88 



I£ we liad taken tlie organic fluid used in the experiment 

 already noted, and if instead of exposing it to the air we had 

 put it into a small flask, and after boiling it for some time we 

 had hermetically sealed up the mouth of the flask or had put 

 into its neck a stopper of baked cotton wadding, it is probable 

 that the fluid would have remained unchanged for an indefinite 

 time. The boiling would kill all the Bacteria in the fluid, and 

 the cotton would prevent the entrance of any fresh organisms 

 from the air. If we had repeated the boiling for a few days 

 in succession the probability would be reduced to certainty, 

 assuming of course that all the necessary precautioLs were 

 taken. A fluid so prepared is said to be sterilized, and may 

 be used as a culture fluid for the growth of Bacteria. If we 

 pass into it an infinitesimal quantity of a liquid containing 

 any special form of bacterium, the bacterium will grow and 

 multiply, chiefly by fission, and will commonly also form 

 minute refractive granules, which in process of time will be 

 liberated from the containing cell and fall to the bottom o£ 

 the flask. These granules or spores are the seed from which 

 new growths of Bacteria exactly like the original stock can be 

 readily started. And just as we find it easy to kill young 

 plants by cold or heat or injur}", although the seed of such 

 plants will bear all kinds of rough usage without harm, so, al- 

 though fully developed Bacteria can be easily killed by heat 

 and other means, the spores are almost indestructible. Germi- 

 cides have but little effect on them. Alcohol wdll not kill 

 them, and they have been known to germinate even after from 

 eight to ten hours boiling. 



"We have seen that there are differences in form and size 

 among the Bacteria. It is probable there are numerous 

 varieties of species, and even of genera, which the best micro- 

 scopes yet provided will not enable us to distinguish sharply 

 one from another. We can form some opinion of this by the 

 differences in the changes they produce in the fluids or sub- 

 stances in which they flourish. Some of them are probably 

 harmless enough, but we have knowledge of many which a.re 

 highly destructive, and which even produce powerful poisons 

 in solids and liquids brought under their influence. I need 

 only remind you of the change of the sugar of grape juice 

 into alcohol by the action of one form of these organisms, 

 of the conversion of alcohol into acetic acid by another form, 

 and of the change of one of the constituents of milk iuto lactic 

 acid by another form. The Bacteria, though invisible, are 

 always workiug, playing, even in their destructiA'eness, an 

 essential part in the progressive alternatious of organic life. 



"We have as yet noticed the Bacteria in their relation to 

 dead organic matter only, but we must now approach the 



