+Immunity.+ To be able to resist the invasion of any species of bacteria, one is said to be refractory to or immune against that variety of germs. Serum therapy is based on the demonstrated fact of immunity, and of the possibility of producing it by injecting the serum of immunized animals. In many infectious diseases, one attack protects an individual for a lifetime and one form of disease may protect against even a more virulent form, as vaccination protects against smallpox. It is a fact that if the serum of an animal which has been rendered immune to a certain disease be injected into a susceptible animal, the same immunity can be produced temporarily in the second animal. Serum therapy proves that the injected serum will not only confer immunity against the infection, but will enable the animal to throw off an already existing infection. +Sterilization.+ The question how to destroy microorganisms is one of the most important in bacteriology. Exactly how chemical antiseptics act in suspending the growth in living organisms and yet leaving them capable of restoration, is not understood. The explanation is offered that the antiseptics enter into combination with the capsule of the cell and can be freed from it by breaking up this chemical combination. It has always been evident that very minute quantities of germicidal substances, and some substances which are not germicidal, would prevent the growth of bacteria, so that it is not surprising that chemical disinfectants should act in this prolonged inhibitory way. It must be remembered that in operative surgical work, germs which will not develop are, for practical purposes, as good as dead; therefore such results do not invalidate the present methods of sterilization for operations. They naturally stimulate interest in the discovery of better methods of sterilization and especially in the thorough application of the methods upon which we are now depending, in order to obtain the best possible results from them. There are three ways of destroying microorganisms: (1) by deprivation of food and water, (2) by chemicals (including toxins), (3) by heat. +Chemical Antiseptics.+ For practical disinfection, chemicals and heat need only concern us. The power of these substances is greatly decreased by heat, grease, oil, mucus, and even blood will cover germs with a coating which prevents chemical germicides from reaching them. Among the ordinary germicides, bichloride of mercury, iodin, alcohol and carbolic acid, are of the greatest importance. A source of error in the direct application of these experiments is the fact that many of these chemicals are decomposed or rendered inert, by combinations with the albuminoids of blood and pus, mercuric bichloride being transformed into an indifferent substance and even carbolic acid being altered.