BACTERIA IN THE SOIL. 643 



and down. But for the purpose of obtaining pure cultures from sin- 

 gle colonies of the various species present, we should prefer to wash 

 the earth in distilled water and to allow the sediment to settle before 

 taking a portion of the water to add to the nutrient medium. 



In some experiments made in 1881 Koch ascertained that in soil 

 which had not been disturbed but few bacteria were to be found at 

 the depth of a metre; and this fact has since been established by the 

 extended researches of Frankel, who devised a special boring instru- 

 ment for obtaining samples of earth from different depths. Miquel, 

 in 1879, estimated the number of bacteria in one gramme of earth 

 collected in the park of Montsouri, Paris, at a depth of twenty centi- 

 metres, at 700,000; and in a cultivated field which had been treated 

 with manure, at 900,000. The following results were obtained by 

 Adametz : One gramme of earth from a sandy soil contained at the 

 surface 380,000, at a depth of twenty to twenty-five centimetres 

 400,000 ; the same quantity of clayey soil contained at the surface 

 500,000, at a depth of twenty to twenty-five centimetres 460,000. 



In experiments made by Beumer (1886) and by Maggiora (1887) 

 considerably greater numbers were found, but the last-named ob- 

 server, in some instances at least, kept the earth for some time after 

 collecting it, which may have materially influenced the result. 

 Beumer obtained from a specimen of sandy humus taken from a 

 depth of three metres 45,000,000 to the gramme ; at four metres, 

 10,000,000; at five metres, 8,000,000; at six metres, 5,000,000. 

 These specimens were obtained from the vicinity of hospitals at 

 Greifswald. In a churchyard, at a depth of four metres, the num- 

 ber in one experiment was 1,152,000, and in another 1,278,000. 



Frankel has given special attention to the examination of undis- 

 turbed soil not in the immediate vicinity of dwellings. In samples 

 from a fruit orchard near Potsdam he found that the superficial 

 layers contained from 50,000 to 350,000 germs per cubic centimetre. 

 The greatest number was not immediately upon the surface, but at 

 from one-quarter to one-half metre below the surface. The num- 

 ber was found to be greater in summer than in winter, the maximum 

 being in July and August. At a depth of three-quarters of a metre 

 to a metre and a half there was a very great and abrupt diminution in 

 the number of germs. From 200, 000 at one-half metre the number fell 

 to 2,000 at a depth of a metre, from 250,000 at three-quarters of a 

 metre to 200 at one metre, etc. , and at a depth of one and one-half 

 metres, in some instances, no more living germs were obtained. In 

 other experiments a few colonies developed from earth obtained at a 

 depth of three or four metres, but these were slow in making their 

 appearanc'e, and often several days, or even weeks, elapsed before 

 they became visible in Esmarch roll tubes. In experiments with sur- 



