The number of bacteria of all kinds in water varies enormously. After a long rain the last part of the precipitation may be almost free from impurities, including bacteria. The same is also true of water from snow collected on high mountain peaks, yet even in these cases absolute freedom from bacteria is not secured. The water that comes from great depths after filtration through rocks and beds of sands often has its bacterial content reduced to almost zero. The same is true of the waters of certain springs. When, however, water comes from any source near the surface of the soil its bacterial content is always likely to be high. The number of bacteria in the same water may vary from day to day. Warm and sultry days cause a great increase in the rapidity of bacterial growth, so that in changing from cold to warm weather suddenly the number of bacteria in any given water may rapidly change.