94 



INSECTICIDES, PUNGICIDES, AND WEED KILLERS. 



those obtained by prnssic acid in similar conditions. This gas is so 

 I)oisonous that it can never be used in actual practice. 



14. Arsenious Sulphide, As^Sg (yellow orpiment). — Loarer, in 

 1872, sprayed vines attacked by the phylloxera with a bouillie con- 

 taining 0-1 per cent of this product, but without success. 



15. Arsenious Acid, AsoOy. — Arsenious acid, or arsenic, is ob- 

 tained, commercially, by roasting arsenit'erous minerals, the arsenides, 

 and sulpharsenides of nickel, cobalt, and iron, in capacious muffles, of 

 refractory material, round which the flame from the fiie circulates. - 



2FeAsS + 30 = 2FeS + AsoOg. 



The arsenious anhydride given off is led into cold, superimposed 

 chambers, where it condenses as a white crystalline powder. It is 

 removed lay raking it out and purified by distilling it in wrought-iron 

 vessels. 



Properties. — White arsenic is a white or colourless, inodorous 

 solid ; recently fused, it forms a vitreous, transparent, amorphous mass. 

 White arsenic only dissolves at 13° C. in water, in the ratio of 1*2- 

 1-3 per cent, whilst the vitreous acid dissolves in the ratio of 4 per 

 cent. Its taste is at first faint, then bitter and nauseous. Arsenious 

 acid is a violent poison for all animals, and in a dose of 1 decigramme, 

 say 1^ grains, kills man. It is a violent escharotic of the mucous 

 membrane, and of all the tissues in general, which it inflames, and 

 rapidly destroys. Absorbed by the digestive channels, it gives rise ta 

 gastro-intestinal symptoms, which are often followed by paralysis. 

 In very small doses it is a powerful stimulant, which encourages 

 growth. This property causes it to be much used in medicine. 

 Arsenic is as poisonous to plants as to animals. It burns the leaves. 

 Arsenites soluble in water are the more active the greater their solu- 

 bility ; those which are insoluble in water consequently have no 

 injurious action on plants. 



Herbaceous plants die when they are watered with a 0*5 per 

 cent solution. Deciduous trees are also very sensitive to the action 

 of arsenic. To destroy certain insects, a solution of 200 grammes in 

 100 litres (2 lb. per 100 gallons) were tried, but even in that propor- 

 tion the arsenic is still too prejudicial to the leaves. 



TABLE XVI. — Shoioing Sensitiveness of Leaves of Various Fruit Trees to Solu- 

 tions of Arsenious Acid of Various Strengths. 



Parts by weight in 100,000 parts by volume. 



