It has happened with this subject as with many others, that similar ideas have independently pre sented themselves to different minds about the same period. In 'leibig and Wohler's Annalen' for May a paper appeared by M. Mayer which I had not read when my third edition was published, but which I have now read in the translation by Mr. Youmans of New York. It deduces very much the same conclusions to which I had been led, the author starting partly from d priori reasoning and partly from an experiment by which water was heated by agitation, and from another, which had, however, previously been made by Davy, viz. That ice can be melted by friction, though kept in a medium which is below the freezing point of water. In 1843 a paper by Mr. Joule on the mechanical equivalent of heat appeared, which, though not in terms touching on the mutual and necessary dependence of all the Physical Forces, yet bears most importantly upon the doctrine.