FUNGICIDES 291 



in color and on exposure to the air oxidizes to an inactive 

 resin. This explains the necessity of keeping the powder 

 in air tight containers, otherwise it loses its efficacy. Pyre- 

 thrum is not poisonous to man. 



(/) Tobacco. — An old remedy for certain delicate insects 

 like plant lice has been a simple decoction of tobacco leaves 

 or waste. The water extracts the alkaloid nicotine (Section 

 86, e) which is active in destroying insects. Besides a liquid 

 extract, powdered tobacco and the smoke of tobacco are 

 efficacious in some instances. The latter contains some 

 nicotine, but in addition decomposition products which 

 have some toxic effect. Commercial preparations are on 

 the market, many of them containing nicotine sulphate as 

 the active constituent. It will be remembered (Section 85) 

 that alkaloids are basic in character and will unite to form 

 salts with mineral acids. 



(g) Whale Oil Soap. — Only the name is left of what used 

 to be the soap made from wdiale oil. Now almost any kind 

 of cheap fish oil is used and saponified with potassium or 

 sodium hydroxide. The potash soap is soft; it is more 

 readily soluble in hot water, and the solution does not 

 harden when cold. It is also more penetrating and effective. 

 One pound of soap in 4 to 10 gallons of water are the pro- 

 portions ordinarily used. The sticky soap solution clogs 

 up the pores of the insects and causes death. 



217. Fungicides. — (a) Ammoniacal Copper Carbonate 

 is a solution of basic copper carbonate, CuC0 3 .Cu(OH)2, or 



/OH 



Cu<^ 



">C=0 



Cu< 

 x OH 



dissolved in ammonia to form cuprammonium carbonate, 

 Cu(XH 3 ) 4 C03.H 2 0, 



NH 3 — NHs— OH 



/ 

 Cu OH 



\ / 



NHa— NHs— O— C=0 



