The ideal disinfectant is one which, while capable of destroying the germs of disease, does not injure the bodies and material upon which the germs may be found; it must also be penetrating, harmless in handling, inexpensive, and reliable. The ideal disinfectant has not as yet been discovered. For successful scientific disinfection it is necessary to know: (1) the nature of the specific germs of the disease; (2) the methods and agents of its spread and infection; (3) the places where the germs are most likely to be found; (4) the action of each disinfectant upon the germs; and (5) the best methods of applying the disinfectant to the materials infected with germs of disease. Disinfection is not a routine, uniform, unscientific process; a disinfector must be conversant with the basic principles of disinfection, must make a thorough study of the scientific part of the subject, and moreover must be thoroughly imbued with the importance of his work, upon which the checking of the further spread of disease depends. _Physical Disinfectants_ The physical disinfectants are sunlight, desiccation, and heat. =Sunlight= is a good disinfectant provided the infected material or germs are directly exposed to the rays of the sun. Bacteria are killed within a short time, but spores need a long time, and some of them resist the action of the sun for an indefinite period. The disadvantages of sunlight as a disinfectant are its superficial action, its variability and uncertainty, and its slow action upon most germs of infection. Sunlight is a good adjunct to other methods of disinfection; it is most valuable in tuberculosis, and should be used wherever possible in conjunction with other physical or chemical methods of disinfection.[20] =Desiccation= is a good means of disinfection, but can be applied only to very few objects; all bacteria need moisture for their existence and multiplication, hence absolute dryness acts as a good germicide. Meat and fish, certain cereals, and also fruit, when dried, become at the same time disinfected. _Heat_ is the best, most valuable, all-pervading, most available, and cheapest disinfectant. The various ways in which heat may be used for disinfection are burning, dry heat, boiling, and steam. =Burning= is of course the best disinfectant, but it not only destroys the germs in the infected materials, but the materials themselves; its application is therefore limited to articles of little or no value, and to rags, rubbish, and refuse. =Dry Heat.=--All life is destroyed when exposed to a dry heat of 150 deg. C. for one hour, although most of the bacteria of infection are killed at a lower temperature and in shorter time. Dry heat is a good disinfectant for objects that can stand the heat without injury, but most objects, and especially textile fabrics, are injured by it.