Flame. — When heat is produced by the chemical combination of two substances, the combining bodies may become heated to incandescence. When this takes place the temperature rise is usually such as causes the combining bodies to change to the gaseous state. This spreads the zone of union over a considerable area, which is occupied by the combining gases, and these being in an incandescent state give us what we call flame. When a flame is produced by the union of two gases its shape is that of a hollow cone. The interior is filled with unburnt gas, then immediately outside is the luminous zone of perfect combustion, which entirely envelops the interior. Between the unburnt gas in the interior and the outer envelope of flame, there is a zone where the combustion is incomplete. These zones are shown in Fig. 57.

Flames have characteristic appearances which depend on the nature of the uniting gas. Thus the flame of burning hydrogen is almost colourless, while that of burning sulphur is deep blue.

Before a flame can be produced the temperature of the combustible body must be raised to what is described as the point of ignition. Thus, certain gases may be ignited by means of a red-hot wire, while others require a white heat.

Firedamp can be ignited at a temperature of 1200 degrees Fahrenheit, but it takes a little time, whereas if the temperature is 1800 degrees Fahrenheit ignition is instantaneous. The intensity of the source of heat is also important, as it is easy to show that a mixture of gas and air that will just become inflammable when brought into contact with the flame from a lighted match will, if ignited by an electric spark, explode sharply.

Burning begins at the heated point, and from this point sufficient heat is produced by the combustion of the particles of gas to ignite the immediately adjoining particles. In this way the flame is propagated through the mass of gas and air, the rate depending on the nature of the source of ignition, and the relative proportions of gas and air forming the mixture. If the burning particles of gas do not develop sufficient heat to ignite the adjoining particles, then combustion does not proceed throughout the mass. If the temperature of the flame produced by burning gas is lowered sufficiently, then combustion ceases. These facts may be demonstrated thus : — If an alcohol lamp and a bunsen burner be taken and their flames compared, it will be found that they have one property in common, that is, neither will deposit soot on any cold object placed in them. If a piece of fine-meshed wire gauze be brought down on the top of the bunsen flame, the flame will not pass through, but if a light be applied above a flame will appear and continue to burn. If the gauze be held two or three inches above the burner, the gas may be lighted above the gauze, the flame burning on the top without passing down through. Note that in each case the flame will continue to burn without igniting the gas above or below, until the gauze becomes red-hot, but immediately the gauze attains the temperature of ignition the flame strikes through.

The same effects, due to the lowering of temperature, may be shown by bringing a spiral of fairly thick copper wire into the flame of a candle. If the wire be quickly introduced the flame is immediately extinguished. If the wire is heated before being inserted in the flame, the flame is not extinguished. We thus see that a cold metallic body brought into a flame lowers its temperature, and if the body contains a number of small apertures through which the flame has to pass, its temperature may be lowered sufficiently to prevent the ignition of a combustible mixture on the other side. These are the scientific truths upon which the construction of the miner's safety lamp is based.

We have already seen that an explosive mixture of gas and air must be heated to a certain point before it will explode. If, by any possible means, we can prevent the mixture from being raised to this temperature, then no ignition will take place. If then a lamp flame be so enclosed as to have no direct access to the atmosphere unless through finely meshed wire gauze, we have a means of taking a light into a combustible atmosphere without causing immediate explosion. This is practically what Sir Humphry Davy did when he invented the safety lamp. Modern safety lamps have been greatly improved, but there is one essential point that should be impressed on all miners. Safety lamps are not intended to enable the miner to work in an explosive atmosphere, but to prevent the immediate ignition of an explosive atmosphere, and to disclose its presence to the miner.

Safety Lamps. — In a safety lamp the flame of the burning illuminant is isolated from direct contact with the mine air by surrounding it with a glass cylinder, which is fixed on the top of the oil vessel, and surmounted by two cylinders of wire gauze (Fig. 58). With this arrangement the air to feed the flame passes through the gauzes and down the side of the glass, while the products of combustion pass up the centre. If the gauze be now covered with a metal shield, and the passages provided for the entry of the air and the escape of the products of combustion be restricted in area, the lamp will be extinguished if placed in an inflammable atmosphere. This is due to excessive combustion inside the lamp, and the accumulation of the products of combustion, which are prevented by the restricted openings from escaping as fast as they are formed.

