Milk is of course seeded with a considerable variety of organisms at first. The liquefying and inert species are the most abundant, the distinctively lactic acid class occurring sparsely, if at all. As milk increases in age, germ growth begins to occur. More or less development of all types go on, but soon the lactic species gain the ascendency, owing to their being better suited to this environment; they soon outstrip all other species, with the result that normal curdling generally supervenes. The growth of this type is largely conditioned by the presence of the milk sugar. If the sugar is removed from milk by dialysis, the liquid undergoes putrefactive changes due to the fact that the putrefactive bacteria are able to grow if no acid is produced. ~Relation of temperature to growth.~ When growth does once begin in milk, the temperature at which it is stored exerts the most profound effect on the rate of development. When milk is not artificially cooled, it retains its heat for some hours, and consequently the conditions become very favorable for the rapid multiplication of the contained organisms, as is shown in following results obtained by Freudenreich[41]: _No. of bacteria per cc. in milk kept at different temperatures._ 77 deg. F. 95 deg. F. 5 hrs. after milking 10,000 30,000 8 " " " 25,000 12,000,000 12 " " " 46,000 35,280,000 26 " " " 5,700,000 50,000,000 [Illustration: FIG. 14. Effect of cooling milk on the growth of bacteria.] Conn[42] is inclined to regard temperature of more significance in determining the keeping quality than the original infection of the milk itself. Milk which curdled in 18 hours at 98 deg. F., did not curdle in 48 hours at 70 deg., and often did not change in two weeks, if the temperature was kept at 50 deg. F. Where kept for a considerable period at this low temperature, the milk becomes filled with bacteria of the undesirable putrefactive type, the lactic group being unable to form acid in any appreciable amounts. Running well water can be used for cooling, if it is possible to secure it at a temperature of 48 deg.-50 deg. F. The use of ice, of course, gives better results, and in summer is greatly to be desired. The influence of these lowered temperatures makes it possible to ship milk long distances[43] by rail for city supplies, if the temperature is kept low during transit.