The same is true of the appearance and growth on different culture media. (Chapter XX.) CHAPTER XIII. DISINFECTION--STERILIZATION--DISINFECTANTS. The discussion of the physiology of bacteria in the preceding chapters has shown that a number of environmental factors must be properly correlated in order that a given organism may thrive. Conversely, it can be stated that any one of these environmental factors may be so varied that the organism will be more or less injured, may even be destroyed by such variation. It has been the thorough study of the above-mentioned relationships which has led to practical methods for destroying bacteria, for removing them or preventing their growth when such procedures become necessary. The process of killing all the living organisms or of removing them completely is spoken of as _disinfection_ or as _sterilization_, according to circumstances. Thus the latter term is applied largely in the laboratory, while the former more generally in practice outside the laboratory. So also disinfection is most commonly done with chemical agents and sterilization by physical means, though exceptions are numerous. The original idea of disinfection was the destruction of “infective” organisms, that is, organisms producing disease in man or animals. A wider knowledge of bacteriology has led to the application of the term to the destruction of other organisms as well. Thus the cheese-maker “disinfects” his curing rooms to prevent abnormal ripening of cheese, and the dairy-worker “disinfects” his premises to avoid bad flavors, abnormal changes in the butter or milk. _Sterilization_ is more commonly applied to relatively small objects and _disinfection_ to larger ones. Thus in the laboratory, instruments, glassware, apparatus, etc., are “sterilized” while desks, walls and floors are “disinfected.” The surgeon “sterilizes” his instruments, but “disinfects” his operating table and room. The dairy-workers mentioned above sterilize their apparatus, pails, milk bottles, etc. Evidently the object of the two processes is the same, removing or destroying living organisms, the name to be applied is largely a question of usage and circumstances. Any agent which is used to destroy microörganisms is called a “disinfectant.” Material freed from _living_ organisms is “sterile.” The process of _preventing the growth_ of organisms without reference to whether they are killed or removed is spoken of as “_antisepsis_,” and the agent as an _antiseptic_. Hence a mildly applied “disinfectant” becomes an “antiseptic,” though it does not necessarily follow that an “antiseptic” may become a disinfectant when used abundantly. Thus strong sugar solutions prevent the development of many organisms, though they do not necessarily kill them.