explanation, widely accepted in the 18th century, of the phenomena of heat and combustion in terms of the flow of a hypothetical weightless fluid known as caloric. The idea of an imaginary fluid to represent heat helped explain many but not all aspects of heat phenomena. It was a step toward the present conception of energy—i.e., that it remains constant through many physical processes and transformations; however, the theory also deterred clear scientific thinking. The caloric theory was influential until the mid-19th century, by which time many kinds of experiments, primarily with the mechanical equivalent of heat, forced a general recognition that heat is a form of energy transfer.