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 It is a noteworthy fact that the possibility of the continued existence of the human race depends fundamentally upon the question of heat. If heat, or what is equivalent to heat, does not last, then man cannot last either. There is no shirking this plain truism. It is therefore necessary to review carefully the possible sources of heat and see how far they can be relied upon to provide a continuous supply.

Of course it is obvious that the available heat generally comes from the sun. It may be used directly, or it may be and often is used indirectly, for nothing can be more certain than that it is sun heat in a modified form which radiates from a coal fire in the drawing-room or from a log fire in the backwoods. As the sun shines on the growing vegetation, the leaves extract the warmth from the sunbeams. The organism wants carbon, and to obtain it decomposes the carbonic acid gas of which a certain proportion is always present in the air. To decompose this gas requires the expenditure of heat or of what is equivalent to heat. But this does not show itself in raising the temperature of the carbon and oxygen after they have been dissociated. Their temperature may be no higher than was that of the carbonic acid from which they have come, but the heat has been expended in the process of forcing the several molecules asunder from the close and intimate union of their combined condition.

As the growing plant must have carbon, it draws that carbon from the atmosphere, and the heat that is required to effect the decomposition of the carbonic acid is obtained from sunbeams. When the carbon thus derived by the plant comes ultimately to be burned it reunites with the oxygen of the air, and in the act of doing so evolves an