of pressure. Under diminished pressure the temperature falls, and although there is less loss of heat by radiation in rarefied air than in air at the ordinary pressure, it is possible that the general lowering of the temperature of the flame may modify the chemical decompositions in the direction already referred to.

Flames other than those of hydrocarbons, however, and in which no solid matter can exist, are found to become luminous when the density of the flame gas is increased by pressure. Thus, the flame of carbon monoxide in oxygen at ordinary pressures emits a moderate light; but when exposed to a pressure of two atmospheres the luminosity is greatly increased. Even the non-luminous flame of hydrogen burning in oxygen becomes luminous under a pressure of two atmospheres, and when examined by the spectroscope is found to give a

continuous spectrum. It has been found, as a general rule, that dense gases and vapours, when heated, become incandescent or luminous at much lower temperatures than those of low specific gravity; thus, if different gases be raised to incandescence by the passage through them of electric sparks, under similar conditions, it is seen that the light emitted by the glowing vapour varies with the density of the gas. The luminosity of glowing oxygen (density, 16) is greatly superior to that of hydrogen (density, 1), while the light emitted when the sparks are passed through chlorine (density, 35.5) is considerably in advance of either. And it is found that in one and the same gas the luminosity of the spark increases as the density is increased by artificial compression. Other things being equal, it may be said that the denser the vapours which are present the more luminous is the flame.

(3.) The introduction of solid matter into flames.

Non-luminous flames may be rendered luminous by the intentional introduction into them of solid matter, which, by being raised to a sufficiently high temperature, will become strongly incandescent. Thus, the ordinary limelight owes its luminosity to the incandescence of the fragment of lime, which is raised to a bright white heat by the high temperature of the non-luminous oxy-hydrogen flame. The lime is not vaporised at the temperature of the flame, the light being entirely due to the glowing solid matter.

The " Welsbach" burner, already referred to, is another example of the same order, the luminosity in this case being due to the introduction into an ordinary non-luminous Bunsen flame of a fine gauze mantle made of alumina or other metallic oxide (Fig. 86). When such a mantle is raised to incandescence by the heat of the gas flame, it emits a bright white light, strongly resembling that of an ordinary Argand gas flame. A flame may also be rendered luminous by the intentional precipitation within it of carbon, which, by its ignition and its combustion, produces a high degree of luminosity. Thus, if a small quantity of alcohol be boiled in a flask, and a jet from which chlorine is issuing be then lowered through the burning vapour into the flask, as shown in Fig. 87, the chlorine will burn in the alcohol vapour with a luminous flame; and the precipitated carbon (which is thrown out of combination by the action of the chlorine upon the alcohol), ascending into the previously non-luminous alcohol flame, will render it brightly luminous.

From these considerations it will be evident that the luminosity of a flame may be due, first, to the presence of vapours sufficiently dense to become incandescent at the temperature of the flame; or, second, to the presence of solids rendered incandescent, either by the heat of the flame gases alone, or in conjunction with their own combustion; or, third, from the simultaneous operation of all these causes. Ordinary gas and candle flames come under the last of these heads. The decompositions that go forward in these flames not only give rise to dense vapours which become incandescent, but also to the precipitation of solid carbon, which by its ignition and combustion adds to the luminosity of the flame.

The Bunsen Flame. The construction of the Bunsen lamp is too well known to need description. The gas, issuing from a small jet situated at the base of a metal tube, and mixing with air which is drawn in through openings in the tube, burns at the top of the chimney with the familiar non-luminous flame. The existence of this flame in its ordinary condition depends upon two main causes; first, upon the fact that in the immediate neighbourhood of a jet of gas issuing from a small orifice, there is a reduction of pressure; and, second, upon the relation between the velocity at which the gases pass up the tube and the rate of propagation of combustion in the mixture of air and coal gas. Upon the first of these causes depends the entrance of air into the "air-holes" of the lamp, and upon the second depends the continuance of the flame in its position upon the top of the tube.

As the coal gas issues from the small jet at the base of the chimney, instead of the gas escaping through the side-holes, air is drawn into the tube by virtue of the reduced pressure produced immediately round the jet. That this area of reduced pressure actually exists in the neighbourhood of the jet of a Bunsen may be proved by attaching a delicate manometer to the air-hole of such a lamp, as shown in Fig. 88. As the gas is turned on, the liquid in the horizontal

tube will be sucked towards the lamp, showing that the issuing gas causes a partial vacuum in its immediate neighbourhood.*

In order that the flame shall remain at the top of the tube, there must be a certain relation between the velocity of the issuing gases and the rate of propagation of combustion in the mixture; for if the latter be greater than the former, the flame will travel down the tube and ignite the gas at the jet below. By gradually reducing the supply of gas to the flame, and so altering the proportion of gas and air ascending the tube, the mixture becomes more and more explosive, until a point is reached when the velocity of inflammation is greater than the rate of efflux of the gases, and the flame travels down the tube, and the familiar effect of the flame "striking down" is obtained.

