Modern man's conception of the same process is revealed in the answer one invariably receives from both layman and scientist when they are asked what they understand by combustion. It is described as a process through which oxygen combines with the combustible substance. And yet this side of combustion, first observed by J. Priestley (1771), is neither the one for the sake of which man produces combustion in the service of his everyday life, nor is it at all observed by ordinary sense-perception. Nevertheless, to describe the obvious fact, that combustion is liberation of heat from the combustible substance, will hardly occur to anyone to-day. This shows to what extent even the scientifically untrained consciousness in our time turns instinctively to the tangible or weighable side of nature, so that some effort is required to confess simply to what the eye and the other senses perceive.