These are shallow collections of rain-water. Such pools abound in vegetable growths of all kinds. From their shallowness they are soon warmed by the heat of the sun, and then ensue decomposition, fermentation, and decay of dead matter, overtaxing altogether the purifying power of the dissolved oxygen. The result is a fluid more or less charged with badly-smelling gases and dissolved vegetable matter, which, though small in amount, not exceeding sometimes more than five or ten grains per gallon, is in a state of change and liable to set up disease in those who incautiously drink or are more or less compelled occasionally to drink beverages prepared from such waters. The dark-colored peaty pools on mountains are less liable to do harm, especially if the water is merely peaty and not much concentrated by evaporation. Pond-water may be sometimes little else than rain-water with five or ten grains per gallon of dissolved solids, and occasionally fit for drinking. Pond-water of this character is not often met with. On the other hand, it may be mere diluted sewage, disgusting alike to eyes and nose. In all cases avoid this kind of water and keep on the side of safety.