Proceeding upon his hypothesis of atoms and vacua as the only objective existences, he tried to show what particular modifications of atoms, in figure, size, and position, produced upon the sentient the impressions of different colours. He recognised four fundamental or simple colours--white, black, red, and green--of which all other colours were mixtures and combinations.[227] White colour (he said) was caused by smooth surfaces, which presented straight pores and a transparent structure, such as the interior surface of shells: where these smooth substances were brittle or friable, this arose from the constituent atoms being at once spherical and loosely connected together, whereby they presented the clearest passage through their pores, the least amount of shadow, and the purest white colour. From substances thus constituted, the effluvia flowed out easily, and passed through the intermediate air without becoming entangled or confused with it. Black colour was caused by rough, irregular, unequal substances, which had their pores crooked and obstructed, casting much shadow, and sending forth slowly their effluvia, which became hampered and entangled with the intervening medium of air. Red colour arose from the effluvia of spherical atoms, like those of fire, though of larger size: the connection between red colour and fire was proved by the fact that heated substances, man as well as the metals, became red. Green was produced by atoms of large size and wide vacua, not restricted to any determinate shape, but arranged in peculiar order and position. These four were given by Demokritus as the simple colours. But he recognised an infinite diversity of compound colours, arising from mixture of them in different proportions, several of which he explained--gold-colour, purple, blue, violet, leek-green, nut-brown, &c.[228] [Footnote 227: Theophrastus, De Sensu, s. 73 seq.; Aristotel. De Sensu, c. iv. p. 442, b. 10. The opinions of Demokritus on colour are illustrated at length by Prantl in his Uebersicht der Farbenlehre der Alten (p. 49 seq.), appended to his edition of the Aristotelian or Pseudo-Aristotelian treatise, [Greek: Peri\ Chrôma/tôn] (Munich, 1849). Demokritus seems also to have attempted to show, that the sensation of cold and shivering was produced by the irruption of jagged and acute atoms. See Plutarch, De Primo Frigido, p. 947, 948, c. 8.] [Footnote 228: Theophrastus, De Sensu, s. 76-78. [Greek: a)/peira ta\ chrô/mata kai\ tou\s chulou\s kata\ ta\s mi/xeis--ou)de\n ga\r o(/moion e)/sesthai tha)/teron tha)te/rou.]] [Side-note: Vision caused by the outflow of effluvia or images from objects. Hearing.]