252 THE FLOATING-MATTER OF THE AIR. 



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 power- 

 ful microscope ; it is seen swarming with organisms 

 resembling those in the putrid milk. These organisms, 

 which receive the common name of bacteria,^ are the 

 agents of all putrefaction. Keep them and their germs 

 from your meat and it will remain for ever 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, never- 

 theless, 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 bacteria. 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. To determine 

 this point 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 ap- 

 pearance 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 pro- 

 posed to you ? ' I am not in a condition,' you would 

 say, ' to aflSrm 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 thistle-seeds form, at all 

 events, part of the powder.' Supposing a succession of 

 such powders to be placed in your hands with grains 

 becoming gradually smaller, until they dwindle to the 



' Doubtless organisms exhibiting grave specific differences are 

 grouped together under this common name. 



