SPONTANEOUS GENERATION. 283 



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 fiurther revealed by Helmholtz in 1843. 

 By means of a membrane he separated a sterilized 

 putrescible liquid from a putrefying one. The steril- 

 ized infusion remained perfectly intact. Hence it was 

 not the liquid of the putrefying mass — for that could 

 freely diffuse 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 experimenters employed plugs of cotton- wool 

 to filter the air supplied to their infusions. Fed with 

 such air, in the great majority of cases the putrescible 

 liquids remained perfectly sweet after boiling. Milk 

 formed a conspicuous exception 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 

 investigators. Its title was Heterogenie, and its author 

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

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

 only of scientific but of metaphysical fervour, he threw 

 his whole energy into the inquiry. Never did a subject 

 require the exercise of the cold critical faculty more 

 til an this one — calm study in the unravelling of com- 

 plex phenomena, care in the preparation of experiments, 

 care in their execution, skilful variation of conditions, 

 and incessant questioning of results until repetition had 

 placed them beyond doubt or question. To a man of 

 Pouch et's temperament the subject was full of danger 