The same result may be brought about, and the effect more closely observed, by extending the chimney of the lamp by means of a wide glass tube. As the supply of gas is reduced, or the quantity of air introduced is increased, the flame will be seen to shrink in size and finally descend the tube. By adjust

ment it may be caused either to explode rapidly down the tube or to travel quite slowly, or even to remain stationary at some point in the tube, which is slightly constricted, and where, therefore, the flow of the issuing gas is slightly accelerated. †

The non-luminosity of a Bunsen flame is due to the combined operation of three causes, namely, oxidation, dilution, and cooling. It was formerly supposed that the destruction of the luminosity of a gas flame by the admixture of air with the gas before burning was entirely owing to the influence of the oxygen in bringing about a more rapid and complete state of oxidation, that the hydrocarbons were at once completely burnt up by the additional supply of oxygen so provided. It has been shown, however, that not only is this effect brought about by air, but also by the use of such inert gases as nitrogen, carbon dioxide, and even steam. The following table (Lewes) shows the relative volumes of various gases that are required to destroy the luminosity of a gas flame :

FIG. 88.

That the atmospheric oxygen effects the result by a direct oxidising action, and is not acting merely as nitrogen does, is proved by the fact that mixtures of oxygen and nitrogen, containing a higher proportion of oxygen than is

Chemical Lecture Experiments," new ed., 498-502. + Ibid., 506.

present in air, destroy the luminosity more rapidly than is effected by air. Thus, when mixtures containing nitrogen and oxygen in the proportion of 3 to I, 2 to I, I to I by volume are employed, the volumes of the mixtures required to destroy the luminosity of one volume of coal gas are respectively 2.02, 1.49, and 1.00.

It has been shown that when coal gas is diluted with nitrogen a higher temperature is necessary to effect its decomposition; hence the action of the atmospheric nitrogen in causing the loss of luminosity of a gas flame is in part due to the higher temperature that is required for the formation of acetylene, which, as already mentioned, is the first step in the decomposition and condensation of the hydrocarbons in the gas.

As already mentioned, the luminosity of a flame is very much influenced by alterations of temperature; and just as the non-luminosity of the outer mantle of an ordinary flame is partly due to the cooling action of the air which is dragged into the flame from the outside, so the want of luminosity of the Bunsen flame is in part due to the cooling influence of the large volume of air that is drawn up into the interior of the flame. That the gases which are drawn into a flame reduce the luminosity by virtue of their cooling action is borne out by the fact that the higher the specific heat of the diluent (and therefore the greater its power to abstract heat from the flame) the less of it is required to effect the destruction of the luminosity; thus, as already mentioned, less carbon dioxide than nitrogen is necessary to render a flame nonluminous: the specific heat of nitrogen is 0.2370, while that of carbon dioxide is 0.3307.

The specific heat of oxygen is also slightly greater than that of nitrogen, being 0.2405; but the cooling effect of dilution with this gas is enormously overpowered by the increased temperature due to its oxidising action upon the combustible materials of the flame.

Experiments made upon the actual temperatures of various regions of a Bunsen flame, rendered non-luminous by admixture with different gases, the results of which are seen in the following table (Lewes), show the cooling effect of these diluents upon the flame :

Temperature of Flame from Bunsen Burner, burning 6 cubic feet of Coal Gas per Hour.

In the case of air, it will be seen that the first effect is to cool the flame; but in the upper region, where the oxidising action of the oxygen is felt, the temperature rapidly rises to a maximum at a point about half-way between the tip of the inner and outer cones. In the flames rendered non-luminous by the two inert gases, the highest temperature is only reached at the outer limit, where the full amount of oxygen for combustion is obtained from the outer atmosphere.

On account of the wide range of temperature exhibited by the various regions of a Bunsen flame, it constitutes a most valuable analytical instrument, for, by the judicious use of the different parts of the flame, it is often possible to detect the presence of several flame-colouring substances in a mixture. Thus, if a mixture of sodium and potassium salts be introduced upon platinum wire into the cooler region of the flame near its base, the more volatile potassium compound will impart its characteristic violet tint to the flame before the sodium salt is volatilised sufficiently to mask the colour, by the strong yellow it itself gives to the flame. In this way many mixtures may readily be differentiated.

If a piece of copper wire be held horizontally across a Bunsen flame, so as to cut the inner cone, it will be seen that the wire in contact with the edges of the flame becomes coated with copper oxide, while the portion in the centre remains bright. On moving the wire so as to bring the oxidised portion into the inner region, the oxide will be reduced, the metal once more becoming bright. The outer area of a flame, where oxygen is in excess, is called the oxidising flame; while the inner region, in which heated and unburnt hydrogen or hydrocarbons exist, is spoken of as the reducing flame. These regions exist in all ordinary flames. The oxidising action of the outer flame of a candle, for example, is illustrated in the behaviour of the wick. So long as the wick remains in the inner region of the flame it is not burnt; and in the early days of candles, as the tallow gradually consumed, the wick remained standing straight up, and by degrees extended into the luminous area of the flame, where, owing to the deposition of soot upon it, it frequently developed a cauliflower-like accretion, which greatly impaired the luminosity of the flame, and which necessitated the use of snuffers. In the modern candle, owing to a method of plaiting the wick, it is caused to bend over (as shown in Fig. 83), and so thrusts its point into the oxidising region, where it is continually burnt away.