Another series of observations revealed the same sort of rapid increase of bacteria. On the date of collection the micro-organisms per cc. in a deep-well water (in April) were seven. After one day's standing at room temperature the number had reached twenty-one per cc. After three days under the same conditions it was 495,000 per cc. At blood-heat the increase would, of course, be much greater, as a higher temperature is more favourable to multiplication. But this would depend upon the degree of impurity in the water, a pure water _decreasing_ in number on account of the exhaustion of the pabulum, whereas, for the first few days at all events, an organically polluted water would show an enormous increase in bacteria. Furthermore, it is desirable to remember that organisms, in an ordinary water, do not continue to increase indefinitely. There is a limit to all things, even to numbers in bacteriology. Cramer, of Zurich, examined the water of the Lake after it had been standing for different periods, with the following results:-- Hours and Days of No. of Micro-organisms Examination. per cc.[12] 0 hours 143 24 " 12,457 3 days 328,543 8 " 233,452 17 " 17,436 70 " 2,500 The writer's own experience is entirely in agreement with this cessation of multiplication at or about the end of a week, and the later decline. _Method of Examination._ At the outset of a systematic study of a water it is well to observe its physical characters. The colour, if any, should be noted. Suspended matter and deposit may indicate organic or inorganic pollution. If abundant or conspicuous, a microscopic examination of the sediment may be made. The reaction, whether acid, neutral, or alkaline, must be tested, and the exact temperature taken. Any and every fact will help us, perhaps not so much to determine the contents of the water as to interpret rightly the facts we deduce from the entire examination. [Illustration: LEVELLING APPARATUS FOR KOCH'S PLATE]