In this sentence, then, Thomas of Aquin was proclaiming the doctrine of the indestructibility of matter. It was not until well on in the nineteenth century that the chemists and physicists of modern times realized the truth of this great principle. The chemists had seen matter change its form in many ways, had seen it disappear apparently in the smoke of fire or evaporate under the influence of heat, but investigation proved that if care were taken in the collection of the gases that came off under these circumstances, of the ashes of combustion and of the residue of evaporation, all the original material that had been contained in the supposedly disappearing substance could be recovered, or at least {314} completely accounted for. The physicists on their part had realized this same truth, and finally there came the definite enunciation of the absolute indestructibility of matter. St. Thomas's conclusion, "Nothing at all will ever be reduced to nothingness," had anticipated this doctrine by nearly seven centuries. What happened in the nineteenth century was that there came an experimental demonstration of the truth of the principle. The principle itself, however, had been reached long before by the human mind, by speculative processes quite as inerrable in their way as the more modern method of investigation. When St. Thomas used the aphorism, "Nothing at all will ever be reduced to nothingness," there was another signification that he attached to the words quite as clearly as that by which they expressed the indestructibility of matter. For him _nihil_ or nothing meant neither matter nor form, that is, neither the material substance nor the energy which is contained in it. He meant, then, that no energy would ever be destroyed as well as no matter would ever be annihilated. He was teaching the conservation of energy as well as the indestructibility of matter. Here once more the experimental demonstration of the doctrine was delayed for over six centuries and a half. The truth itself, however, had been reached by this medieval master-mind, and was the subject of his teaching to the university students in Paris in the thirteenth century. These examples should, I think, serve to illustrate that the minds of medieval students were occupied with practically the same questions as those which are now taught to the university students of our day, and that the content of the teaching was identical with ours. {315}