The Principles of Biology

force falling on units that are but little dissimilar does not readily segregate them ; but that it readily segregates them if they are widely dissimilar. Thus, these two extreme contrasts, the one between physical mobilities, and the other between chemical activities, fulfil, in the highest degree, a certain further condition to facility of differentiation and in-, tegration, § 2. Among the binary combinations of these four chief organic elements, we find a molecular mobility much less than that of these elements themselves ; at the same time that it is much greater than that of binary compounds in general. Of the two products formed by the union of oxygen with carbon, the first, called carbonic oxide, which contains one atom of carbon to one of oxygen (expressed by the symbol C 0), is an incondensible gas ; and the second 6 THE DATA OF lUOLOGY. carbonic acid, containing an additional atom of oxygen (C O2) assumes a liquid form only under a pressure of nearly forty atmospheres. The several compounds of oxygen with