43. Vital phenomena are the phenomena manifested _in_ organisms when external agencies disturb their molecular equilibrium; and _by_ organisms when they react on external objects. Thus everything done in an organism, or by an organism, is a vital act, although physical and chemical agencies may form essential components of the act. If I shrink when struck, or if I whip a horse, the blow is in each case physical, but the shrinking and the striking are vital. Every part of a living organism is therefore vital, as _pertaining_ to Life; but no part has this Life when isolated; for Life is the synthesis of all the parts: a federation of the organs when the organism is complex, a federation of the organic substances when the organism is a simple cell. 44. All definitions, although didactically placed at the introduction of a treatise, are properly the final expression of the facts which the treatise has established, and they cannot therefore be fully apprehended until the mind is familiarized with the details they express. Much, therefore, which to the reader may seem unintelligible or questionable in the foregoing definition, must be allowed to pass until he has gone through the chapters which follow. CHAPTER III. ORGANISM, ORGANIZATION, AND ORGANIC SUBSTANCE. 45. There is a marked difference between organ_ic_ and organ_ised_ substances. The organic are non-living, though capable of living when incorporated in organized tissue (albumen is such a substance); or they may be incapable of living because they have lived, and are products of waste, e. g. urea. The organized substance is a specific combination of organic substances of various kinds, a combination which is organization. Any organized substance is therefore either an independent organism, or part of a more complex organism. Protoplasm, either as a separate organism or as a constituent of a tissue, is organized substance. Organic substances are numerous and specific. They are various combinations of proximate principles familiar to the chemist, which may conveniently be ranged under three classes: The first class of organic substances comprises those composed of principles having what is called a mineral origin; these generally quit the organism unchanged as they entered it. The second class comprises those which are crystallizable, and are formed _in_ the organism, and generally quit it in this state as excretions. The third class comprises the colloids, i. e. substances which are coagulable and not crystallizable, and are formed in and decomposed in the organism, thus furnishing the principles of the second class. All the principles are in a state of solution. Water is the chief vehicle of the materials which enter and the materials which quit the organism; and bodies in solution are solvents of others, so that the water thus acquires new solvent properties.