Thermodynamics: Early Concepts of Heat

James Richard Fromm

Energy is a comparatively recent concept, enclosing the far older concepts of heat and work as well as more modern concepts such those of electricity and magnetism. The oldest concept is probably that of heat, whose relationship to chemistry we now take up.

Early concepts of heat did not distinguish it from fire. In the Middle Ages, and especially in the period of alchemy (see also) which followed them, much effort was put into applied furnace design - to the point where an alchemical laboratory could be easily identified by its banks of furnaces for different purposes. The sand bath, ash bath, water bath, and kerotakis were used for lower-temperature work, although the concept of temperature did not yet exist. Indeed, concepts of heat had not, through the sixteenth century, made significant progress beyond the concepts of Aristotle enunciated almost two millennia before. Heat was considered to be a property of matter, and the variability in the heat contained by matter was well known. No quantitative measurements related to heat had been made, although the alchemists and smiths had developed considerable skill in practical heat control and furnace design since it was recognized that heat had a significant and controllable effect on matter exposed to it.

The seventeenth century (1600 - 1700) was characterized by extensive but random experimentation and discussion of heat and things related to it. Experimental studies of heat were not yet quantitative. It was discovered that a sheet of glass blocked the heat of a fire, but not that of the sun; that the speed with which a body warms up in the rays of the sun increases as the object becomes darker; and that heat will travel through a vacuum while sound will not. Attempts were made to detect the heat of moonlight and to ignite inflammable materials using lenses of ice. This variety of experimentation arose from an equal variety of theories related to heat.

There were also some advances in related fields in the seventeenth century. In technology, there had been comparatively little development beyond blacksmith level; some specialist metal and glass workers were skilled in practical heat control, but they had little to no fundamental understanding. Coal fires were slowly beginning to replace wood fires, and coke was first used to smelt iron in 1709 in England. In meteorology, the important contribution of the seventeenth century was the understanding that we live at the bottom of an ocean of air which exerts a considerable pressure on everything underneath it. Particularly important, both in understanding the ocean of air and for practical sea travel, were studies of the tides (Newton) and the trade winds (Edmund Halley, astronomer and geophysicist, 1656 - 1742), as well as of evaporation and rainfall.

In the seventeenth century, the earliest significant advance in the understanding of heat was made by Francis Bacon. In 1617, Bacon argued persuasively, and from empirical evidence rather than authoritative deduction, that heat was not an entity or a material substance but a form of motion. This was more an attempt to classify heat than to understand it - Bacon's understanding of "heat is a form of motion" was identical to his understanding of "lion is a form of cat"- but the idea of heat as a motion was taken up later in the century. Galileo Galilei (1564 - 1642) indicated that the sensation of heat was caused by the rapid motion of certain atoms. That is, "heat" was a sensation or an illusion produced by the senses which translate the motion of the atoms of the external or real world into the internal or sensual world of comfort or discomfort, warmth or its absence, somewhat like the tickle at the end of a feather. This sensation of heat is correlated, however, to changes which occur in the real world, since as bodies get "hotter" they expand, melt, or vaporize. These real changes are due to changes in the atomic motion, and are sensible to measurement by means of the thermometer. The philosopher Rene Descartes (1596 - 1650) tried to explain heat, along with everything else, solely in terms of the properties of motion and extension. His ideas were carried on by the Cartesian philosophic school and, by the end of the seventeenth century, a development of this was generally accepted by physicists: heat is not a subtle form of matter, or heat is motion only.

The chemist Robert Boyle (1627-1691) was originally Cartesian, but came slowly to a different position. His general idea was that there existed a material subtle fluid called heat; that motion could be involved but it would have to be motion of the constituent corpuscles of the heat fluid. By the end of the seventeenth century, Boyle's position was that generally held by chemists: the motion of atoms does not give rise to heat, heat is a quantity of a subtle material fluid.

Copyright 1997 James R. Fromm