640 AGRICULTURAL ANALYSIS 



small amount of water and filter the lead through the very small- 

 est filter paper, using as little wash-water as possible. Place 

 this filtrate in an erlenmeyer flask, and determine arsenic as 

 described above for total arsenic oxid, using the same amount of 

 reagents and the same dilutions. 



Soluble solids or impurities. Evaporate 200 cubic centime- 

 ters of the water extract obtained above to dryness in a weighed 

 platinum dish, dry to constant weight at the temperature of the 

 boiling water bath and weigh. The soluble solids so obtained 

 represent principally any sodium acetate or sodium nitrate pres- 

 ent, with a very small quantity, perhaps, of lead acetate or 

 nitrate and some soluble arsenic, probably in the form of lead 

 arsenate. 



The above methods for determining the constituents of com- 

 mercial lead arsenate have been carefully tested by Haywood, 

 as well as by several other chemists all of whom have reported 

 that exceptionally good results were obtained. 



535. Insecticides for External Sucking Insects. This group of 

 insecticides includes soaps, caustic soda and potash, lime-sulfur- 

 salt mixtures, kerosene emulsions, dilute nicotine solutions, hy- 

 drocyanic acid gas, vapors of carbon bisulfid, etc. 



536. Soaps. Soap may be used to destroy soft bodied insects, 

 such as plant lice, and in strong solutions, as a winter wash to 

 destroy scale insects. Fish oil soap, prepared by the action of 

 caustic potash or soda on fish oil is one of the most effective 

 soaps, and is the one which the chemist is most often called 

 upon to examine. The potash soap is the better of the two, as 

 it does not clog the spraying machine when the solution becomes 

 cold. 



It is usually necessary to know only three constituents of a soap 

 in order to judge of its value for spraying purposes, namely: 

 moisture, total fatty matter, and total soda or potash. The mois- 

 ture and alkali are usually determined and the total fatty matter 

 approximately estimated by difference. 



Following are the methods of analysis usually employed: 17 

 Moisture. Tare accurately a 100 cubic centimeter beaker, the 

 17 Bureau of Chemistry, Bulletin 107, 1907 : 31. 



