Finally, Arthus and Pages have shown that blood does not coagulate when deprived of its calcium salts by the addition of alkaline oxalates, fluorides, or citrates, and that the blood thus treated recovers its coagulability on the addition of a soluble salt of calcium. The coagulation of milk is also a calcium salt precipitation. Coagulation therefore would seem to be merely the colloidal precipitation of a salt of calcium. Diffusion and osmosis are the elementary phenomena of life. All vital phenomena result from the contact of two colloidal solutions, or of two liquids separated by an osmotic membrane. Hence the study of the physics of diffusion and osmosis is the very basis of synthetic biology. A living being exhibits two sorts of movements, those which are the result of stimulus from without, and those which are determined by an excitation arising from within. In the higher animals the stimulus or exciting energy coming from the entourage may be infinitely small when compared with the amount of energy transformed. Moreover, the response to an identical excitation may so vary as to give to these different responses an appearance of spontaneity. There is in reality no spontaneity, since the difference in response is governed by previous external impressions which have left their record on the machinery. There is in fact no such thing as a spontaneous action, since every action of a living {156} being has as its ultimate cause a stimulus or excitation coming from without. The movements of the second category are also conditioned by an excitation, but the stimulus comes from within the organism. These movements consist principally of changes of nutrition, or movements of the circulation and respiration; they are rhythmic in character and are probably produced by the same chemico-physical causes which determine rhythmic movements outside the living body. Just in the same way osmotic growths present two sorts of movements, external movements and those which are connected with their nutrition. A free osmotic growth swimming in the mother liquor will alter its position and form under the influence of the slightest exterior excitation or vibration. It responds to every variation of temperature, or to a slight difference of concentration produced by adding a single drop of water, and reacts to every exterior influence by displacement or deformation.