The author remarked that he could not better open up his theme than by explaining what was meant by disinfection. He would do so by an illustration from Greek literature. When Achilles had slain Hector, the body still lay on the plain of Troy for twelve days after; the god Hermes found it there and went and told of it--"This, the twelfth evening since he rested, untouched by worms, untainted by the air." The Greek word for taint in this sense was _sepsis_, which meant putrefaction, and from this we had the term "antiseptic," or that which was opposed to or prevented putrefaction. The lecturer continued: I have here in a test tube some water in which a small piece of meat was placed a few days ago. The test tube has been in rather a warm room, and the meat has begun to decompose. What has here taken place is the first step in this inquiry. This has been the question at which scientific men have been working, and from the study of which has come a valuable addition to surgical knowledge associated with the name of Professor Lister, and known as antiseptic. What happens to this meat, and what is going on in the water which surrounds it? How long will it be before all the smell of putrefaction has gone and the water is clear again? For it does in time become clear, and instead of the meat we find a fine powdery substance at the bottom of the test tube. It may take weeks before this process is completed, depending on the rate at which it goes on. Now, if we take a drop of this water and examine it with the microscope, we find that it contains vast numbers of very small living creatures or "organisms." They belong to the lowest forms of life, and are of very simple shape, either very delicate narrow threads or rods or globular bodies. The former are called bacteria, or staff-like bodies; the latter, micrococci. They live upon the meat, and only disappear when the meat is consumed. Then, as they die and fall to the bottom of the test tube, the water clears again.