After a few high-sounding but somewhat disconnected introductory sentences, and a complimentary allusion to “the theories of a celebrated medical philosopher, Dr. Beddoes,” he proceeds to put Lavoisier’s question, “La lumière, est-elle une modification du calorique, ou bien le calorique est-il une modification de la lumière?” to the test of experiment. This he does by repeating Hawksbee’s old experiment of snapping a gunlock “armed with an excellent flint” in an exhausted receiver. The experiment fails in his hands; such phenomena as he observes he misinterprets, and he at once concludes that light and heat have nothing essentially in common. “Nor can light be as some philosophers suppose, a vibration of the imaginary fluid ether. For even granting the existence of this fluid it must be present in the exhausted receiver as well as in atmospheric air; and if light is a vibration of this fluid, generated by collision between flint and steel in atmospheric air, it should likewise be produced in the exhausted receiver, where a greater quantity of ether is present, which is not the case.” Since, then, it is neither an effect of caloric nor of an ethereal fluid, and “as the impulse of a material body on the organ of vision is essential to the generation of a sensation, _light is consequently matter of a peculiar kind_, capable when moving through space with the greatest velocity, of becoming the source of a numerous class of our sensations.” By experiments, faultless in principle but wholly imperfect in execution, he next seeks to show that caloric, or the matter of heat, has no existence. His reasoning is clear, and his conceptions have the merit of ingenuity, but any real acquaintance with the conditions under which the experiments were made would have convinced him that the results were untrustworthy and equivocal; and yet, in spite of the dubious character of his observations, he arrived at a theory of the essential nature of heat which is in accord with our present convictions, and which he states in the following terms:-- “Heat, or that power which prevents the actual contact of the corpuscles of bodies, and which is the cause of our peculiar sensations of heat and cold, may be defined a peculiar motion, probably a vibration, of the corpuscles of bodies, tending to separate them.” This conception of the nature of heat did not, of course, originate with him, and it was a question with his contemporaries how far he was influenced by Rumford’s work and teaching. On this point Dr. Beddoes’s testimony is direct and emphatic. He says:--