Page:Lectures on Ten British Physicists of the Nineteenth Century.djvu/64

 dynamical theory of heat. His first contribution, read before the Royal Society of Edinburgh in 1849, was a critical account of Carnot's memoir "Réflexions sur la puissance motive du feu." Joule's measurements were at first almost ridiculed, and had few hearty supporters; but one of these was Thomson. Carnot's theory of the heat-engine assumed that heat is a species of matter; Thomson set to himself the task to modify the theory to suit the doctrine that heat consists in the motion of the small particles of a body. His great stumbling block in the way of accepting the dynamical theory of heat was the difficulty of accurately defining temperature. Founding on Carnot's work Prof. Thomson put this matter upon a perfectly satisfactory scientific basis. Before he propounded his absolute scale of temperature, purely empirical scales founded on the behavior of various gases, liquids, and solids, had each its advocate, and there seemed to be no satisfactory reason for preferring one to another. Once he propounded the absolute scale, no question has ever since been raised but that it is the only rational scale to adopt as the absolute one. To carry out this idea he made experimental investigations in conjunction with Joule on the thermodynamic properties of air and other gases, and as a result showed how to define a thermodynamic scale temperature having the convenient property that air thermometers and other gas thermometers agree with it as closely as they agree with one another.

His thermodynamic investigations led to the doctrine of the dissipation of energy announced by him in 1852. "During any transformation of energy of one form into energy of another form there is always a certain amount of energy rendered unavailable for further useful application. No known process in nature is exactly reversible, that is to say, there is no known process by which we can connect a given amount of energy of one form into energy of another form, and then, reversing the process, reconvert the energy of the second form thus obtained into the original quantity of energy of the first form. In fact, during any transformation of energy from one form into another, there is always a certain portion of the energy changed into