Page:The Harvard Classics Vol. 51; Lectures.djvu/122

 112 once more to form water and carbonic acid that this energy is liberated, as in the human body itself, and utilized by man. The resulting water and carbonic acid can then be used over again by the plant. The nature of this cycle of matter was clearly recognized by Lavoisier. This is the basis of nearly all our industry and commerce.

THE WAVE THEORY OF LIGHT The next great achievement of physical science is commonly regarded as the establishment of the wave theory of light by Young and Fresnel. This view had been put forth in the seventeenth century in a very weighty form by Huygens, and it had even been held before him by the versatile Hooke. On the assumption that light is propagated as undulations, Huygens had given a most satisfactory account of the laws of reflection and refraction; and he had had good success even in his application of the theory to the very difficult problem of double refraction in Iceland spar. Huygens, however, did not succeed in establishing his hypothesis, and Newton's preference for the so-called emission or corpuscular theory of light weighed heavily against the theory of waves.

Newton himself never quite rejected the wave theory of light, and, in truth, at many points in his writings seems strongly to favor it. But there are propositions in his works which led his followers to the positive assertion of the emission hypothesis. The great mathematician Euler, on the other hand, adopted, in the eighteenth century, the undulatory theory. Between his purely theoretical views and the Newtonians there was great controversy.

Again at the beginning of the nineteenth century the undulatory theory was set forth, this time, however, on the basis of exact observations upon the colors of thin plates, by Thomas Young, one of the most versatile men of genius of the country. The contributions of Young were destined to prevail, but, in spite of their soundness, they were treated with contempt by his contemporaries and forgotten for twenty years, until revived by the confirmations of Fresnel. Fresnel, moreover, gradually developed the mathematical theory of this intricate subject, and at length, supported by Arago, he won over the