The miner must not be allowed to open his lamp, and for this reason all safety lamps are fitted with some form of locking device. A safety lamp of a type which has passed the Home Office test is shown in Fig. 59.

All miners should be able to test for the presence of firedamp, and it is essential that those men whose duty it is to examine the workings should have an intimate knowledge of how to do so. The men who are appointed to undertake this important duty are called Firemen or Deputies.

How to test for Firedamp. — The presence of small quantities of inflammable gas in the air of the mine produces a distinct effect on the flame of the safety lamp. This effect is caused by the particles of gas in the vicinity of the flame being ignited and uniting with the oxygen of the air. If then a safety lamp with its flame at the normal height be placed in a mixture of gas and air, the flame becomes elongated and drawn out into a sharp point. A trained observer can at once notice this, and confirm his observation by lowering the lamp flame until it ceases to be luminous. He will now see an aureole or cap of burning gas on the top of the lowered flame. The observation of this cap furnishes the simplest and most direct method of detecting the presence of gas, and also affords a ready means of estimating the approximate percentage of gas in the atmosphere under test.

Since the cap is produced by the combustion of gas in the vicinity of the flame, it is evident that so long as the temperature of the flame is sufficiently high to ignite the particles of gas, the size and distinctness of the cap will depend on (a) the temperature and volume of the testing flame, and (b) the amount of firedamp in the air being tested.

Percentage of Gas in Air. — The number of parts of inflammable gas in every hundred parts of a mixture of gas and air can be estimated with a considerable degree of accuracy by observation of the cap formed on the lowered flame of a safety lamp. Such observations are limited in practice to quantities of gas between 1 and 4¼ per cent. Below 1 per cent. the cap is difficult to distinguish, while with 4¼ per cent. of gas the mixture has almost reached the point of inflammability, and any quantity above this simply produces a long thread of burning gas which extends to the crown of the gauze.

A great deal of importance is now attached to the estimation of percentages of gas, but it is equally important to observe the conditions under which the gas is found, because it is only when the gas is mixed with and carried in the general body of the air that a percentage estimate is of real service.

If we assume an air current containing gas to be passing along an airway some distance from the point where the gas has entered the air, the gas will then have mixed completely with the air, and will show a cap which will be approximately of the same height all over the section of the airway. This enables a very fair estimate of the amount of gas given off to be made, and the safety of the mine determined.

On the other hand, assume a longwall working where gas is lying next the face of the brushing or ripping in one of the roads. In this case the gas is in the form of an accumulation which will be almost pure next the roof, and will gradually diffuse into the underlying air until it disappears. This condition is due to the air being nearly stationary in such a place, and the gas being lighter remains next the roof, only passing away by diffusion. If now the flame of the safety lamp be lowered, and the lamp raised gently into the cavity, a gas cap will show. The height of the cap and the corresponding percentage of gas will depend on how far the lamp is raised. It is thus quite apparent that no really serviceable information, beyond the fact that gas is present, will be obtained as the percentage will vary with the position of the lamp. Under such circumstances the fireman must trace the gas out to its tail end or boundary, and thus determine the extent of the accumulation. He will then be able to form an opinion about the volume of gas he has to remove, and the precautions he must take to have this done safely. To sum up, when the fireman has to deal with an accumulation of gas in places that are not swept by an air current, he should endeavour to discover the extent of the accumulation, rather than trouble about a percentage that depends upon the position selected for the test. On the other hand, if the gas is mixed with, and travelling in the air current, he must place his dependence upon a percentage estimate.

