BACTERIA 



that the belief has survived this long period of time 

 in the popular mind. Long before the activities of 

 bacteria were connected with various phenomena, 

 such as infectious diseases, souring of milk, etc., it 

 was thought that these changes were brought about 

 by chemical action. Like many of the early theories, 

 this one contained a half truth, for a great many 

 of the changes brought about by bacteria are really 

 due to chemical action initiated by the organisms. 

 In other words, the bacteria set free certain sub- 

 stances which actually cause the changes to take 

 place. 



Let us make our statement clear by a simple 

 experiment. To a little fresh milk we add a weak 

 acid, the milk curdles at once and by dipping a 

 piece of litmus paper (obtained at any chemist's) 

 into the mixture, it will turn red, showing the 

 presence of acid. Litmus, by the way, is obtained 

 from a lichen; in the presence of acid it is red, 

 an alkali, the opposite of an acid, turns it blue. In 

 a neutral solution, that is to say one that is neither 

 acid nor alkaline, litmus is of a purplish hue. 



To continue our experiment, we allow another 

 sample of the same milk to stand for a day or two 

 in a warm, dark place and again the milk will be 

 curdled. A test with the litmus will show that the 

 solution is acid. The bacteria themselves have not 

 curdled the milk but they have liberated a sub- 

 stance, called a ferment, which has split up part 

 of the milk into an acid, amongst other things and 

 that acid has actually done the curdling. For this 



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