The rapidity with which bacteria increase in number is sometimes phenomenal. Even in water where the bacterial food is not present in great abundance the increase is very rapid. In milk, which is a most favorable medium for rapidity of growth, the rate of increase is sometimes quite incredibly large. At ordinary room temperature the number of bacteria in any given amount of water increases at different rates. Franklin found, as quoted by George Newman (Bacteriology and the Public Health) that a water which contained in round numbers 1,000 organisms at the time of the collection of the sample had 6,000 at the end of 6 hours, 7,000 at the end of 24 hours, and 48,000 at the end of 48 hours. In another case where water was collected from a deep spring the number of organisms per cubic centimeter was found to be 7. In 24 hours the number had increased to 21. After three days the enormous number of 495,000 were found. This was at the usual temperature of the month of April. The kind of bacteria has also to be considered in this connection. Some varieties multiply much more rapidly than others.