To make a gas test, the flame of the safety lamp is carefully drawn down until it shows only a small yellow tip, which will appear to be surrounded by a faint blue line. When lowered to this point the flame is not more than 1/10 of an inch above the wick tube. Should firedamp be present in the air, a light bluish halo is formed over the flame. This halo or cap is readily noticed when the air contains about one per cent. of firedamp, and appears as a cone faintly outlined at the base, and with no visible tip. As the percentage of firedamp increases from one per cent to two per cent. the cap becomes more distinct, until at two per cent. it is sharply outlined, and the pointed tip of the cone becomes visible at a height of about of an inch above the yellow speck in the lowered flame of the lamp. From 2 to 3 per cent, the cap increases in height, and becomes more distinct until at 3 per cent, it stands about 5/8 of an inch above the yellow tip of the testing flame. Above 3 per cent, the cap increases until at 4 per cent, it has reached a height of 1 1/8 inches. From this point onwards the cap shows signs of spiring, and as it approaches 5 per cent, it forms a long blue thread reaching to the top of the lamp gauze.

The heights of cap for various percentages are shown in the accompanying diagram (Fig. 60).

If in the course of an examination for firedamp the lamp is placed in a mixture that fills it and burns up into the gauze, it should be carefully and quietly withdrawn and steadily removed to a safe atmosphere. If circumstances are such as to prevent the lamp being removed to a place of safety, it may be extinguished by wrapping it up in a jacket or muffler or anything that will surround and close the ventilating holes and thus smother the flame. It is well to remember that any undue movement, such as jerking the lamp or trying to shake it out, may cause the flame of the gas burning inside to pass through the gauze and thus cause an explosion. If, however, the lamp be carefully handled there is little risk.

Sulphuretted Hydrogen. — This gas is formed by the combination of 2 parts by weight of Hydrogen with 32 parts by weight of Sulphur, the symbol being HS, and the relative weight 17. It is a gas with no colour, no taste, but a very disagreeable smell. It is extremely poisonous. Air containing less than 0.5 per cent. of this gas will, if breathed, soon cause death. Although an extremely dangerous gas, accidents due to its presence in the mine are rare. This is due to the ease with which its presence can be detected, and also to the fact that it is not frequently found in the mine. Traces of this gas are frequently found along with firedamp, certain pit waters sometimes contain a little of it in solution, and it is given off along with other gases when coal heats spontaneously. It may also be produced by blasting where gunpowder is the explosive used. Sulphuretted hydrogen forms explosive mixtures with air, but this property is of little consequence to the miner, as the presence of the gas is proclaimed by its smell long before the danger point is reached.

Carbon Monoxide. — This gas is a compound composed of 12 parts by weight of carbon combined with 16 parts by weight of oxygen, its symbol being CO. It has a relative weight of 14, very nearly the same as air. It has no colour, no taste, but a slight smell, which, however, is of no use as a distinguishing feature, and may be disregarded by the miner. Carbon monoxide is inflammable, and burns with a pale blue flame, which shows a marked tendency to flicker. This flame can be observed on the top of a charcoal fire, where the carbon dioxide first formed is turned into carbon monoxide by the further action of red-hot carbon. Thus

CO₂ + C(red hot) = 2CO.

Although an inflammable gas it is never dangerous in the mine because of this property, as it is so intensely poisonous that no person could live for more than a few seconds in an atmosphere containing explosive quantities of the gas. Carbon monoxide forms a stable compound with the haemoglobin of the blood, and if breathed even in very small quantities it induces headache, giddiness, loss of muscular power, faintness, and ultimately death.

The chief causes of the occurrence of carbon monoxide in the mine are blasting with certain classes of explosives, the outbreak of underground fires, and explosions where coal dust takes part. Dr. Haldane found by experiment that a person could remain for half an hour without discomfort in an atmosphere containing as much carbon monoxide as produced signs of illness with a mouse in two or three minutes. This suggested a test for the gas, and now if a suspected atmosphere has to be entered in the mine a mouse or a small bird imprisoned in a cage is carried by the exploring party. As soon as the bird shows signs of being unable to sit on its perch, or the mouse begins to sprawl about on its belly, the atmosphere is getting dangerous, and it is time for the person or party to retire to a place where the air is known to be pure.