All the rays are absorbed when they fall on to a perfectly dull, black surface, and their energy is converted into heat. This heating effect provides the best way of measuring the energy in the different parts of the spectrum, and of thus constructing curves similar to those given in Fig. 16. The instrument moat commonly used is called Langley's bolometer. It consists of a fine strip of blackened platinum, which can be placed in any part of the spectrum at will and thus absorb the waves over a very small range of wave-length. It is heated by {33} them, and the rise in temperature is found by measuring the electrical resistance of the strip. The electrical resistance of all conductors varies with the temperature, and since resistance can be measured with extreme accuracy this forms a very sensitive and accurate method. +Spectrum of an Incandescent Solid or Liquid.+--The spectra given by different sources of light show certain marked differences. An incandescent solid or liquid gives a continuous spectrum, _i.e._ all the different wave-lengths are represented, but the part of the spectrum which has the greatest energy is different for different substances and for different temperatures: cf. arc and gas flame in Fig. 16. This is quite in keeping with the idea already suggested that in solids and liquids there are electrons of almost every period of vibration. When they are agitated by being heated, a mixture of simple waves of all periods will be sent out giving a very irregular wave. Gases may also become incandescent. Thus when any compound of sodium is put into a colourless flame the flame becomes coloured an intense yellow. This is due to the vapour of sodium, and the agitation of the electrons in it is probably due to the chemical action in which the compound is split up into sodium and some other parts. We may also make the gas incandescent by enclosing it at low pressure in a vacuum tube and passing an electrical discharge through it. The glow in the tube gives the spectrum of the gas. Incandescent gases give a very characteristic kind of spectrum. {34} It consists usually of a limited number of narrow lines, the rest of the spectrum being almost perfectly dark. The light therefore consists of a few simple waves of perfectly definite period. This would suggest that in the atom of a gas there are only a few electrons which are concerned in the emission of the light waves. Thus the spectra of gases and of incandescent solids are represented in character by the curves in Fig. 18. [Illustration: FIG. 18.]