868 



SCIENCE 



LN. S. Vol. XX.XIII. No. 857 



eliminate the injury, but may prolong the 

 time required for it to take place. Also, ar- 

 senicals of several types, applied as dust 

 sprays (without water), have produced injury 

 under our climatic conditions. At the same 

 time and under the same conditions arsenicals 

 have been applied that produced no injury or 

 a very small amount. Such variation from 

 perfect foliage neutrality to serious injury 

 was found in a series of arsenate of lead 

 samples. The samples which produced no 

 injury were found to still retain their non- 

 injurious properties when mixed up for spray- 

 ing in any of the various waters in common 

 use in the locality. These waters vary from 

 40 to 150 parts per 100,000 of salts in solu- 

 tion ; chlorides, carbonates and sulphates form- 

 ing the bulk of the salts. 



Thus, all degrees of injury were obtained 

 when samples of that material commonly 

 known as arsenate of lead were applied to 

 foliage with the same water. Such results 

 indicated a radical difference in the chemical 

 properties of the various samples. Chemical 

 authorities mention several plumbic arsenates, 

 but consider them as being alike insoluble in 

 water. However, " Handbuch der Anorgan- 

 ischen Chemie" (O. Dammer), Vol. 11, Part 

 2, pp. 565 and 566, states that pyro-arsenate 

 (Pb„As,Oj) is soluble in ammonia while the 

 ortho-arsenate [PbjCAsOJJ is not. This 

 brief statement requires interpretation and 

 may be expanded as follows:" The acid arsen- 

 ates are stable under acid conditions, but are 

 transposed into the ortho-arsenate, the most 

 stable compound, under neutral and alkalin 

 conditions. This transposition involves the 

 liberation of arsenic oxide or soluble arsenr 

 ates. The significance is at once apparent. 

 When arsenate of lead is applied as a spray it 

 is subjected to neutral and alkalin conditions. 

 This is especially true if the water used in 

 spraying contains alkalies. That is, the con- 



- The credit for this interpretation belongs to 

 Mr. E. E. Luther, at that time (1906) an assistant 

 in the laboratory at Watsonville. This discovery 

 not only explained the injurious properties of cer- 

 tain types of lead arsenate, but also indicated the 

 means by which such injury could be overcome. 



ditions favorable to transposition of the acid 

 arsenates into the ortho-compound obtain. As 

 fast as the neutral waters of fogs, dews and 

 rains wash away the liberated arsenic oxide, or 

 when the latter is absorbed by the plant tis- 

 sues themselves, the conditions are restored 

 for more to be formed. The ultimate result 

 is the complete transposition of the acid 

 arsenates to the ortho-compound and the lib- 

 eration of the excess arsenic oxide. 



At the time when these conclusions were 

 reached we had the records of a large number 

 of foliage tests with arsenate of lead samples. 

 Checking these off showed that without excep- 

 tion those simple plumbic arsenates which 

 produced no injury contained lead oxide and 

 arsenic oxide in the correct proportions to 

 produce the ortho-arsenate. 



These results were obtained in 1906 and in 

 the early part of 1907, and while the evidence 

 was then sufficient to exclude practically all 

 doubt, it was thought best to wait until sev- 

 eral years of commercial spraying and supple- 

 mentary experimental work should give incon- 

 testable grounds to announce the deductions 

 as fully supported by experimental evidence. 



To continue further with deductions which 

 are probably correct. Some authors recognize 

 pyro-arsenate (Pb^As,©,) and others only 

 hydrogen-arsenate (PbHAsO^) as occurring 

 in wet precipitates. Our results apparently 

 show that both compounds may occur in com- 

 mercial lead arsenate. Foliage tests show that 

 pure hydrogen-arsenate behaves differently 

 from mixtures containing considerable pro- 

 portions of the ortho-arsenate. These mix- 

 tures of ortho and pyro are more rapidly in- 

 jurious than the pure hydrogen-arsenate. 



A chemical explanation of this fact is ap- 

 parent. Lead-hydrogen-arsenate is Pb,As„0, 

 to which one molecule of water has been added ; 

 that is, Pb,As,0, + H,0 = (PbHAsO,),. In 

 other words, PbHAsO^ is a product of the 

 hydrolysis of Pb^As^O,. The ILO may even 

 be regarded as water of crystallization as the 

 hydrogen-arsenate is crystalline rather than 

 amorphous in structure. This rearrangement 

 of the molecule gives opportunity for rapid 

 transposition to the ortho-arsenate, and would 



