METHODS OF EXAMINATION 117 



inhabited localities. In cultivated lands the number of organisms 

 augments with the activity of cultivation and the strength of the 

 fertilisers used. In all soils the maximum occurs in July and 

 August. 



But the condition which more than all others controls the quantity 

 and quality of the contained bacteria is the degree and quality of 

 the organic matter in the soil. The quantity of organic matter 

 present in soil having a direct effect upon bacteria will be materially 

 increased by placing in soil the bodies of men and animals after 

 death. Dr Buchanan Young two or three years ago performed some 

 experiments to discover to what degree the soil bacteria were affected 

 by these means. " The number of micro-organisms present in soil 

 which has been used for burial purposes," he concludes, "exceeds 

 that present in undisturbed soil at similar level, and that this excess, 

 though apparent at all depths, is most marked in the lower reaches 

 of the soil."* The numbers were as follows : 



Virgin soil, 4 ft. 6 in. = 53,436 m.o. per gram of soil. 

 Burial soil (8 years), 4 ft. 6 in. = 363,411 m.o. per gram of soil. 

 (3 ), 6 ft. 6 in. = 722,751 



Methods of Examination of Soil. Two simple methods are generally adopted. 

 The first is to obtain a qualitative estimation of the organisms contained in the soil. 

 It consists simply in adding to test-tubes of liquefied gelatine or broth a small 

 quantity of the sample, finely broken up with a sterile rod. The test-tubes are now 

 incubated at 37 C. and 22 C., and the growth of the contained bacteria observed 

 in the test-tube, or after a plate culture has been made on gelatine, agar, or glucose- 

 litmus agar. The second plan is adopted in order to secure more accurate quanti- 

 tative results. One gram or half-gram of the sample is weighed on the balance, 

 and then added to 100 c.c. or 1000 c.c. of distilled sterilised water in a sterilised 

 flask, in which it is thoroughly mixed and washed. From either of these two 

 different sources it is now possible to make sub-cultures and plate cultures. The 

 procedure is, of course, that described under the examination of water (p. 463 at seq.), 

 and Petri's dishes, Koch's plates, or Esmarch's roll cultures are used.t Many of 

 the commoner bacteria in soil will thus be detected and cultivated. Spores may 

 be isolated, as is described under Examination of Sewage. But it is obvious that this 

 by no means covers the required ground. It will be necessary for us here to con- 

 sider the methods generally adopted for growing anaerobic bacteria, that is to say, 

 those species which will not grow in the presence of oxygen. This anaerobic 

 difficulty may be overcome in a variety of ways. 



Methods of Anagrobic Cultivation 



1. The oxygen may be displaced by some other gas, and though coal-gas, 

 nitrogen, and carbon dioxide may all be used for this purpose, it has become the 

 almost universal practice to grow anaerobes in hydrogen. The hydrogen is readily 

 obtained by Kipp's or some other suitable apparatus for the generation of hydrogen 



* Proc. Royal Soc. of E din., xxxvii., pt. iv., p. 759. 



t See also Report of the Medical. Officer to the Local Government Board (1897-98), 

 A. C. Houston, pp. 251-307. 



