FERMENTATION AND DISEASE. 139 



sour, separating like blood into clot and serum. Place a drop of this 

 sour milk under a powerful microscope and watch it closely. You 

 see the minute butter-globules animated by that curious quivering 

 motion called the Brownian motion. 1 But let not this attract your 

 attention too much, for it is another motion that we have now to seek. 

 Here and there you observe a greater disturbance than ordinary 

 among the globules ; keep your eye upon the place of tumult, and 

 you will probably see emerging from it a long, eel-like organism, toss- 

 ing the globules aside and wriggling more or less rapidly across the 

 held of the microscope. Familiar witli one sample of this organism, 

 which from its motions receives the name of vibrio, you soon detect 

 numbers of them. It is these organisms which, by decomposing the 

 milk, render it sour. This vibrio is in fact the butyric-acid ferment, 

 as the yeast-plant is the alcoholic ferment. Keep the vibrio and its 

 germs out of your milk and it will never turn sour. But, instead of 

 becoming sour, milk may become putrid. This is due to the action of 

 another living ferment. Examine your putrid milk microscopically, 

 and you find it swarming with organisms much shorter than the 

 vibrios, and manifesting sometimes a wonderful alacrity of motion. 

 Keep this smaller organism and its germs out of your milk and it will 

 never putrefy. Expose a mutton-chop to the air and keep it moist ; in 

 summer weather it soon stinks. Place a drop of the juice of the fetid 

 chop under a powerful microscope ; it is seen swarming witli organ- 

 isms resembling those in the putrid milk. These organisms, which 

 receive the common name of bacteria, 2 are the agents of all putrefac- 

 tion. Keep them and their germs from your meat and it wall remain 

 forever sweet. Thus we begin to see that within the world of life to 

 which we ourselves belong there is another living world requiring 

 the microscope for its discernment, but which, nevertheless, has the 

 most important bearing on the welfare of the higher life-world. 



And now let us reason together as regards the origin of these bac- 

 teria. A granular powder is placed in your hands, and you are asked 

 to state what it is. You examine it, and have, or have not, reason to 

 suspect that seeds of some kind are mixed up in it. But you prepare 

 a bed in your garden, sow in it the powder, and soon after find a 

 mixed crop of docks and thistles sprouting from your bed. Until this 

 powder was sown neither docks nor thistles ever made their appear- 

 ance in your garden. You repeat the experiment once, twice, ten 

 times, fifty times. From fifty different beds after the sowing of the 

 powder you obtain the same crop. What will be your response to 

 the question proposed to you ? "I am not in a condition," you would 

 say, " to aifirm that every grain of the powder is a dock-seed or a 

 thistle-seed; but I am in a condition to affirm that both dock and 



1 Which I am inclined to regard as an effect of surface tension. 



2 Doubtless organisms exhibiting grave specific differences are grouped together 

 under this common name. 



