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CH. VIII.] may consider the voice, Plato observes, as percussion (sound, that is,) transmitted, by the air, through the ears, brain and blood, to the sentient principle. But as the nature and properties of the air were then, from the want of experimental science, unknown, they were for any hypothesis; and yet there is evidence that Aristotle, not to add Plato, did regard the air as essential to sound and voice. Aristotle, while agreeing with most philosophers in ranking air among the four elements, "sees a difficulty in determining what its nature may be in the universe around the earth, or what its order in relation to the other elements of bodies." He was aware of the air holding water in solution, and observes that, whether water be or be not produced, equally, from the whole air, that which is around the earth must be not air only but vapour, which is again to be condensed and become water. Thus, "we maintain," he adds, "that fire and air, water and earth are producible out of one another, and that each of them is present, in potentiality, in each of the others; as is the case with all bodies, which have a base into which each of them is ultimately reducible." He has distinguished the air we inspire from that which we send forth (ἐκπέμπειν) and to which he has given a specific appellation (τὸ πνεῦμα); but owing to the of determining either its nature or its office, (although it is the subject of a special treatise,) no