The following study is in three parts. First, and very briefly, I would like to sketch a range of ideas characteristic of thermodynamics in the nineteenth century. In Part II, these will then be seen to have a decisive influence upon Nietzsche's thought - especially concerning the relationship between the famous notions of Will-to-Power and the Eternal Return. In the third part, in turn, these Nietzschean ideas will be found illuminating for a consideration of the problem of metamorphosis in Thomas Mann's The Magic Mountain.