364 MISCELLANEOUS PRODUCTS USED IN AGRICULTURE. XVI. 



considerable quantities of arsenic and evolve arsenuretted 

 hydrogen. Thus one of the most delicate tests for the 

 presence of arsenic (Abba's test) consists of introducing into 

 the suspected substance a strong culture of Penicillium brei'i- 

 caule and observing the garlic-like odour evolved. As little as 

 5 J^ of a milligramme can thus be detected/ 1 ' 



In culture solution as little as 0*0002 % of arsenious acid 

 will destroy plants. f Arsenic acid and arsenates (compounds 

 of As 2 O 5 ) are much less injurious, for plants will grow in solu- 

 tions containing as much as 0'02%|. 



Arsenious oxide is Sometimes used in medicine as a nerve 

 tonic, and by repeated small doses a person may acquire the 

 power of taking, without danger, quantities which far exceed 

 the normal lethal doses. In such cases, however, ill effects 

 upon the system are generally produced by continual dosing 

 with arsenic. The administration of arsenic in small quan- 

 tities often produces a plumpness and sleekness of the skin. 

 For this reason it is often secretly given to horses by farm 

 servants, often with fatal results. Considerable publicity has 

 lately been given to the prevalence of this practice in the 

 north of Yorkshire, and many cases of serious losses of horses 

 have occurred from this cause. 



Arsenious oxide finds a more legitimate use in sheep dips, 

 especially for foot-rot and as a vermin poison. 



Bleaching Powder Chloride of Lime, Ca(OCl)Cl, is used 

 chiefly as a disinfectant. It acts by evolving hypochlorous 

 acid, HC10, which is a strong oxidising agent and thus able 

 to destroy putrescible matter and micro-organisms. The 

 hypochlorous acid is set free by the carbon dioxide of the 

 air, thus: 



2CaCl(OCl) + C0 2 + H 2 O - 2HC10 + CaCl 2 + CaCO. 



* Scholtz, Jouv. Chem. Soc. 1900, abst. ii. 244 ; also Abel and Buttenberg, Jour. 

 Chem Soc. 1900 abst. ii. 299. 



t Arsenic is often found in sulphuric acid and therefore in sulphate of ammonia 

 and in superphosphate. Usually, however, care is taken to employ acid free from more 

 than traces of arsenic in the preparation of these substances. According to Haselhoff 

 (Jahresbericht ii. Agricultur-Chemie 1900, 126) superphosphate made with sulphuric 

 acid from German pyrites contains about 0'0o%of arsenic; with acid from Spanish 

 pyrites, as much as 6'14!>% of arsenic. He concludes that little danger exists of the 

 arsenic in superphosphate being sufficient to do harm. 



I Stoklasa, Ann. Agron. 1897, 471; J.C.S. 1898, abst. ii. 131. 



