Inoculation, vaccination, is the boxing-master’s method of utilizing the self-protective instinct of the dusky warrior. Knowing that his pupil will for a long while continue to cover an injured spot, he asks himself: “Where is he most likely, when it comes to a serious contest, to be hit?” Then he gives him a gentle tap in that particular place. Does he need to know how to defend himself against small-pox? Give him cow-pox. Is he likely to receive a knock-down blow from typhoid fever? Just show him what it feels like to have a gentle shake. Educate his protoplasm to make antityphoid ferment, by giving him the typhoid germ in such an attenuated form that it cannot do him any harm. The chemistry of protoplasm is a science which is growing rapidly, or, to speak less arrogantly and more correctly, our knowledge of the ways of protoplasm, the Chemist, has greatly increased during the last few years. We can but watch protoplasm at work. Our experiments, so called, are but windows which we open in the walls of his laboratory. We cannot take the work out of his hands. The methods of mineral chemistry are useless in this search for knowledge. And, naturally, the longer we watch, the more details do we discover in what seemed at first a generalized procedure. We recognize that several manipulations are required in the carrying out of a reaction which hitherto we believed to take place in a single stage. This is not the place in which to give an account of a subject regarded as belonging, owing to its applications, to the province of pathology. But Nature is one, however many be the companies into which we divide the explorers of her secrets. We have attempted the merest outline of the observations made up to the present, and have submitted the results for the sake of the light which they throw upon the way in which ferments are prepared as they are wanted to meet the needs of normal every-day digestion and metabolism, rather than for the purpose of showing the methods by which protoplasm combats disease.