MICRO-ORGANISMS AND ENZYMS 289 



140 F., when exposed to this heat for ten minutes, 

 and most of them are destroyed at 185 F. Many 

 spores are killed at temperatures only above 212 

 F., and even then require heating one to three" hours. 

 (Fig. 43.) Dry heat is less effective than moist heat. 

 Live steam, therefore, affords a most effective means 

 of destroying bacteria. All bacteria are rendered in- 

 active at low temperatures and some may be killed 

 by intense cold. Many bacteria may retain life on 

 being dried and become active again when placed 

 under favorable conditions of moisture and tem- 

 perature. 



(5) Action of sunlight, chemicals, etc. Sunlight 

 kills many bacteria when they are exposed directly 

 to the sun's rays for a few hours. Bacteria are either 

 checked in growth or killed by many different chem- 

 ical compounds. Those compounds that simply 

 retard the rapidity of growth of bacteria are called 

 antiseptics, among which are carbolic acid, salt, salt- 

 peter, etc. ; those that destroy bacterial life are called 

 disinfectants, among which are mercuric chlorid (cor- 

 rosive sublimate), formaidehyd (formalin), potas- 

 sium bichromate, chloroform, etc. The activity of 

 each kind of bacteria is stopped by an accumulation 

 of products formed by it and, in some cases, by the 

 products of activity of other bacteria. Thus, most 

 kinds of lactic acid bacteria stop growing when about 

 0.9 per cent acid is formed, and much less than this 

 amount of lactic acid also prevents the growth of 

 many other bacteria. 



(6) Changes produced. In the course of their 

 growth, bacteria produce great changes in the 

 materials in which they grow ; and the process by 

 which these changes are brought about are known, 



