734 A MANUAL OF BACTERIOLOGY 



To make anything like an accurate quantitative examination is 

 almost impossible. Weighed amounts of the soil, after thorough 

 pulverisation in an agate mortar, may be introduced into sterile 

 test-tubes and thoroughly exhausted by repeated washing with 

 sterile water or broth, plate cultivations being made with the 

 washings. 



Various forms of boring apparatus have been devised for 

 withdrawing soil from different depths. 



Sewage 1 



Sewage is exceptionally rich in organisms, but the numbers 

 present are variable. Jordan in Massachusetts found an average 

 of 708,000 per cubic centimetre. Laws and Andrewes found from 

 905,000 to 1 1,216,000, the latter being the highest number obtained. 

 The number of organisms naturally varies at different seasons 

 and with the amount of dilution. The organisms present are 

 very varied, but moulds, yeasts, and sarcinse only occasionally 

 occur. A few micrococci are met with and streptococci are 

 present in considerable numbers, at least 1,000 per cubic 

 centimetre, but bacilli, especially liquefying forms, largely pre- 

 dominate. The commonest species are the B. fluorescence lique- 

 faciens and varieties, several varieties of Proteus, the B. fila- 

 mentosus, varieties of the B. mesentericus, B. mycoides, B. subtilis, 

 B. cloacae, and the colon bacillus. The latter numbers from 20,000 

 to 2,000,000 per cubic centimetre, and the other bacilli mentioned 

 number 200,000 to 2,500,000 per cubic centimetre. Many 

 anaerobic sporing bacilli are also found, especially the B. Welchii, 

 the spores of which number from 30 to 2,000 per cubic centimetre, 

 averaging 500-600. Foreign bacteria introduced into sewage are 

 probably soon suppressed by the predominant species of the 

 sewage. 



The air of well-ventilated sewers differs but little from that of 

 the external air, and the organisms in it contrast with those of 

 sewage by the abundance of moulds. Specific organisms may, 

 however, gain access to it (p. 426). 



The powerful liquefying and solvent actions of the bacteria 



1 See various Reports to the London County Council by Clowes, Houston, 

 Laws and Andrewes ; Klein, Houston, Reps. Med. Off. Loc. Gov. Board for 

 1897-1904 ; Rep. of the Sewage Commission. 



