 Liebig had focused attention on the question of whether an organism’s metabolism produces all its heat and mechanical energy. If so, then appealing to a vital force to explain these phenomena would be superfluous. Through experiments on frogs’ muscles using electrical currents, Helmholtz showed that the heat the frogs’ muscles produced was accounted for by metabolism and muscular action. He published his results in 1845, again in Müller’s journal, in “On Metabolism during Muscular Activity” (see Bevilacqua 1993, 297ff). Helmholtz realized, again in 1845, that the question of whether forces could be accounted for by mechanical means had a much more general application. The presupposition of vitalism, that there is an inexhaustible “vital force” that powers living bodies, had led some investigators to posit that there is an inexhaustible force, whether mechanical or not, that can power a machine indefinitely: a perpetual motion machine, perpetuum mobile. The realization was prompted by Helmholtz’s study of the 18th century classics of mathematical analysis: Euler, Bernoulli, D’Alembert, and Lagrange. As Helmholtz remarked years later, by the end of the 18th century these mathematicians had established the impossibility of a perpetual motion machine powered by mechanical forces: