tiOW DOES THE ARSMIC ACT 121 



their endurance, and there is considerable evidence to indicate this, 

 it is only for a short time. If the use be discontinued 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 given to the horse, for 

 they find it improves the coat of the horse. If a horse, however, has 

 been doped on arsenic for a long time it seems necessary to continue 

 the practice; otherwise, the animal rapidly "loses his condition." 



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

 ments have shown that although during the first few weeks the 

 bacterial activity of soils containing small quantities of arsenic is 

 much greater than it is in a similar soil without arsenic, this activity 

 continues to get less and less, until at the end of several weeks it is 

 no greater than in soil containing no arsenic. It is interesting to 

 note that if proper aeration is maintained bacterial activity never 

 becomes lower than in untreated soil. 



Now why this stimulating influence of arsenic upon soil bacteria? A 

 similar condition has been found to exist when soils are treated with 

 carbon bisulphid, chloroform, or other disinfectants, or even when 

 the soil is heated. Many theories have been offered to account for 

 it, but probably the most interesting is the one held by Russell and 

 Hutchinson. They maintain that within the soil are microscopic 

 plants, bacteria, and also microscopic animals, protozoa. The 

 minute animals are continually feeding upon the minute plants, 

 with the result that the bacterial plants cannot multiply as they 

 could in the absence of the protozoa. Now when a weak solution 

 of an antiseptic is applied to the soil it kills many of the protozoa, 

 and the bacteria being no longer preyed upon by their natural foe 

 rapidly multiply. As the antiseptic evaporates the few remaining 

 protozoa start to multiply and soon are able to keep in check the 

 bacterial flora of the soil. So within the soil one species preys upon 

 another. It is possible that microscopic forms of life wage within 

 the soil battles as terrific as those waged by the higher forms of life 

 upon the earth's surface. 



It is likely that this is one of the ways in which arsenic stimulates- 

 the bacterial activities of the soil. It acts more readily upon the 

 protozoa than upon the bacteria. After the arsenic has been in the 

 soil for some time it may become insoluble or some of it may be 

 changed by molds into a gas arsine and pass into the air. Then the 

 few protozoa which have not been destroyed by its presence rapidly 

 multiply and soon hold the bacteria in check. 



This, however, is not the only way in which arsenic acts, for pure 

 cultures of the Azotobacter have been obtained from these soils, and 

 it is found that these are so stimulated that they bring about greater 

 changes in the presence of arsenic than they do in its absence. This 

 is due to the action of the arsenic upon these minute specks of living 



