THE recent issue as one of the volumes of Ostwald's “Klassiker der exakten Wissenschaften “of Robert Mayer's two papers of 1842 and 1845, on the subject now known as the conservation of energy, will prove a great boon to those interested in the early history of that great generalisation. Traces of the idea may be found amongst the ancients, and Descartes held that it was a self-evident truth. But in the middle of the seventeenth century the term energy had but a vague significance, even in the simple case of a moving body, and the doctrine of conservation, when held, meant little or nothing for physical science. Towards the middle of the nineteenth century interest in the question appears to have been widespread. Séguin in France in 1839 calculated the mechanical equivalent of heat from the fall of temperature of steam when expanding against external pressure; Joule in England in 1840 showed that when a battery of cells drives a motor the consumption of zinc in the cells is proportional to the work done by the motor; and Mayer in Germany, after explaining how the term energy was to be understood, stated the generality of the law in his first paper in 1842, and with greater clearness in his pamphlet of 1845.