312 FRAGMENTS OF SCIENCE 



organisms were derived not from the air, but from some- 

 thing contained in the air, which was destroyed by a suffi- 

 ciently high temperature. There never was a more deter- 

 mined opponent of the doctrine of spontaneous generation 

 than Schwann, though a strange attempt was made a year 

 and a half ago to enlist him and others equally opposed 

 to it on the side of the doctrine. 



The physical character of the agent which produces 

 putrefaction was further revealed by Helmholtz in 1843. 

 By means of a membrane he separated a sterilized putres- 

 cible liquid from a putrefying one. The sterilized infu- 

 sion remained perfectly intact. Hence it was not the 

 liquid of the putrefying mass for that could freely dif- 

 fuse through the membrane but something contained in 

 the liquid, and which was stopped by the membrane, that 

 caused the putrefaction. In 1854 Schroeder and Von 

 Dusch struck into this inquiry, which was subsequently 

 followed up by Schroeder alone. These able experiment- 

 ers employed plugs of cotton-wool to filter the air supplied 

 to their infusions. Fed with such air, in the great major- 

 ity of cases the putrescible liquids remained perfectly 

 sweet after boiling. Milk formed a conspicuous excep- 

 tion to the general rule. It putrefied after boiling, though 

 supplied with carefully filtered air. The researches of 

 Schroeder bring us up to the year 1859. 



In that year a book was published which seemed to 

 overturn some of the best established facts of previous in- 

 vestigators. Its title was ' ' Heteroge'nie, " and its author 

 was F. A. Pouchet, Director of the Museum of Natural 

 History at Rouen. Ardent, laborious, learned, full not 

 only of scientific, but of metaphysical fervor, he threw his 

 whole energy into the inquiry. Never did a subject re- 



