In the large cities or even in the larger towns the cisterns have been replaced with more modern and more satisfactory means of water supply. Rain water is soft and therefore is most desirable for laundry purposes and is also very desirable for cooking. It is not considered as satisfactory for drinking, however, as ground water or properly filtered surface water. It always contains gases such as nitrogen, oxygen, and carbon dioxide. The amount of solids varies. The storage of water in cisterns forms a good breeding place for a certain kind of mosquito, stegomyia calopus, which is supposed to cause yellow fever; but regardless of the supposed pathogenicity of this mosquito it is quite objectionable. Surface Water Surface water is derived from ponds, lakes, rivers and creeks; in fact, any water which is in contact with the atmosphere is known as surface water. Surface water forms really the most desirable and satisfactory source of public water supply. =Streams= form natural sewers for the regions which they drain and ponds and lakes form convenient dumping places for the sewage carried by the streams, therefore it will readily be seen that the surface water is liable to great pollution from these sources. It therefore becomes necessary to purify it by some means before it can be used for domestic purposes. At one time it was thought that streams purified themselves in their flow, but this is now disputed and has been proven to be erroneous and should be strenuously denied. There are many factors at work in Nature which purify water in its natural state. Before civilization brought about so many artificial and unnatural conditions it was true that, with the small amount of waste material emptied into the streams, Nature would, through her natural processes, keep the waters of the streams in a state of natural purity. But in the present day with not only the sewage of our large cities pouring into the rivers and lake, but also the waste material from factories and different industries, it becomes necessary to use some artificial methods for water purification. =Lakes=, from a sanitary standpoint, form a more desirable source of public water supply than rivers. There are several reasons for this. First: there is possible a greater dilution of the impurities that reach the lake. Second: there is greater opportunity for sedimentation which is a very important process in water purification. Third: the water is softer than river water and freer from organic impurities. The greatest problem that a city getting its water supply from a lake has to solve is how to keep its sewage from polluting its own water supply. To prevent this, it is necessary to place the intake for the water supply far out in the lake. The danger from pollution has become so great in some places, as in Chicago, that special canals have been constructed to carry the sewage into other channels rather than empty it into the lake.