THE IMPORTANCE OF OXYGEN 241 



covered an entirely new world of living microorganisms. He dis- 

 covered bacteria and protozoa, which many investigators said were 

 the organisms from which more complex organisms originated. 

 By means of the microscope he was able to prove that the weevils 

 found in granaries were hatched from minute eggs deposited on 

 the wheat grains by winged insects. 



Needham and Spallanzani. About 1770, Needham, an English- 

 man, became interested in these experiments. He boiled meat 

 extract in glass flasks which he closed securely with corks. He 

 thought he had killed all the life present with the boiling process. 

 In every case, he found that great numbers of microorganisms 

 appeared sooner or later. He decided that if life appeared, it 

 must originate spontaneously. He started the spontaneous gen- 

 eration controversy anew. The Abbe Spallanzani, an Italian, sus- 

 pected that Needham had not been very careful in conducting his 

 experiments and that germs in the air might have entered the flask. 

 He repeated the same experiments, but used glass flasks that 

 could be hermetically sealed in a flame while the infusion was still 

 hot. No organisms appeared. Needham objected to Spallan- 

 zani' s experiment, saying that the prolonged hieating had destroyed 

 the nutritive value of the substance. Spallanzani then per- 

 mitted some air to enter the glass and almost at once microscopic 

 organisms appeared, showing that the nutritive qualities of the 

 material were still there. 



The importance of oxygen. About this time a scientist named 

 Priestley discovered oxygen and its importance to life. Scientists 

 now asked whether the boiling of tKe closed flasks had not changed 

 the oxygen so that it had lost its life-giving properties. But no 

 more scientific experiments were performed until 1836, when an ex- 

 periment was devised that permitted clean air to enter the culture 

 medium continuously. Organisms in the air were removed by 

 passing the air through a series of tubes containing substance which 

 would kill all living matter. No organisms appeared in the flask. 



