220 ELEMENTARY AGRICULTURAL CHEMISTRY 



be effective. This solution would contain 0'167 per cent, of 

 the real formaldehyde. 



in. Insecticides. — By this term is understood a substance 

 which can be used to kill insects or creatures similar to insects. 

 The destruction may be accomplished in three ways : 



(A) By poisoning the food eaten by the insects, or by 

 absorption through their skin. 



(B) By poisoning the air breathed by the insects. 



(C) By suffocating the insects by stoppage of their 

 breathing apparatus. 



(A) Poisoning the Food. 



Under Class A large numbers of chemical compounds may be 

 included ; in fact, nearly all which are poisonous to the higher 

 animals are fatal to insects. 



Among those most largely used for the purpose of destroying 

 objectionable insects are the following : — 



Arsenic. — This substance is never used in the pure elementary 

 state, but in the form of its oxid©, arsenious oxide, Asfl^, or 

 some compound containing this. Indeed, in common language 

 "arsenic" or "white arsenic" is generally used to designate 

 what the chemist would call arsenious oxide. This is a hea'vy 

 white substance, not readily soluble in water, but dissolving 

 easily in alkalies — e.g., caustic soda or sodium carbonate solu- 

 tion, when the arsenious oxide is converted by the soda into 

 sodium arsenite, or arsenite of soda. 



Arsenious oxide is used as a vermin poison, and is fatal to 

 most forms of animal and plant life. Certain low forms of 

 vegetables, however, can develop in presence of considerable 

 quantities of arsenic. This is the case with many moulds. 

 But to higher plants arsenical solutions are quickly fatal, even 

 when highly diluted. 



In very small doses arsenic acts as a tonic upon animals, and 



