106 LOUIS PASTEUK. 



Lussac's experiment was wrong in another particular. 

 He proved the fallacy of the assumption that the 

 smallest quantity of air was always capable of pro- 

 ducing microscopic organisms. 



More thickly spread in towns than in the country, 

 the germs become fewer in proportion as they recede 

 from human habitations. Mountains have fewer 

 than plains, and at a certain height they are very 

 rare. 



Pasteur's experiments to prove these facts were 

 extremely simple. He took a series of bulbs of about 

 a quarter of a litre in capacity, and, after having half 

 filled the bulbs with a putrescible liquid, he drew out 

 the necks by means of the blowpipe, then he caused 

 the liquid to boil for some minutes, and during the 

 ebullition, while the steam issued from the tapering 

 ends of the bulbs, he sealed them with the lamp. Thus 

 prepared, the bulbs can be easily transported. As 

 they are empty of air that which they originally 

 contained having been driven out with the steam- 

 when the sealed end of a bulb is broken off, the air 

 rushes into the tube, carrying with it all the germs 

 which this air holds in suspension. If it is closed 

 again immediately afterwards by a flame, and if the 

 vessels are then left to themselves, it is easy to 

 recognise those in which a change occurs. Now, 

 Pasteur established that, in whatever place the opera- 

 tion might be carried on, a certain number of bulbs 



