in soap bubbles. A mixture of the two gases contained in a small gas-holder is made to bubble through a solution of soap contained in a mortar, until the vessel is filled with a froth of bubbles. The delivery tube is then removed, and in a darkened room the mixture exploded, when it will be seen that there is no luminous flash attending the combustion.

The froth should be ignited by means of a small spirit flame, which may conveniently be obtained by plugging a fragment of cotton wool into one end of a piece of glass tube, which is then dipped into alcohol and ignited.

469. The effect of increased temperature upon the luminosity of flames may be illustrated by heating coal-gas to redness before its combustion. This may be

shown by elongating the tube of a Bunsen lamp by means of a platinum tube about 12 centimetres long, and of such a bore that it will just fit upon the Bunsen. The lamp is supported by a clamp in a position shown in the figure, and the

FIG. 148.

supply of air so adjusted that the lamp burns with a flame which just begins to show luminosity at the tip. On heating the platinum tube by means of a powerful Bunsen, the flame will be seen to increase greatly in luminosity.

470. By heating both the coal-gas and air to redness before combustion. This may be illustrated by means of the apparatus already described (Marsh Gas, Experiment 436). A flame of coal-gas, burning under the ordinary conditions, and of the same size as that obtained in the furnace, should be placed alongside for comparison.

471. A ready method for roughly comparing the luminosity of different flames, suitable for lecture purposes, is the shadow test. For this purpose a rod, conveniently a retort stand, is

P

placed near to a small transparent screen (either made of tracingpaper stretched over a light wooden frame, or, better, a sheet of opal glass), and the two lights which are to be compared are so arranged that the shadows of the rod cast by each flame fall

in close juxta-position upon the screen (fig. 149). The feebler light is then moved nearer to the screen until the two shadows are of equal intensity. The relative luminosity of the two flames will be inversely as the square of their distances from the screen, which is ascertained by a measure.

gas

472. The increased luminosity obtained by heating marsh and air to redness may also be shown.

473. By burning boiling phosphorus in red-hot chlorine. When phosphorus is introduced into chlorine gas, it spontaneously ignites and burns with a flame of very feeble luminosity; but if the two elements are strongly heated before combination, the luminosity is greatly increased. To effect this, a bulb is blown near to one end of a combustion tube, which is heated to redness in a furnace. Upon the end of the tube is fitted a cork, over which a wide glass tube about 30 centimetres long will easily pass, the open end of which delivers into a draught flue. The other end of the combustion tube is connected to a chlorine apparatus, and a stream of gas passed through the apparatus.

When all the air has been displaced, the passage of chlorine through the tube is stopped; the wide tube is then withdrawn and a piece of phosphorus pushed into the bulb by means of a glass rod, and the tube quickly replaced. The phosphorus is then heated by means of a Bunsen flame, and when it is in active ebullition the stream of chlorine is again diverted into the tube; the red-hot chlorine thus coming in

contact with the boiling phosphorus, the latter element will be seen to burn with a comparatively high degree of luminosity.

It is well to cover the wide tube with black paper, so that, if any air is drawn in between the cork and the tube, the light produced by its combination with the phosphorus vapour shall not be seen, and so mask the effect of the combustion in the bulb.

FIG. 151.

474. To show the diminution in luminosity of a coal-gas flame by cooling it. A cold block of iron, conveniently an ordinary domestic 'flat-iron,' is brought against a flame of gas burning from a fish-tail burner in such a way that the flame just plays against the cold metal; it will be seen that the luminosity is almost entirely destroyed.

475. This experiment may be modified in the following

way-The flat-iron is replaced by a stout platinum dish, which is supported by a clamp in the position shown in fig. 152. A

FIG. 152.

small gas flame, burning from a fine jet, is made to impinge against the dish, whereby the luminosity of the flame is nearly destroyed. A small blowpipe flame is then directed into the dish so as to strongly heat the metal at the

point where the
pinges upon it.

outer flame imAs the platinum

becomes hot the luminosity of the flame is restored very nearly

to its original brightness.

476. The shadows cast by flames. When certain luminous flames are placed between a strong light and a screen, it will be seen that they cast a distinct shadow, which will be found to be coincident with the luminous portion of the flame; this fact

is considered by some chemists as a strong argument in support of the solid particle theory. The phenomenon may be shown with either a candle flame or a flame of burning coal-gas. A strong ray from a naked light is allowed to fall upon a small

transparent screen either of tracing-paper or opal glass, and a lighted candle placed in the path of the beam so that its shadow may fall upon the screen (fig. 153); there will be seen in the central part of the shadow of the flame a dark shadow; by holding a pair of dividers against the luminous portion of the candle flame, it will be seen that this dark shadow is coincident with it. If the flame be disturbed by slight draughts and so made to smoke, the shadow of the luminous portion will be seen to extend into the shadow cast by the smoke. A flame of coal-gas burning from a straight jet shows the same phenomenon as a candle flame. If the gas be burnt from a fish-tail burner, and the flame be placed broadside to the screen, no shadow is observed; on turning the flame edgeways to the screen it is of sufficient thickness to cast a very distinct shadow.

477. Instead of casting a shadow by means of a naked light, which shadow must of necessity be rather small and therefore

not easily seen by a large audience, it is possible to project a magnified image of the flame upon the screen in the usual manner by the use of lenses.