Antiseptics are employed externally in order to destroy microbes before their entrance into the body, and are administered internally with a like object, or for the purpose of at least preventing the free development and multiplication of the microbes.

They are employed externally in surgical operations, with the object of destroying any organisms which might find a nidus in the wound, and there give rise to the formation of poisonous substances. Both these substances and the bacteria themselves will not only have an injurious local action in the wound, but by undergoing absorption may prove injurious or fatal to the organism as a whole. The antiseptic plan of treatment has been empirically practised in a limited manner for a very long period without its principle being recognised : for the well-known Friar's balsam has antiseptic properties. It is to Lister that we owe the introduction of such a mode of treatment, not based upon mere empiricism, but upon scientific knowledge. The reason why it had fallen into disuse probably was that some of the antiseptic substances used for dressing wounds in the Middle Ages were irritants as well as antiseptics. Those who employed them did not know the reason why they were beneficial, and supposed that their virtue was due to their irritating properties. The ointments were accordingly made more and more irritating : and thus more harm than good was done, until they were discarded by Ambrose Pare. The antiseptic most commonly employed is carbolic acid. Not only are all the instruments to be employed disinfected by a watery solution, but the operation itself is conducted under a spray of the dilute acid, so as to render innocuous any organisms which may be present in the air. The wound is then covered with an antiseptic dressing. Whenever this requires to be removed it must always be done under the spray. The reason of these great precautions is obvious : if any germs, however few, gain an entrance they will soon multiply and prove as deadly as a great number, the only difference being one of time.