Among the researches of Demokritus there were some relating to animal generation, and zoology; but we cannot find that his opinions on these subjects were in peculiar connection with his atomic theory.[219] Nor do we know how far he carried out that theory into detail by tracing the various phenomenal manifestations to their basis in atomic reality, and by showing what particular magnitude, figure, and arrangement of atoms belonged to each. It was only in some special cases that he thus connected determinate atoms with compounds of determinate quality; for example, in regard to the four Empedoklean elements. The atoms constituting heat or fire he affirmed to be small and globular, the most mobile, rapid, and penetrating of all; those constituting air, water, and earth, were an assemblage of all varieties of figures, but differed from each other in magnitude--the atoms of air being apparently smallest, those of earth largest.[220] [Footnote 219: Mullach, Demokr. Fragm. p. 395 seqq.] [Footnote 220: Aristotle, Gen. et Corr. i. 8, p. 326, a. 5; De Coelo, iii. 8, p. 306, b. 35; Theophrastus, De Sensu, s. 64.] [Side-note: His account of mind--he identified it with heat or fire diffused throughout animals, plants, and nature generally. Mental particles intermingled throughout all the frame with corporeal particles.] In regard to mind or soul generally, he identified it with heat or fire, conceiving it to consist in the same very small, globular, rapidly movable atoms, penetrating everywhere: which he illustrated by comparison with the fine dust seen in sunbeams when shining through a doorway. That these were the constituent atoms of mind, he proved by the fact, that its first and most essential property was to move the body, and to be itself moved.[221] Mind, soul, the vital principle, fire, heat, &c., were, in the opinion of Demokritus, substantially identical--not confined to man or even to animals, but diffused, in unequal proportions, throughout plants, the air, and nature generally. Sensation, thought, knowledge, were all motions of mind or of these restless mental particles, which Demokritus supposed to be distributed over every part of the living body, mingling and alternating with the corporeal particles.[222] It was the essential condition of life, that the mental particles should be maintained in proper number and distribution throughout the body; but by their subtle nature they were constantly tending to escape, being squeezed or thrust out at all apertures by the pressure of air on all the external parts. Such tendency was counteracted by the process of respiration, whereby mental or vital particles, being abundantly distributed throughout the air, were inhaled along with air, and formed an inward current which either prevented the escape, or compensated the loss, of those which were tending outwards. When breathing ceased, such inward current being no longer kept up, the