2o8 BACTERIA AND SOIL FERTILITY 



time as they did in the absence of arsenic. The Paris green, 

 however, is poisonous to this group of organisms in the minut- 

 est quantities. This is due to the copper and not to the arsenic 

 found in the compounds. 



How Does the Arsenic Act? — We may, therefore, conclude 

 that arsenic stimulates all the beneficial bacteria. But how 

 does it act? Will it stimulate for a short time and then allow 

 the microorganism to drop back to its original or to a lower 

 level as does alcohol and various other stimulants when given to 



Fig. 42. — Beans grown on arsenic treated soil. No. 41 received no 

 arsenic the others small increasing quantities. Note the greater 

 growth in 45 and 46 due to the arsenic. (After Stewart.) 



man? Or will it act as does caffeine — continue to stimulate? 

 From the results on men and horses the former might be expected, 

 for although the arsenic eaters of India and Hungary maintain 

 that the eating of arsenic increases their endurance — and there 

 is considerable evidence to indicate this — it is only for a short 

 time. If the use be not continued the arsenic eaters cannot endure 

 as much physical exertion as can others who are not addicted to 

 the drug. Many European horse dealers place small quantities 

 of arsenic in the daily corn of the horse for they find it improves 

 its coat. If a horse, however, has been doped on arsenic for any 

 length of time it seems necessary to continue the practice; other- 

 wise, the animal rapidly loses his condition. 



Similar results might be expected with the bacteria, and ex- 

 periments have shown that although during the first few weeks 

 the bacterial activity of soils containing small quantities of ar- 



