Harmful Preservatives.--Certain chemicals and drugs, used as preservatives, seem to be on the border line of harmfulness. Such are benzoic acid, borax, or boracic acid. Such drugs _may_ be harmless in small quantities, but unfortunately in canned goods we do not always know the amount used. The national government in 1906 passed what is known as the Pure Food Law, which makes it illegal to use any of these preservatives (excepting benzoic acid in very small amounts). Food which contains this preservative will be so labeled and should not be given to children or people with weak digestion. Unfortunately people do not always read the labels and thus the pure food law is ineffective in its working. Infrequently formaldehyde or other preservatives are used in milk. Such treatment renders milk unfit for ordinary use and is an illegal process. Disinfectants.[22]--Frequently it becomes necessary to destroy bacteria which cause diseases of various kinds. This process is called _disinfecting_. The substances commonly used are carbolic acid, formalin or formaldehyde, lysol, and bichloride of mercury. Of these, the last named is the most powerful as well as the most dangerous to use. As it attacks metal, it should not be used in a metal pail or dish. It is commonly put up in tablets which are mixed to form a 1 to 1000 solution. Such tablets should be carefully safeguarded because of possible accidental poisoning. Footnote 22: Experiment to determine the most effective disinfectants. Use tubes of bouillon containing different strength solutions of formaldehyde, lysol, iodine, carbolic acid, and bichloride of mercury. Results. Conclusions. Formaldehyde used in liquid form is an excellent disinfectant. When burned in a formalin candle, it sets free an intensely pungent gas which is often used for disinfecting sick rooms after the patient has been removed. Carbolic acid is perhaps the best disinfectant of all. If used in a solution of about 1 part to 25 of water, it will not burn the skin. It is of particular value to disinfect skin wounds, as it heals as well as cleanses when used in a weak solution. Its rather pleasant odor makes it useful to cover up unpleasant smells of the sick room. The fumes of burning sulphur, which are so often used for disinfecting, are of little real value. [Illustration: This shows how organic matter is broken down by bacteria so it may be used again by green plants.] Bacteria cause Decay.--Let us next see in what ways the bacteria directly influence man upon the earth. Have you ever stopped to consider what life would be like on the earth if things did not decay? The sea would soon be filled and the land covered with dead bodies of plants and animals. Conditions of life would become impossible and living things on the earth would cease to exist.