20 BACTERIOLOGY. 



for the mysterious appearance of these raiiiute 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 preexistence of similar liv- 

 ing forms in the infusion or upon the walls of the vessel 

 containing it, or by the infusion having been exposed 

 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 certain substances required to be heated for a much 

 longer time than was necessary to render other sub- 

 stances free from living organisms, and even under the 

 most careful precautions decomposition would occasion- 

 ally appear. 



In 1762 Bonnet, who was deeply interested in this 

 subject, had suggested, in reference to the results ob- 

 tained by Needham, the possibility of the existence of 

 " germs, or their eggs," which have the power to resist 

 the temperature 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 only 

 be due to, either the existence of more resistant species 

 of bacteria or to more resistant stages into which certain 

 bacteria have the property of passing. After much 

 work he demonstrated that certain of the rod-shaped 

 organisms possess the power of passing into a resting 



