Besides being exposed to danger from falls of roof and sides, the miner has to face risks due to the occurrence of irrespirable gases in the mine. To remove or lessen these risks an adequate current of fresh air must be circulated throughout the workings. Further, a scientific knowledge of the nature, properties, conditions of occurrence, and methods of testing for these gases must be possessed by every miner in a degree sufficient to ensure his personal safety. In addition persons who intend to become firemen, undermanagers, or managers, should undergo a course of training in the chemistry of the mine gases, as such persons in their official capacity become responsible for the safety of the mine and the workmen as a whole.

As an introduction to the chemistry of the mine gases a few fundamental facts must be considered.

Elements, Compounds, and Mixtures. — In nature all substances are either elements, or compounds, or mixtures, of two or more elements or compounds.

Those substances, which so far as we know, consist of one kind of matter only are called elements; e.g. copper, iron, oxygen, and nitrogen are elements, and the reason for considering them as such is, that they cannot by any known process be broken up into simpler bodies. Over eighty different elements are known at present.

Two or more elements which react on each other chemically will form a new substance which differs from either of its constituents, and such a substance formed by the chemical union of two or more elements is called a chemical compound. Copper oxide and carbon dioxide are common examples.

It should be noted that when a compound is formed, the elements forming it lose their identity, and the properties of the substance formed differ from those of the individual constituents. In each case where a compound is formed, the elements which go to form it enter in definite proportions, hence all similar compounds contain exactly the same proportion of constituent elements.

A mixture may be formed by two or more substances, which may be either elements or compounds, the essential condition being that no chemical union takes place between them. In a mixture each separate substance retains its own identity, and the composition is not necessarily definite.

The Indestructibility of Matter. — All natural substances, however varied in character, are described as matter, or that which fills space. Matter may exist in three states — solid, liquid, and gaseous, and many substances can be made to change from the one physical condition to the other by a change of temperature, ice, water, and steam being the most familiar example.

A solid has definite size and shape, and it requires the application of a force to separate its particles or change its shape.

A liquid has definite size, but it always adapts itself to the shape of the vessel in which it is held. A very small force separates its particles.

A gas is a substance whose bulk tends to increase indefinitely as the pressure confining it in a certain space is removed. A gas is not possessed of any definite size or shape.

In whatever form we find matter it is indestructible; we may readily cause it to change its form, but we cannot by any process of manipulation add to or diminish its weight.

Weight. — This is one of the fundamental properties of matter, and is due to the attraction of gravity, which is a pull exerted by the earth on all bodies. If unsupported a body will fall towards the centre of the earth, if supported the body exerts a pressure on its support, and this pressure is described as the weight of the body.

Matter is impenetrable. — When a space is occupied by any given quantity of matter, it is occupied to the absolute exclusion of everything else, that is to say, that no two substances can occupy the same space at the same time.

Symbols. — In writing the names of elements and compounds, it is customary to use symbols. The symbol is usually the initial letter of the name of the element : thus C for carbon, 0 for oxygen, H for hydrogen, N for nitrogen, and S for sulphur are the symbols of the five elements which in various combinations form the mine gases.

When a compound is formed, it is represented by symbols grouped so as to show the elements that it contains, and also the amount of each element present; thus carbon dioxide, which contains one part of carbon combined with two parts of oxygen, is written CO₂.

The symbols give still more information, as they represent the proportion by weight of each element in the compound. Thus C represents 12 parts by weight of carbon, and 0 16 parts by weight of oxygen, and hence the symbol CO₂ tells us that in all carbon dioxide contains 12 parts of carbon and 32 parts of oxygen by weight.

Atoms and Molecules. — An atom is the smallest particle of matter that can take part in a chemical reaction. It is assumed that an atom cannot exist in a free or uncombined state.

A molecule is the smallest particle of matter that is assumed to exist in a free state, or otherwise to possess all the properties of a mass of the substance.

Chemical Equations. — When certain substances are brought together under suitable conditions, a chemical reaction takes place, and a new substance is formed. The results of such reaction may be expressed as an equation.

Thus	C + O2 = CO2

reads

twelve parts by weight of Carbon	added to	thirty-two parts by weight of Oxygen	produces	forty-four parts by weight of Carbon dioxide
12	+	(2 x 16)	=	(44)

It should be noted that the volume (number of molecules) after the change may be less than the volume before, so that the sign of equality does not signify equality of volume; but as matter is indestructible we must have equality of weight, and in this sense only is the symbolic representation of a chemical reaction an equation.

Boyle's Law. — This law tells us that the volume of a gas varies inversely as the pressure, provided the temperature is kept constant. Thus, if the volume of a gas be halved, the pressure must be doubled, or if the volume be increased to three times the original volume, the pressure must be reduced to one-third of what it originally was, and so on for any change that may be made. It is usual to express Boyle's law algebraically thus:

Let	V1=Original volume of the gas,
	V2=New volume of the gas after change of pressure,
	P1=Original pressure of the gas,
	P2=New pressure of the gas,

then P₁V₁=P₂V₂, and from this equation, should any change take place, the new pressure or volume can be calculated.

