20 BAGTERIOLOBY. 



Conclusive as this work may appear, there still ex- 

 isted a number of doubters who required further proof 

 that " spontaneous generation " was not the explanation 

 for the mysterious appearance of these minute living 

 objects, and it was not until some time later that Tyn- 

 dall, in his well-known investigations upon the floating 

 matters in the air, demonstrated again that the presence 

 of living organisms in decomposing fluids was always 

 to be explained either by the pre-existence of similar 

 living forms in the infusion or upon the walls of the 

 vessel containing it, or by the infusion having been ex- 

 posed to air which had not been deprived of its organisms. 



Throughout all the work bearing upon this subject, 

 from the time of Spallanzani to that of Tyndall, certain 

 irregularities were constantly appearing. It was found 

 that particular substances required to be heated for a 

 much longer time than was necessary to render other 

 substances free from living organisms, and even under 

 the most careful precautions decomposition would occa- 

 sionally appear. 



In 1762 Bonnet, who was deeply interested in this 

 subject, suggested, in reference to the results obtained 

 by Needham, the possibility of the existence of " germs, 

 or their eggs," which have the power to resist the tem- 

 perature to which some of the infusions employed in 

 Needham's experiments had been subjected. 



More than a hundred years after Bonnet had made 

 this purely speculative suggestion it became the task of 

 Ferdinand Cohn, of Breslau, to demonstrate its accuracy. 



Cohn repeated the foregoing experiments with like 

 results. He concluded that the irregularities could onlv 

 be due to either the existence of more resistant species 

 of bacteria or to more resistant stages into which certain 



