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229. We are now able to recognize the difference between the relations to energy of a living being, such as man, and a machine, such as a steam-engine.

There are many points in common between the two. Both require to be fed, and in both there is the transmutation of the energy of chemical separation implied in fuel and food into that of heat and visible motion.

But while the one—the engine—requires for its maintenance only carbon, or some other variety of chemical separation, the other—the living being—demands to be supplied with organized tissue. In fact, that delicacy of construction which is so essential to our well-being, is not something which we can elaborate internally in our own frames—all that we can do is to appropriate and assimilate that which comes to us from without; it is already present in the food which we eat.

230. We have already (Art. 203) been led to recognize the sun as the ultimate material source of all the energy which we possess, and we must now regard him as the source likewise of all our delicacy of construction. It requires the energy of his high temperature rays so to wield and manipulate the powerful forces of chemical affinity; so to balance these various forces against each