IV. Visibility, tangibility, solidity, and weight are relative properties, and not absolute realities. V. The infinitely small. The experiments made in beating gold-leaf show that ten thousand leaves are contained in the thickness of a millimetre. A millimetre has been divided on a glass plate into a thousand equal parts; and infusoria exist, which are so small that their entire bodies, placed between two of these divisions, do not touch either of them. The members and organs of these beings are composed of cellules, these of molecules, and these of atoms. Twenty cubic centimetres of oil spread over a lake will cover four thousand square metres, so that the layer of oil thus expanded measures only one two hundred thousandth of a millimetre in thickness. Spectral analysis of light discloses the presence of a millionth of a milligramme of sodium in a flame. The sense of smell perceives 1/604000000 a milligramme of mercaptan in the air breathed. The dimensions of atoms must be less than a millionth of a millimetre in diameter. [Waves of light are comprised between 4 and 8 ten millionths of a millimetre, from violet to red; 2300 are required to fill a millimetre. In the duration of a second the ether through which light is transmitted makes 700,000,000,000,000 oscillations, each of which is mathematically defined.] VI. The intangible, invisible atom, scarcely conceivable to our mind accustomed to superficial judgments, constitutes the only true matter; and what we call matter is but an effect produced on our senses by the motion of atoms,--that is to say, an incessant possibility of sensations. The result is, that matter, like the manifestations of energy, is only a mode of motion. If motion should stop; if force should be annihilated; if the temperature of bodies should be reduced to absolute zero,--matter, as we know it, would cease to exist. VII. The visible universe is composed of invisible bodies. What we see is made up of things which are not seen. There is but one kind of primitive atom. The constituent molecules of different bodies--iron, gold, oxygen, hydrogen, etc.--differ only in the number, grouping, and motion of the atoms which compose them. VIII. What we call "matter," vanishes when scientific analysis thinks to grasp it. But we find as the support of the universe and the origin of all form, Force,--the dynamic element. By my will I can unsettle the Moon in her course. The movements of each atom on our Earth are the mathematical resultant of the undulations of the luminiferous ether which come to it in time from the abysses of infinite space. IX. The human being has for essential principle the soul. The body is visible and transitory. X. Atoms are indestructible.