Sound is the influence of air in motion upon the hearing or auditory nerves. Light, as we have seen, is the ether in motion, the vibrations striking the nerves of the eye. There are musical and unmusical sounds. The former are audible when the vibrations of the air reach our nerves at regular intervals. Unmusical sounds, or irregular vibrations, create _noise_. Now, musical tones bear the same relation to the ear as colours do to the eye. We must have a certain number of vibrations of ether to give us a certain colour (_vide_ table). “About four hundred and fifty billion impulses in a second” give red light. The violet rays require nearly double. So we obtain colours by the different rate of the impingement of impulses on the retina. The eyes, as we have already learned, cannot receive any more rapidly-recurring impressions than those producing violet, although as proved, the spectrum is by no means exhausted, even if they are invisible. In the consideration of Calorescence we pointed this out. These invisible rays work great chemical changes when they get beyond violet, and are shown to be heat. So the rays which do not reach the velocity of red rays are also heat, which is the effect of motion. Thus we have HEAT, LIGHT, and SOUND, all the ascertained results of vibratory motion. The stillness of the ether around us is known as “Darkness”; the stillness of the air is “Silence”; the comparative absence of heat, or molecular motion of bodies is “Cold”! In the first part we showed how coins impart motion to each other. VELOCITY OF LIGHT WAVES. _According to_ SIR J. HERSCHEL. Colour of the No. of Undulations No. of Undulations Spectrum. in an inch. in a second. Extreme Red 37,640 458,000,000,000,000 Red 39,180 477,000,000,000,000 Intermediate 40,720 495,000,000,000,000 Orange 41,610 506,000,000,000,000 Intermediate 42,510 517,000,000,000,000 Yellow 44,000 535,000,000,000,000 Intermediate 45,600 555,000,000,000,000 Green 47,460 577,000,000,000,000 Intermediate 49,320 600,000,000,000,000 Blue 51,110 622,000,000,000,000 Intermediate 52,910 644,000,000,000,000 Indigo 54,070 658,000,000,000,000 Intermediate 55,240 672,000,000,000,000 Violet 57,490 699,000,000,000,000 Extreme Violet 59,750 727,000,000,000,000