The Hebrew word אור _aur_, signifies not only light, but _fire_. God created this powerful agent on the first day, and diffused it through every part of nature; because without it no operation could be carried on and perfected. T. Bartholine quotes Aristotle as saying, “That fire is the efficient cause of all things.” Robison says, “Heat is susceptible of fixation--of being accumulated in bodies, and, as it were, laid up till we have occasion for it; and we are as certain of getting the stored-up heat, as we are sure of obtaining from our drawers the things we put in them.”[31] It pervades all bodies: this is not the case with any other substance we know of--not even light. It lies hid in every thing around us. It is a substance which we are ever in want of; it is therefore deposited on every side, and is ready for every exigency. _Caloric_ is the name given by modern chemists, to that substance by whose influence the phenomena of heat are produced, and which had before been distinguished by the terms _igneous fluid_, _matter of heat_, and other analogous denominations. In order to give precision to chemical language, it was necessary to adopt a word by which to distinguish between the substance which produces the sensation we call heat, and the sensation itself; these being connected as cause and effect; for whenever caloric becomes fixed in a body, it loses its property of affording heat. Whatever is the nature of that quality in bodies called heat, we are assured it does resemble the sensation of heat. A man whose mind is destitute of the cultivation of science, if endued with common sense, never imagines the sensation of heat to be in the fire; he only imagines that there is something in fire which occasions this sensation. Though we are well acquainted with the effects of fire, we know but little of its nature. It is so active, as well as powerful a principle, that it eludes all our researches. We may, however, define it to be the phlogiston or inflammable principle, which pervades in a greater or less degree all substances. Boerhaave thinks it is a fluid of a nature peculiar to itself; that it was created such as it is, and cannot be altered in its nature or its properties; that it naturally exists in equal quantities in all places; and that it is wholly imperceptible to our senses, being only discoverable by such effects as in its operation it produces.[32]