ACTION OF LEAD ARSENATE ON FOLIAGE. 45 



which compound, under the usual conditions, is more injurious to 

 foliage than lead arsenate. 



Duggar reports an extreme case in which bright sunshine follow- 

 ing rain caused the appearance of "shot holes" in peach foliage. 

 Others have also reported injury under these conditions, and it has 

 been attributed to the concentration of the sun's rays on one spot by 

 means of the drops of water acting as a lens and causing burning. A 

 disease of the peach, shown to be of bacterial origin, has also been 

 reported, 6 which produces "shot holes" in the foliage and which is 

 much worse in wet seasons. 



The work here reported has shown that pure lead arsenate applied 

 ;o tender foliage like the peach will, in some cases, cause serious 

 njury, indicating, therefore, that there is some influencing condition 

 not as yet satisfactorily determined which causes the material to be 

 decomposed and the arsenic to go into solution. This fact led to 

 >ther experiments in the effort to discover the cause of this decom- 

 position. 



CAUSE OF THE DECOMPOSITION OF LEAD ARSENATE. 



EXPERIMENTS ON THE ACTION OP THE CARBON DIOXID OP THE AIR. 



The first idea that presented itself as a possible explanation for this 

 decomposition of lead arsenate was that the carbon dioxid of the air 

 might act on the lead arsenate, forming lead carbonate, and thus 

 iberate the arsenic acid. . This theory, however, did not seem to be 

 very plausible from a chemical point of view and also owing to the 

 ack of uniformity in the injury reported in different years and at 

 different places, but it was decided to determine the point. In order 

 ;o do so the following experiments were carried out: 



Experiment 1. One gram of lead arsenate, made from sodium arsenate and lead 

 acetate, was treated with 1,000 cc of cold distilled water which had been previously 

 >oiled to expel carbon dioxid. This was allowed to stand ten days, being shaken 

 eight times each day, and was then filtered. At the end of ten days the amount of 

 arsenic in the solution was determined. 



Experiment 2. One gram of lead arsenate made from sodium arsenate and lead 

 nitrate was treated in the same way. 



Experiment 3. Same as Experiment 1, except that unboiled distilled water was used 

 and carbon-dioxid gas was run into the solution for about one-half hour each day for 

 ten days. 



Experiment 4. Same as Experiment 3, except that lead arsenate was used as in 

 Experiment 2. 



Experiment 5. Same as Experiment 3, except that the solution was kept at about 

 50 ('. during tho day. 



Experiment 6. Same as Experiment 4, heating to 50 C. each day. (This ia 

 probably a higher temperature than the material would ever attain on the tree.) 



"New York Cornell Agr. Exper. Sta., 1899, Bui. 164. 



. Rep. Conn. Agr. Exper. Sta., 1903, p. 337; Mycologia, 1909, 1: 23. 



