The schizomycetes are unable to extract nitrogen from the air or the soil, like the higher vegetables, and must, therefore, be provided with a higher nitrogenous compound, such as is produced by vegetable and animal life. Some of them are able to live upon dead organic matter, while others cannot exist without living tissues to feed upon and are therefore true parasites. There are some which are able to live upon either dead or living tissues and are known as _facultative parasites_, a class which includes a majority of pathogenic germs. Some organisms require albuminous matter, others need carbohydrates; they all require water, carbon, nitrogen, oxygen, and certain inorganic materials, especially lime and potassium. All organisms require water. If dried, no form will multiply, and many forms will die. The fluids and tissues of the individual may or may not afford a favorable soil for the germs of a disease, or, in the same person afford it at one time, and not at another. Some individuals seem to possess indestructible immunity from, and others are especially prone to, certain contagious diseases. Impairment of health, by alterating some subtle condition of the soil, may make a person liable who previously was exempt. +Effect of Oxygen.+ Some bacteria need free oxygen; some can live either with or without free oxygen, while others cannot live at all in the presence of free oxygen. Those requiring oxygen are called _aerobic_; those which can live with or without it are called _facultative aerobic_; those which do not live in free oxygen are called _anaerobic_. Bacteria are very sensitive to temperature, few being able to live in a temperature below 68°F. or 29°C. or above 104°F. or 40°C. The pathogenic varieties thrive best at about the normal temperature of the blood. Direct sunlight retards their growth and may kill them. Freezing renders bacteria motionless and incapable of multiplication, but it does not kill them; they again become active when the temperature is raised. The absurdity of employing cold as a germicide is evident when it is known that a temperature of 200°F. below zero is not fatal to germ life, cell activities by such a temperature only being rendered dormant. The high temperatures are fatal to bacteria, moist heat being more destructive than dry heat, and adult cells are more easily killed than spores. A temperature less than 212°F. will kill many organisms and boiling will kill every pathogenic organism that does not form spores. Some spores are not destroyed after prolonged boiling and some will withstand a temperature of 120°C. As a practical fact, however, boiling water kills in a few minutes all cocci, most bacilli, and all pathogenic spores, though anthrax and tetanus are harder to kill than are the spores of other bacteria.