The descriptions of nearly all the processes occurring in this volume have been revised in type by the managers of the various works and in every case I have been careful to specify the names of those gentlemen who have thus rendered me such essential service. Amongst foreign metallurgists, my thanks are most especially due to those of Sweden for their prompt and hearty cc operation. It affords me much pleasure to acknowledge my obligations to the friends who have assisted me in my investigations. While this is only an act of simple justice, it is also one of policy; for men work with a much better heart when they know that the credit of their labours will not be appropriated by others without acknowledgment. I cannot refrain from mentioning in this place the name of one friend, who has rendered me most willing, and I may add, most valuable aid, — it is that of my colleague Mr. Richard Smith. The descriptions of the methods of assaying the ores of copper and zinc are by Mr. Smith, who has been almost daily occupied, during the last ten years, in instructing students in the art of assaying.