Milk may be preserved for several days if "pasteurized" or "sterilized." Pasteurization consists of heating milk to a temperature of about 167 deg. F., and maintaining it at that degree for twenty minutes. Sterilization means keeping the milk at a temperature of 212 deg. F. for two hours and a half. Immediately after either process the milk should be cooled, then placed in absolutely clean, covered bottles and kept on ice. These methods are not only harmless but actually beneficial in that they destroy any disease germs that might be present. Chemical preservatives are occasionally found in milk. They may be suspected if the milk is alkaline in reaction and has a disguised taste. The ones most commonly used are boric and salicylic acids and formaldehyde; the two former can only be detected by chemical tests too delicate and intricate to be used by the housewife. Formaldehyde may be tested for by using a solution of one drop of a ten per cent solution of ferric chloride to one ounce of hydrochloric acid.[6] Fill a small porcelain dish one-third full of this solution; add an equal volume of milk and heat slowly over a flame nearly to the boiling point, giving the dish a rotary motion to break up the curd. If formaldehyde is present, the mass will show a violet color, varying in depth with the amount present; if it is absent, the mass turns brown. =Butter.=--Good butter has a fresh, sweet odor and an agreeable taste. It should be of the same color and consistency throughout, easily cut and adherent and not crumbly when molded into shapes. Pure butter is very light in color; nearly all that is sold is colored, in order to meet the popular demand for "yellow" butter; annatto and other vegetable and mineral substances are sometimes employed for this purpose. These coloring matters are generally harmless but may be detected by dissolving a portion of the butter in alcohol; the natural color will dissolve, while foreign coloring will not. Butter should consist of eighty-five per cent fat, with the remainder water, casein, and salt. The most common methods of adulteration consist in an excess of water and the addition of oleomargarine. If an excess of water has been added it may be shown by melting the butter; the water and fat will separate in two distinct layers. Oleomargarine has a distinctive meaty smell, like that of cooked meat, and lacks the characteristic odor of pure butter. If pure butter is melted in a spoon, it will not sputter; if oleomargarine is present, it will. The preservatives sometimes used, namely, boric and salicylic acids and formaldehyde, can only be detected by chemical tests. =Eggs.=--Two methods may be used to detect stale eggs. First: make a solution of one part of table salt to ten parts of water and immerse the suspected egg; if it sinks, it is perfectly fresh; if it remains in the water below the surface, it is at least three days old, and if it floats, it is five or more days old.