consequence of this is, that we have a higher mean temperature, and a lighter column of air in the longer shaft than in the shorter one, at all times when the surface temperature is as low as or lower than the mean temperature of the mine. As the temperature of the atmosphere however increases, the difference of heat between the two shafts diminishes, and ventilation becomes gradually more and more feeble, until at length, when the two shafts become equal, it stagnates altogether. It may then happen (especially in pits of small depths) that the ventilation will be reversed, not indeed for any length of time, but only until that which has now be come the downcast shall, in its turn, by the introduction of the hot surface atmosphere, again become the warmer of the two.

In pits of great depth, however, where the temperature under ground far exceeds any mean, and even equals that of the hottest summer, natural ventilation may continue uninterrupted and of considerable power. The rationale of the motion of a column of air under such circumstances will be explained shortly.

Natural ventilation, however effectual it may be found in deep mines, having few ramifications, accompanied by freedom from inflammable or noxious gases, is quite inadequate to keeping in a safe and healthy state mines where these gases abound, not only on account of its comparative feebleness, but on account of its liability (especially in mines of moderate depth) to be disarranged by changes of atmospheric temperature. We must, therefore, have in all mines artificial means at hand: in some, not perhaps necessary to be used at all times, but immediately applicable under any circumstances affecting the circulation of air produced by natural causes. This then leads us to the consideration of artificial ventilation.

2. ARTIFICIAL VENTILATION.

a. THE WATERFALL.-This may be effected by allowing a portion of water to fall down the downcast shaft, which produces a very good current of air if the water be in any considerable quantity, and the fall be great.

The effect of a waterfall, consisting of a quantity of water passing through two holes, one inch in diameter each, and flowing sixty-three fathoms, may be judged by the following experiment made at Blackboy Colliery, May 8, 1845. The colliery was at this time ventilated by a nine-feet furnace, and the experiment was made in one of the working districts, previous to and after subdividing the portion of air applied to its ventilation.

1. BEFORE PLITTING THE AIR.

The quantity passing into the district with the furnace alone

Cubic feet.

8,394 11,565

Increase due to waterfall

3,171

2. AFTER SPLITTING THE AIR.

The quantity passing into the district with the furnace alone

was

After putting on the waterfall it was

Increase due to waterfall

Cubic feet.

11,313

: 13,687

2,374

The reason of this reduced increase will be explained hereafter, when we discuss the important question of resistance. Although in cases of emergency the waterfall may be applied

as a useful ventilating power, yet it is subject to great objection on two accounts; for it occasions the return into the mine of a considerable quantity of water, which is to be again drawn out, and it so produces a dampness in the air which is very injurious to the timber used in the mine, causing its rapid decay. After an explosion, when the furnace is inaccessible, it is a ready way, as causing a circulation of air through the mine; but excepting in extreme cases, or when it is necessary to suspend for repairing the ordinary ventilating power, it is a means of ventilation which cannot be recommended.

b. THE FURNACE.-The system of ventilation usually adopted, consists of a furnace or fire placed near to the bottom of the upcast shaft, for the purpose of rarefying the air contained therein, and should be adopted in the first instance in hot weather, on the failure of the ventilation produced by natural causes.

The theory of the action is as follows:-First, that the velocity of the ventilating current increases with the temperature of the upcast shaft, because the velocity of the current increases with the length of the motive column, which, other things being equal, depends on the temperature of the upcast shaft. The converse of this, although not for the same reason, is also true; viz., that the temperature of the upcast shaft, other things being equal, depends on the velocity of the air current; or, in other words, if by using an addition to the furnace, any mechanical or other means, we increase the quantity of air, we shall find such increase to be in a greater ratio than is due to the mere mechanical agent employed, inasmuch as we shall find a higher mean temperature in the upcast, and the motive column increased in a corresponding degree, owing partly to the fierce combustion of the furnace, and partly to the more rapid travel of the hot air up the shaft, and consequent higher point which it attains before losing its temperature by absorption, radiation upwards, or whatever cause produces that diminution in heat which takes place so rapidly. Second, the quantity of air depends upon the air channels being shortened and made of as large area as possible.

The shortening of the mine channels may be effected in a great degree by splitting the air, as it is called, or dividing the main current of the downcast shaft into subordinate currents, each having a separate district to ventilate, instead of causing the main current to travel in an undivided state through the whole

of the ramifications of the mine, besides the effect of shortening the run of the air, the other requisite, viz. that of enlarging the average area of the air channel is also produced by splitting the air. In splitting the air two points require particular attention. 1. Not to carry the principle too far, otherwise each split will be feeble and inefficient.

2. To have large channels before splitting the air, and also after the divisions have been reunited.

If any district of a mine evolves so large a quantity of firedamp, that its being mixed with the rest of the return air would raise the whole current to the firing point, the split of air which ventilates such district must of course be taken into the upcast shaft by some other means than by that of the furnace. A drift is therefore driven in fiery mines from one of the returns into the upcast shaft, by means of which any division of air which is of a dangerous character, may be conveyed into it. The point of delivery into the shaft of such a drift, should not be less than 8 fathoms above the furnace to drift end, so as to preclude the possi bility of the inflammable gas being ignited by any ascending flame. There are cases in which the whole of the return air must pass into the upcast, without contact with any flame, in which case the furnace must be fed entirely with fresh air from the downcast shaft. In order to regulate the quantity of air which it is desirable to pass through each district, a description of door is required, which should be 6 feet wide by 3 feet 6 inches in height, fixed in a frame and divided vertically into halves, one of which is movable behind the other half, which is fixed. This frame is placed in any single return from the districts where the air has the shortest distance to travel, and is opened sufficiently wide to allow the requisite quantity of air to pass through. There ought to be means provided, by lock or otherwise, of securing the slide in the required position, to prevent any ignorant or mischievous person from altering the proper distribution of the air.

In all fire-damp mines requiring, in consequence, an active ventilation, the necessity of having large air-ways is paramount to every other consideration.

The essentials to produce a good furnace ventilation may be shortly summed up as follows:

1. Powerful means of heating the air.

2. Length of heated column, which in shallow mines may be