Filth or dirt has been well defined as matter in the wrong place. Blood on a carpet, for example, is certainly messy and dirty; it ought not to be there. But blood or serum (the fluid part of the blood) in the heart, or in the arteries and veins, is in its rightful place, and it does its duty of nutrition and so forth when it comes into more immediate contact with the tissues in the small tubes we call the capillaries. One of these duties is to exert a protective influence upon the whole body, by destroying the germs of disease which get in, despite all precautions. We are all of us exposed, so long as spitting in public places is not prohibited, to the germs of consumption, but we do not all die of that disease. This is because the white corpuscles of our blood are in good trim, and able successfully to devour the bacteria that enter our interior. It is those people who are run down, and in whom the white corpuscles are 'below par,' that catch the disease. In assisting the white corpuscles to perform this important function, the co-operation of certain substances dissolved in the fluid portion of the blood is also necessary. The most recently discovered of these auxiliary substances are called _opsonins_. The word opsonin is derived from a Greek root which means "to prepare the feast." The opsonin either adds something to the bacterium which makes it tasty to the white corpuscle, or removes (or neutralises) something which previously made it distasteful. White corpuscles will not as a rule ingest and devour bacteria from a pure culture, but they do so eagerly immediately the bacteria are bathed in serum; and the serum which is most efficacious in acting as a sort of sauce is that which has been obtained from an animal which has been previously infected with the same kind of bacteria, and which has recovered from the ailment such bacteria have set up. This is not mere fancy: the whole sequence of events can be easily followed on a glass slide kept at body temperature and examined with a microscope.