10 



pounds were broken np by the oxygen of the atmosphere into others 

 of a simpler constitution. The late Professor Miller speaks of "the 

 readiness with which the albuminoid bodies putrefy and undergo 

 sponfamous chemical changes, when moist," as being " one of 

 their most striking characters." I hope to be able to show you 

 presently that not only albuminoid principles, but all other organic 

 substances, have no such spontaneous tendency at all. 



The Microscopists then took up the question. The discovery 

 in 1835 of the Yeast plant gave a great impetus to this inquiry. 

 It was readily demonstrated that if two portions of wort or other 

 saccharine liquid are separated by some porous material, such as 

 filtering paper, and yeast is added to one, fermentation occurs 

 only in that portion which actually comes in contact with the 

 Yeast plant, and not at all in the other; thus showing, at any 

 rate, that the presence of the Yeast plant is something more than 

 an accidental circumstance, as of course all the liquid portions of 

 the wort can readily pass from one side of the paper to the other. 



With the increased perfection of our microscopes came the 

 discovery that the presence of the Yeast plant in fermenting beer 

 is only a small part of a gi-eater and more general truth, for it 

 was soon ascertained that decomposing organic matter, under all 

 ordinary circumstances, contains large numbers of minute living 

 organisms. 



For the sake of convenience these investigations were con- 

 ducted for the most part by studying the changes manifested in 

 the filtered infusions of various animal and vegetable substances. 

 Turnip, for instance, has been much used, as it yields an infusion 

 nearly as bright and clear as water. Broths made from pork, 

 fowl, and Liebig's extract of beef, have also furnished suitable 

 animal fluids. 



If such an infusion is exposed to ordinary conditions it soon 

 becomes cloudy, and teems with minute organisms, the number 

 of which increases pari passu with the decomposition. The first 

 question which excited the curiosity of scientific men was, from 

 whence come these organisms ? Do they arise de novo from the 

 contents of the infusion, or are they derived from germs intro- 

 duced from without ? In other words, does their presence sup^wrt 

 or not the theory of spontaneous generation ? j\Iany years ago 

 Spallanzani, and after him Pastern-, conducted exj^erimeuts to 



