All practical illu-minants are made of carbon brought to incandescence (glowing). The types of illuminants fall into two classes: first, particles heated by the combustion of their own carbon, such as candles, lamps, and gas flames; second, particles of carbon heated by outside means, such as mantle gas-burners, electric incandescent lamps, electric arc lamps, etc.

Fig. 64.   Refraction of Light.

Fig. 64. - Refraction of Light.

A flame is caused by the glowing of solid particles that have been volatized, converted into vapor, and rendered luminous by intense heat. The flame of a common lamp or candle is produced by the oil or melted tallow rising between the fibers of the wick through capillary attraction (attraction which causes liquids to go up into minute openings). When the wick is ignited, the oil is heated to a state of vapor, which inflames as the oil first raised is used in burning. Other portions are attracted up the fibers, become vapor, and are burned likewise. In this way a constant, and steady combustion is maintained. The flame of a lamp is hollow, not solid, as the heated vapor must combine with oxygen before combustion can ensue. Hence, only the portions that come in contact with the air are transformed into flame. The vapor that rises from the wick in the center rises unburned. The hollow part of the flame is indicated by the darker and less luminous portion seen just above the wick.