Besides, it can be proved that the action of the sun is directly indispensable to the maintenance of animal life. In the first place, is it not the fact that solar light and heat exercise an immense influence on the health of animals and of man? To convince ourselves of that, we need only compare men who pass the greater part of their lives in the air and sunshine, with men who live in dark houses, in the narrow streets and lanes of great cities. Not only are these dwellings unwholesome because they are damp, but they are fatal to health because they are not enlivened by the presence of the sun. Light, altogether indispensable to the exercise of respiration in plants, is not indispensable in the same degree to the respiration of animals. It is, however, certain that the products of the respiration of man and animals are less abundant by night than by day. Moleschott has found that the quantity of carbonic acid gas exhaled by an animal is augmented by the intensity of the light of day, and is at its minimum in complete darkness; "which amounts to this," adds that author, "that the light of the sun accelerates molecular action in animals." Thus, the rays of the sun are a primary condition of the existence of animals, because they produce the formation of plants, the essential basis of the alimentation of animals and of man, and because they preside over the fulfilment of many of their physiological functions. We find views of precisely the same order as those we have endeavoured to express, eloquently put forward in Professor Tyndall's work on "Heat:" "And as surely as the force which moves a clock's hands is derived from the arm which winds up the clock, so surely is all terrestrial power drawn from the sun. Leaving out of account the eruptions of volcanoes and the ebb and flow of the tides, every mechanical action on the earth's surface, every manifestation of power, organic and inorganic, vital and physical, is produced by the sun. His warmth keeps the sea liquid, and the atmosphere a gas, and all the storms which agitate both are blown by the mechanical force of the sun. He lifts the rivers and the glaciers up the mountains; and thus the cataract and the avalanche shoot with an energy derived immediately from him. Thunder and lightning are also his transmuted strength. Every fire that burns and every flame that glows dispenses light and heat which originally belonged to the sun. In these days, unhappily, the news of battle is familiar to us, but every shock and every change, is only an application or misapplication of the mechanical force of the sun. * * * * The sun comes to us as heat; he quits us as heat; and between his entrance and departure the multiform powers of our globe appear. They are all special forms of solar power; the moulds into which his strength is temporarily poured, in passing from its source through infinitude."--p. 431.