34 



hunger. In the meantime, however, the protection to the foliage is just 

 as great as though the larva? were killed outright. 



Confirmatory of the efficacy of the arsenite of lead are some experi- 

 ments conducted on the grounds of the Department against the bag- 

 worm on Arbor vitce. Some half dozen trees thickly infested with these 

 larva? were sprayed at the rate of 1 pound to 50 gallons of water. 

 Ten days after the application the cases of the bag worms were invari- 

 ably found to contain dead larvae, and the cessation of feeding of these 

 larva? was almost immediate after the application. The use of these 

 arsenicals on pear and cherry trees was also accompanied by a demon- 

 stration of value as protective applications, in that trees not included 

 in the experiment were much eaten by the pear slug. 



yotes on the composition and other characteristics of the arsenicals. — A 

 great many analyses of the old arsenicals, Paris green and London 

 purple, have been made and are valuable for comparison. Paris green 

 is a definite chemical compound and should have a nearly definite com- 

 position. London purple is a waste product and naturally shows consid- 

 erable variation in composition. Dr. Lintner quotes an analysis showing 

 43.65 per cent of arsenious oxide in this insecticide, whereas other 

 analyses have shown it to contain as little as 19.64 of arsenious oxide. 

 Scheele's green, theoretically, should contain 52.94 per cent arsenious 

 oxide, and the lead arsenites and arsenates occur in several different 

 forms and have a variable component of arsenious oxide. The common 

 form of lead arsenite contains 47.03 per cent; the arsenate such as 

 used by the gypsy moth commission 28.53 per cent, and the lead 

 arsenite used m my own experiments shows by analysis 39.34 per cent 

 of arsenious oxide. In other words, Paris green and Scheele's green 

 are not widely different in the active principle. Lead arsenite in its 

 most poisonous form approaches somewhat closely these two, whereas 

 the arsenate is very much weaker and yet costs more to manufacture. 

 The following solution tests of these arsenicals have been kindly made 

 for me by Prof. George Patrick, of the Division of Chemistry : 



Grains, 

 per gallon. 



No. 1. Paris green, ordinary 0. 87 



No. 2. Paris green, pulverized 1. 18 



No. 3. Arsenite of copper *. 2. 50 



No. 4. London purple • 7 93 



No. 5. Arsenite of lead 94 



The samples were mixed with Potomac water at the rate of 1 pound 

 of the powder to 100 gallons of water, kept in contact for twenty-four 

 hours, with frequent shakings during the first ten hours. The quan- 

 tities of arsenic dissolved are indicated by the figures given above. 



The solubility of the poison depends, as might have been expected, 

 on the fineness of the powder, as indicated by Paris green, ordinary 

 and pulverized. The greater amount of soluble arsenic in Xo. 3 is not 

 more in the same proportion relative to its greater fineness than 

 between the two samples of Paris green. The great solubility of Lon- 

 don purple is shown, and also the slight solubility of the arsenites of 

 lead. The conditions of the experiment represent an exaggeration 

 of what might occur in spraying operations. 



