The wave-length will depend upon the size, form, &c., of the conductors between which the sparking occurs, for the time which the lines of force take to surge backwards and forwards in the conductors will depend upon these things. Other things being equal, the smaller the conductors the smaller the time and therefore the shorter the wave-length. The shortest wave which Hertz succeeded in producing was 24 centimetres long, but since then waves as little as 6 millimetres long have been produced. {91} The waves which are produced in a modern wireless telegraphy apparatus are miles in length. We thus see that there is rather a large gap between the longest heat waves which have been isolated, .006 cms., and the shortest electric waves, .6 cms. The surprising fact, however, is that this gap is so small, for the heat waves are produced by vibrations within a molecule, or at most within a small group of molecules, whereas the electric surgings, even in the smallest conductors, take place over many many millions of molecules. In conclusion, therefore, we see that from the Schumann waves up to the longest heat waves a little over eight octaves of electromagnetic waves have been detected, then after a gap of between five and six octaves the ordinary electrically produced electromagnetic waves begin and extend on through an almost indefinite number of octaves. {92} BOOKS FOR FURTHER READING J. H. Poynting, _The Pressure of Light_. E. Edser, _Heat for Advanced Students_: the chapters on Radiation. E. Edser, _Light for Advanced Students_: the chapters on the Spectrum. B. W. Wood, _Physical Optics_: the chapters on Fluorescence and Phosphorescence, Laws of Radiation, Nature of White Light, and Absorption of Light. {93} INDEX ABSORBING power, 37 -- and radiating power, 38 Absorption, spectra, 34 -- by glass and quartz, 77 -- by air, 78 Addition of waves, 25 Amplitude, 23 BALMAIN, luminous paint, 59 Boltzmann, laws of radiation, 48 CONVECTION currents, 67 Corpuscular theory, 10 -- reflection and refraction by, 11 Crookes' radiometer, 67 DEWAR, temperature and phosphorescence, 62 Diffraction grating, 72 -- dispersion by, 75 -- wire grating, 77 Dispersion, 29, 75 Doppler effect, 69 EFFICIENCY in lighting, 52 Elastic solid theory, 17 Electric field, 18 Electric charges within the atom, 21 Electric oscillations, 19, 83 Electrification, positive and negative, 18 Electromagnetic induction, 80 Electromagnetic waves, 17, 84 Electrons, 30 Energy in simple wave, 25 Energy--wave-length curve, 27 FLUORESCENCE, 58 -- theory of, 60 Foucault, speed of light in different media, 17 Fourier's series of waves, 26, 30 Fraunhöfer lines, 35 Full radiator and absorber, 44, 45 GASES as radiators, 42 HUYGHENS' wave theory, 13 Hertz, experiments on electric waves, 85 -- reflection, 86 -- refraction, 87 -- wave-length by stationary waves, 88