Rainfall is often collected and stored in cisterns and used for beverage purposes. As will be explained further on, rain water is subject to many sources of pollution, particularly by entangling matters floating through the air or collecting debris off the roofs or other surfaces upon which it first falls. This is particularly apt to be the case with the first portions of the rain. In heavy and long-continued rains the portions which fall toward the end of the precipitation are almost pure. They contain very little solid matter in solution but usually are saturated with the nitrogen and oxygen of the atmosphere and contain more or less carbon dioxid; as rain water must also be regarded melted snow either collected upon the roofs of houses or otherwise. The water which comes from the melting of the snow has all the characteristics of the water precipitated directly as rain. When these waters have come in contact with the soil they begin at once to dissolve mineral and organic matters and lose their distinctive character as rain water. The melted snows of high mountain peaks are believed to be the purest forms of rain water that exist.