This is one of a number of lists upon topics of current interest which have been compiled to meet requests by letter. So far as it could be distributed at all it has hitherto been distributed in type written form. The applications have become so numerous that it has now been reduced to print, so as to be available for more general distribution. It has no claim to completeness; nor does it even attempt to exhaust the resources of this Library on the subject. Its purpose is merely to' present some of the authorities of interest to the general inquirer. The special investigator must, of course, go much further.