28 THE INVISIBLE WORLD 



Millions of acres of such lands now lie idle and 

 useless. But isolate the right kind of microbes 

 that is, separate them from all others. Make 

 of these microbes pure cultures that is, 

 cultures containing these microbes only. Make 

 the cultures in the dry form. Reduce them 

 to powder. Mix the powder with the seed 

 and sow it. The germs will rapidly multiply, 

 completely fill the soil, do just the kind of work 

 needful for the seed to germinate and grow, and 

 abundant crops will be the result, 



On the other hand, the bad microbes may be 

 cultivated either intentionally for a good purpose, 

 or unintentionally, producing bad results. 



Isolate the microbe of typhoid fever, taken from 

 the human subject having that disease. Make of 

 it a pure culture. Inject a bit of the culture, hy- 

 perdermically, into the circulation of some sus- 

 ceptible lower animal, like the guinea pig or the 

 rabbit. The animal will have the fever. 



Repeat the same experiment with the microbe 

 of diphtheria, scarlet fever, and so on with all the 

 contagious and infectious diseases. In every in- 

 stance the germ in the culture produces the same 

 disease in the lower animal which the human sub- 

 ject has from whom the germ is taken. 



This is striking proof that in every case the 

 microbe is the cause of the disease, and the only 

 cause. And this knowledge is vitally important 

 because, knowing the cause of the disease, one is 



