2. Cans containing milk of grade C shall be painted red on necks and shoulders and shall have in red the words "Grade C" in large type and the words "for cooking" in plainly visible type affixed to each can. All creameries handling milk of different grades will be required to demonstrate to the Department of Health that they are capable of keeping the grades separate, and must keep records satisfactory to the Department of Health concerning the amount of milk of each grade handled each day. It is to be noted that the grades of milk are based on the bacterial content of the milk and on the opportunity for the milk to become contaminated with pathogenic organisms. From the statements made in a previous chapter it is evident that the number of bacteria in any sample of milk is dependent upon (1) the original amount of contamination, (2) the age of the milk, and (3) the temperature at which it has been held. A high bacterial content is indicative of poor milk, while a low bacterial content can be obtained, in the case of raw milk, only where due attention is paid to cleanliness and cooling. This relation between the quality of milk and its bacterial content has led many cities to adopt numerical bacterial standards, even when grades of milk have not been established. Boston requires that the milk shall not contain more than 500,000 bacteria per cubic centimeter. Rochester, N. Y., has a standard of 100,000 per cubic centimeter, while Chicago requires that the milk on arrival in the city shall not contain more than 1,000,000 per cubic centimeter from May first to September thirtieth, and not over 500,000 between October first and April thirtieth. The sale of milk containing more than 3,000,000 bacteria per cubic centimeter is prohibited.