ence limits so narrow that all attempts to bring the characteristic phenomena of organic life (not to speak of mental action) within them are utterly hopeless. Nevertheless, it is asserted that organic phenomena are the product of ordinary physical forces alone, and that the assumption of vital agencies, as distinct from the forces of inorganic Nature, is wholly inadmissible. In view of this, it seems strange that the validity of the proposition above referred to has never, so far as I know, been questioned, except in the interest of some metaphysical or theological system. It is my purpose in the following essays to offer a few suggestions in this behalf, in order to ascertain, if possible, whether the prevailing primary notions of physical science can stand, or are in need of revision.
One of the prime postulates of the mechanical theory is the atomic constitution of matter. A discussion of this theory, therefore, at once leads to an examination of the grounds upon which the assumption of atoms, as the ultimate constituents of the physical world, rests.
The doctrine that an exhaustive analysis of a material body into its real elements, if it could be practically effected, would yield an aggregate of indivisible and indestructible particles, is almost coeval with human speculation, and has held its ground more persistently than any other tenet of science or philosophy. It is true that the atomic theory, since its first promulgation by the early Greek philosophers, and its elaborate statement by Lucretius, has been modified and refined. There is probably no one, at this day, who invests the atoms with hooks and loops, or (Lucretius, De Rerum Natura, ii., 398, et seq.) accounts for the bitter taste of wormwood by the raggedness, and for the sweetness of honey by the smooth roundness of the constituent atoms. But the "atom" of modern science is still of determinate weight, if not of determinate figure, and stands for something more than an abstract unit, even in the view of those who, like Boscovich, Faraday, Ampère, or Fechner, profess to regard it as a mere centre of force. And there is no difficulty in stating the atomic doctrine in terms applicable alike to all the acceptations in which it is now held by scientific men. Whatever diversity of opinion may prevail as to the form, size, etc., of the atoms, all who advance the atomic hypothesis, in any of its varieties, as a physical theory, agree in three propositions, which may be stated as follows:
1. Atoms are absolutely simple, unchangeable, indestructible; they are physically, if not mathematically, indivisible.
2. Matter consists of discrete parts, the constituent atoms being separated by void interstitial spaces. In contrast to the continuity of space stands the discontinuity of matter. The expansion of a body is simply an increase, its contraction a lessening of the spatial intervals between the atoms.
3. The atoms composing the different chemical elements are of de-