Charles's Law. — A gas expands when heated, and contracts when cooled, the rate at which it does so being uniform. When the pressure remains constant gases expand or contract 1/273 of their volume for each degree Centigrade rise or fall of temperature. Should the temperature be measured in Fahrenheit degrees the rate of expansion or contraction is 1/459 of the volume for each degree rise or fall of temperature. If it were possible to lower the temperature of a gas to — 273 degrees Centigrade or -459 degrees Fahrenheit, and the contraction still continued, the volume of the gas would be reduced to zero. This is as a matter of fact impossible, as the gas would be liquefied before the above temperature was reached, but it is convenient to consider that if the volume kept decreasing, the temperature at which it would have zero volume is described as the absolute zero of temperature. If this point is made the starting-point of a thermometric scale, temperatures so expressed are called absolute temperatures. This enables us to state Charles's Law in a very simple form, namely, that the volume of a gas varies directly as its absolute temperature.

This simple statement may be combined with Boyle's Law, and the final volume of any gas when subjected to a change of both temperature and pressure determined.

Thus let v₁ be the volume of a gas at a temperature of t₁ degrees Centigrade and a pressure p₁ Let v₂ be the volume produced by a change of temperature t₂ degrees Centigrade and a pressure p₂ then

v2 =	v1 x (273 + t2)	X	p1
	(273 + t1)		p2

Elements forming the Mine Gases. — Hydrogen under ordinary conditions of temperature and pressure is a gas with no colour, taste, or smell. It is combustible, and burns in air with an extremely hot flame, which is pale blue in colour, with a slight tinge of brown. The flame is only faintly luminous. Like all other combustible substances, it is itself a non-supporter of combustion. Hydrogen is the lightest substance known in nature, and it is cus tomary to express the weight of other gases in terms of hydrogen as the unit. The weight of 1000 cubic feet of hydrogen at 14.7 lbs. per square inch (atmospheric) pressure, and a temperature of 32 degrees Fahrenheit is 56.06 lbs.

Hydrogen, although not a poisonous gas, is incapable of supporting respiration. It is not found in the mine as free hydrogen, and it is therefore unnecessary to go further into the details of its properties.

Oxygen is a gas with no colour, no taste, and no smell. It does not burn, but is an active supporter of combustion. Its weight under similar conditions of temperature and pressure is 16 times that of an equal volume of hydrogen. Oxygen is the most widely distributed of all the elements. Water contains 90 per cent. of oxygen by weight, or 33.3 per cent. by volume. Air contains 21 parts by volume, or 23 parts by weight in every hundred, and in addition oxygen is found in combination with many mineral substances. Oxygen unites readily with most other elements, and in many cases the union takes place with such energy that it is accompanied by light and heat. When this takes place the reaction is described as combustion. The breathing of all warm-blooded animals is a process of oxidisation. When breathed in a pure state oxygen produces stimulating effects, and it is frequently administered to persons who have been overcome by poisonous or irrespirable gases.

Nitrogen is a gas with no colour, no taste, and no smell. It does not support life or combustion, and is itself incapable of burning. The gas is not chemically active, hence compounds containing nitrogen are comparatively rare, and often of a very unstable character.

Nitrogen is the chief constituent of the atmosphere, of which it forms nearly of the total volume. Nitrogen is also an essential constituent of animal and vegetable tissues, although not entering into them to any great extent. Plants obtain their nitrogen from the soil, while animals obtain it from such animal or vegetable substances as they make use of for food.

Carbon is a solid substance, and so far carbon has never been isolated in the form of a gas, although quite a number of gaseous compounds containing carbon as one of the constituent elements are known. The diamond is a pure crystallised form of carbon, while graphite or plumbago is another mineral form of great purity. Coal, the origin of which has already been dealt with, is another more or less impure form, while carbon is one of the constituent elements of petroleum, limestone, clayband ironstone, and other mineral substances. A small quantity of carbon in the form of carbon dioxide is found in the atmosphere. Carbon is also a constituent of nearly all forms of living matter, and indeed enters into so many combinations that it constitutes a separate branch of chemistry, which is studied under the name of Organic Chemistry, or the Chemistry of the Carbon Compounds. With the exception of the diamond and graphite, which require pure oxygen for their combustion, the other forms, e.g. coal and wood, are combustible in ordinary air, the product of the reaction being carbon dioxide.

Sulphur is a brittle, lemon-yellow coloured solid with no taste and no smell. It burns in air forming sulphur dioxide with the oxygen, a gas which is easily recognised by its pungent odour. When sulphur is heated to 240 degrees Fahrenheit, it melts into a thin, amber-coloured liquid, which gradually thickens as the temperature is increased, until at 320 degrees Fahrenheit it becomes black and viscid. Further heating renders it a little more fluid, until it reaches the boiling point, when it gives off an orange-red vapour. Sulphur is found in the native or pure state, and also largely in combination with other elements, as sulphates and sulphides. It is also found in certain animal and vegetable substances. Many metals when heated to a high temperature, and placed in the vapour of sulphur, combine with it, much heat being evolved, the products so formed being sulphides of the metals.