140 SOILS AND MANUKES 



particular part of it others only on decaying organic 

 matter. 



Some bacteria require oxygen, others grow only in its 

 absence, while others again are indifferent but produce 

 very different results according to the circumstances. The 

 majority grow best in a neutral medium, but some do better 

 when it is faintly acid or alkaline. A few only can with- 

 stand a high degree of acidity or alkalinity ; the others 

 are, therefore, liable ,to inhibit their own action by the 

 acid or alkaline character of their products. 



Temperature. For each type of organism there is a 

 particular temperature at which it grows best, and also 

 a maximum and a minimum temperature beyond which 

 it does not grow at all. The limits of variation in these 

 respects are very wide; most types flourish between 

 20 C. and 30 C., but some of the soil bacteria found near 

 the surface do not develop at temperatures below 60 C. 

 In all cases vitality ceases at a temperature below 100 C., 

 but some form spores which can withstand a temperature 

 considerably higher. 



Antiseptics. Certain chemicals which retard or inhibit 

 the growth of bacteria and other organisms are called anti- 

 septics. Powerful oxidising agents such as chlorine, per- 

 manganate of potash, and mineral poisons, e.g., salts of 

 mercury, copper, silver, etc., are amongst the most effec- 

 tive. Carbolic acid, sulphur dioxide, and some other sub 

 stances are also used as disinfectants on account of their 

 antiseptic action. Salicylic acid, formic aldehyde, borax 

 and common salt have each a similar but much milder 

 effect, and are used as preservatives for food. 



Sterilisation. When all the living organisms in a soil, 

 or other substance, have been destroyed, either by exposure 

 to high temperatures or by treatment with antiseptics, the 

 substance is said to be sterilised. Since, however, bacteria 

 are universally distributed in the air, water and all around, 



