INSECTS AND INSECTICIDES 369 



poisons and those that kill by contact. The former are used 

 against chewing insects and kill by being taken into the diges- 

 tive tract; the latter are not eaten but are applied directly 

 to the insect's body and produce death in various ways, either 

 by suffocation, by corrosive action or by fumes that penetrate 

 the breathing pores of the insect. 



Insects infesting stored seeds and tubers may be killed by 

 fumigating with poisonous gases. 



Arsenic. 



White arsenic, arsenious oxid (x\s203), is a white powder. 

 It is the cheapest form in which arsenic can be obtained. It 

 is soluble in water and therefore very injurious to foliage. 

 Sodium arsenite, a cheap and efficient insecticide, may, however, 

 be prepared from i,t by the foUow^ing method : 



Sal soda 2 pounds 



Water 1 gallon 



Arsenic 1 pound 



Mix the white arsenic into a paste and then add the sal soda 

 and water, and boil until dissolved. x\dd water to replace any 

 that has boiled away, so that one gallon of stock solution is 

 the result. Use one quart of this stock solution to fifty gallons 

 of bordeaux mixture. This material is little used, except for 

 potato spraying, and should always be applied in combination 

 with bordeaux mixture and even w^hen so used care should be 

 taken that there is enough lime in the mixture to neutralize 

 the caustic action of the asenic. 



Paris green. 



Pure paris green, 3Cu(As02)2 • Cu(C2H302)2, is composed 

 of copper oxid, CuO, acetic acid, HC2H3O2, and arsenious oxid, 

 AS2O3, chemically combined as copper-aceto-arsenite as follows: 

 2b 



