In this paper, the author, after having noticed the ordinary quarantine classification of substances, into susceptible, non-susceptible, and doubtful, states as the result of his inquiries conducted in Turkey and the Mediterranean, that the distinctions involved in this arrangement have been made in a very unsatisfactory manner, not after careful research and deliberation, but rather during a period of panic, and hastily, in comparatively remote and ignorant times, and by men, for the then state of knowledge, ill qualified for the task, even had they entered upon it with all the calmness and caution that the subject required.