PREFACE. 



would be sheer trifling gravely to discuss whether 

 the organisms were living or dead, on the strength of 

 the mere activity or languor of the movements which 

 they may be seen to display. Can dead organisms 

 multiply in a closed flask to such an extent as to 

 make an originally clear fluid become quite turbid 

 in the course of two or three days ? 



And if any one wishes to convince himself as to 

 whether such turbidity can occur in a flask which is 

 still hermetically sealed, let him take one that has 

 been prepared in the manner I have elsewhere des- 

 cribed, carefully heat the neck of it in a spirit-lamp 

 flame, and see how the rapid in-bending of the red- 

 hot glass testifies to the preservation of a partial 

 vacuum within. The vacuum in such cases is only 

 partially preserved, because of the emission of a 

 certain amount of gases within the flask such as 

 invariably occurs during the progress of fermenta- 

 tion or putrefaction. 



In these experiments with heated fluids in closed 

 flasks, nothing is easier than to obtain negative 

 results. The same kinds of infusions which if 

 care has been taken to obtain them strong enough- 

 will in a few days teem with living organisms, often 

 show no trace of living things after much longer 

 periods, when the solutions are weak. Again, in 



