All observers have noticed that the rate of increase or decrease in the bacterial count in water, as well as in other media, is irregular. There may be a rapid increase up to a very large number, then a sudden change showing no increase, or even a great falling off in the number of bacteria. This change may be due to various causes; for instance the amount of food which is at the disposal of the bacteria may become exhausted. In the second place, the bacteria by their decay and by the excretions of poisonous matters may render the medium germicidal and thus destroy their own life. Of course changes in temperature are likely also to increase or decrease the rate of multiplication.