It does not mix with water when poured into it.] Another type of slimy milk is produced by the growth of certain types of bacteria which enter the milk after it is drawn from the udder. These may come from various sources. The bacteria concerned belong to two groups: (1) those that grow best in the air and do not form acid; (2) those that grow in the absence of air, throughout the entire mass of milk and which form acid. The slimy condition is noted in the milk only after the milk has been stored for some time; it usually increases with the age of the milk and can be produced in a second sample by transferring a little of the slimy milk to it. The fermentation produced by the aerobic bacteria is most often met in bottled milk and cream during the warmer times of the year. On account of their relation to oxygen, the growth is confined to the surface of the milk and only the upper layer becomes slimy; thus when the cream is removed, the abnormal condition is noted. The sliminess is due to the mass of bacterial growth rather than to the production of any specific substance in the milk. This trouble may be of considerable economic importance to the dealer, as such abnormal milk is objectionable for ordinary use, but as far as is known, it is incapable of affecting the health of the consumer. In numerous outbreaks of this trouble the source of contamination has been traced to infection from well water or a stream, as the organisms causing the trouble are found naturally in water. Keeping the milk in a tank in the pump house sometimes permits of troubles of this sort, the water used for cooling giving opportunity for contamination. Cattle wading in a stream sometimes pollute their udders and so indirectly infect the milk. Such outbreaks rarely persist for any considerable length of time as the common acid organisms soon regain the ascendency.