It has already been explained that by far the majority of bacteria are useful if not essential to human life, and one of the difficulties in employing disinfecting or destroying solutions is that they put an end at the same time to both useless and useful bacteria. As an example, the fermentation processes in the human intestines are accompanied if not produced by certain kinds of bacteria, although on occasion these harmless or useful bacteria may develop into most obnoxious germs, producing unpleasant fermentation. It might be easy enough for a doctor to make a patient swallow some antiseptic solution, like carbolic acid or corrosive sublimate or nitrate of silver, for the purpose of getting rid of certain undesirable bacteria in the intestines, but it does not need a doctor to know that for a patient to swallow such active poisons as these would not merely kill the harmful bacteria and the good ones as well, but probably the patient himself. _Antiseptics._ There is another word often used in connection with bacteria, namely, "antiseptic," and the common significance of this word applies to a substance which interferes with or retards the growth of bacteria without actually destroying them. Doctors, for instance, use antiseptic instead of disinfecting solutions on wounds, not because they do not wish to kill the pus-forming bacteria, but because the antiseptic solution will prevent their growth and not be, as a disinfecting solution, harmful to the cells which he is trying to repair. It would be folly, for example, to inject a strong 50 per cent solution of carbolic acid into a wound on the arm produced by a saw, because all the energy of the vital forces at the seat of the wound are needed for repairs, and there is none to spare for so active a detergent as carbolic acid. An antiseptic, on the other hand, is mild enough so that it does not act on the tissue at all, but merely prevents any undesirable growth of bacteria. _Deodorizers._ There are substances used, perhaps not so much around country houses as around city houses and in water-closets, which are neither disinfectants nor antiseptic, but act as deodorizers only. Such a substance, for example, may be thrown into the kitchen sink, not at all for the purpose of killing bacteria, but for disguising the smell from the cesspool into which the sink-wastes discharge. It has no disinfecting properties and is good for nothing unless the material is so scented as to be agreeable on that score. One of the frauds perpetrated on the public is the preparation and sale of the various appliances designed and regulated to produce a perpetual smell and claimed on that account to be either disinfecting or antiseptic agents. The smell is worth nothing. _Patented disinfectants._