VITALITY OP PUTHEFACTIVE OEGANISMS. 187 



Professor Wyman, of Harvard College, while in 1874 

 it was materially refined and improved upon by Dr. 

 William Roberts, of Manchester. The method is ham- 

 pered by one grave doubt. The air, plus its floating 

 matter, is imprisoned in the sealed bulbs, so that the 

 heat applied has not only to destroy the germs clasped 

 by the infusion, but also those diffused through the 

 supernatant atmosphere. Now it is not certain whether 

 an amount of heat which would be absolutely destruc- 

 tive to a germ embraced by a hot liquid may not be 

 wholly ineffectual when acting on a germ floating in 

 vapour or air. Throughout Spallanzani's and Needham's 

 experiments, throughout those of Wyman and Eoberts, 

 and throughout my own, as reported in this section 

 and the last, this possibility of error runs. Such ex- 

 periments, in short, do not enable us to state with 

 certainty the temperature at which an infusion is steri- 

 lized, because the germs which most pertinaciously 

 oppose sterilization may not belong to the infusion at 

 all, but to the adjacent air. 



The most astonishing cases of resistance to steriliza- 

 tion observed by Wyman were associated with this 

 particular mode of experiment. The fio.it. 



possible action of the uncleansed air, r\ 



moreover, was in his case augmented by / \ 



the fact that he employed quantities / 1 



of liquid, very small in comparison / \ 



with the size of his flasks. In some of / \ 



his earlier experiments the volume of / \ 



air was more than thirty times that of / \ 



the infusion. These relative volumes I I 



are represented in the annexed figure \^____ / 



(fig. 17), copied from Wyman's Memoir ^^^^^^ 

 of 1862.' 



' Silliman's American Journal, vol. xxiv. p. 80. 



