The choice of the above-described agents depends on the conditions. Evidently a barn is not to be disinfected in the same way that a test-tube in the laboratory is sterilized. Among the factors to be considered in making a choice are the thing to be disinfected or sterilized, its size and nature, that is, whether it will be injured by the process proposed, cost of the agent, especially when a large amount of material is to be treated. Among the conditions which affect the action of all agents the following should be borne in mind particularly when testing the disinfecting power of chemical agents: 1. _The kind of bacterium_ to be destroyed, since some are more readily killed by a given disinfectant than others, even though no spores are present. 2. _The age of the culture._ Young bacteria less than twenty-four hours old are usually more readily killed than older ones since the cell wall is more delicate and more easily penetrated, though old growths may be weakened by the accumulation of their products and be more easily destroyed. 3. _Presence of spores_, since they are much more resistant than the growing cells. 4. Whether the organism is a _“good” or “bad” growth_, _i.e._, whether it has grown in a favorable environment and hence is vigorous, or under unfavorable conditions and hence is weak. 5. _The number of bacteria present_, since with chemical agents the action is one of relative masses. 6. _Nature of the substance in which the bacteria are._ Metallic salts, especially bichloride of mercury, are precipitated by albuminous substances and if employed at all must be used in several times the ordinary strength. Solids require relatively more of a given solution than liquids. 7. _State of the disinfectant_, whether solid, liquid or gas, and whether it is ionized or not. Solutions penetrate best and are therefore more quickly active and more efficient. 8. _The solvent._ Water is the best solvent to use. Strong alcohol (90 per cent. +) diminishes the effect of carbolic acid, formaldehyde and bichloride of mercury. Oil has a similar effect. The action is probably to prevent the penetration of the disinfectant. 9. _Strength of solution._ The stronger the solution, the more rapid and more certain the action, for the same reason as mentioned under 5. In fact, every disinfectant has a strength below the lethal at which it stimulates bacterial growth. 10. _Addition of salts._ Common salt favors the action of bichloride of mercury and also of carbolic acid. Other salts may hinder by precipitating the disinfectant. 11. _Temperature._ Chemical disinfectants, as a rule, follow the general law that chemical action increases with the temperature, up to the point where the heat of itself is sufficient to kill.