CHAPTER XVII. 



MISCELLANEOUS PRODUCTS USED IN AGRICULTURE. 



IN this chapter, which is necessarily disconnected and fragmentary,, 

 such substances as find applications in agriculture are briefly de- 

 scribed from their chemical aspect, while some reference is made, in 

 most cases, to the manner and proportion in which they are used.. 

 An alphabetical arrangement has been adopted, since the matters to 

 be dealt with are so numerous and diverse that any connected or con- 

 tinuous description would be impossible. In some cases the sub- 

 stances mentioned have already received notice in the preceding 

 chapters ; when this is the case, reference to the place is given, so as- 

 to avoid unnecessary repetition. 



Arsenious Oxide, As 4 O, ; . This substance is known in three dis- 

 tinct forms, viz., one amorphous or vitreous and two crystalline 

 regular octahedra and trimetric prisms. Ordinary white arsenic 

 consists mainly of the powdered vitreous variety, which, however, 

 tends to pass into the heavier, octahedral form. The specific gravities 

 of the vitreous and octahedral varieties are about 3 '7 and 4'0 respec- 

 tively. Their solubilities in water vary with circumstances. If water 

 be shaken for a long time at 15 with the solids, the amounts dissolved 

 by 100 parts of water are 0'28 of the crystalline and 0*92 of the 

 vitreous varieties, while if saturated solutions at 100 be cooled to 15,. 

 2'18 of the crystalline and 3'33 of the vitreous form remain in solution.. 



Arsenious oxide dissolves readily in solutions of caustic alkalies 

 or of alkaline carbonates, arsenites of the alkali metals being formed. 

 "Fowler's solution '' contains potassium arsenite. Arsenious acid, as 

 the oxide is often called, and its compounds are powerful poisons both; 

 to animals and plants, but, curiously, have much less influence upon 

 micro-organisms. Indeed, certain moulds can develop in the presence 

 of 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 brevicaule and observing the garlic-like odour 

 evolved. As little as -^^ of a milligramme can thus be detected. 1 



In culture solutions as little as 0'0002 per cent of arsenious acid 

 will destroy plants.' 2 Arsenic acid and arsenates (compounds of As 2 O 5 ) 



1 Scholtz, Jour. Chem. Soc., 1900, Abstracts, ii. 244 ; also Abel and Buttenberg, 

 Jour. Chem. Soc., 1900, Abstracts, ii. 299. 



2 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 



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