between mechanical work and heat; that so much work always gives rise, under the same conditions, to so much heat, and so much heat to so much mechanical work.  Thus originated the mechanical theory of heat, which became the starting-point of the modern doctrine of the conservation of energy.  Molar motion had appeared to be destroyed by friction.  It was proved that no destruction took place, but that an exact equivalent of the energy of the lost molar motion appears as that of the molecular motion, or motion of the smallest particles of a body, which constitutes heat.  The loss of the masses is the gain of their particles.

[Sidenote:  Earlier approaches towards doctrine of conservation.]

Before 1843, however, the doctrine of conservation of energy had been approached Bacon’s chief contribution to positive science is the happy guess (for the context shows that it was little more) that heat may be a mode of motion; Descartes affirmed the quantity of motion in the world to be constant; Newton nearly gave expression to the complete theorem; while Rumford’s and Davy’s experiments suggested, though they did not prove, the equivalency of mechanical and thermal energy.  Again, the discovery of voltaic electricity, and the marvellous development of knowledge, in that field, effected by such men as Davy, Faraday, Oersted, Ampere, and Melloni, had brought to light a number of facts which tended to show that the so-called ‘forces’ at work in light, heat, electricity, and magnetism, in chemical and in mechanical operations, were intimately, and, in various cases, quantitatively related.  It was demonstrated that any one could be obtained at the expense of any other; and apparatus was devised which exhibited the evolution of all these kinds of action from one source of energy.  Hence the idea of the ‘correlation of forces’ which was the immediate forerunner of the doctrine of the conservation of energy.

It is a remarkable evidence of the greatness of the progress in this direction which has been effected in our time, that even the second edition of the ‘History of the Inductive Sciences,’ which was published in 1846, contains no allusion either to the general view of the ‘Correlation of Forces’ published in England in 1842, or to the publication in 1843 of the first of the series of experiments by which the mechanical equivalent of heat was correctly ascertained.[I] Such a failure on the part of a contemporary, of great acquirements and remarkable intellectual powers, to read the signs of the times, is a lesson and a warning worthy of being deeply pondered by anyone who attempts to prognosticate the course of scientific progress.

[Sidenote:  What this doctrine is.]