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The time that may be allowed to elapse between the collection of a sample and the beginning of its analysis cannot be stated definitely. It depends on the character of the sample, the examinations to be made, and other conditions. The following are suggested as fairly reasonable maximum limits.

Physical and chemical analysis. Ground waters 72 hours Fairly pure surface waters 48 〃 Polluted surface waters 12 〃 Sewage effluents 6 〃 Raw sewages 6 〃 Microscopical examination. Ground waters 72 hours Fairly pure surface waters 24 〃 Waters containing fragile organisms Immediate examination Bacteriological examination. Samples kept at less than 10°C 24 hours

If a longer period elapses between collection and examination the time should be noted. If sterilized by the addition of chloroform, formaldehyde, mercuric chloride, or some other germicide samples for sanitary chemical examination may be allowed to stand for longer periods than those indicated, but as this is a matter which will vary according to circumstances, no definite procedure is recommended. If unsterilized samples of sewage, sewage effluents, and highly polluted surface waters are analyzed after greater intervals than those suggested caution must be used in interpreting analyses of the organic content, which frequently changes materially upon standing.