,2o5 THE CONNECTICUT POM O LOGICAL SOCIETY. 



These results, together with our other field observa- 

 tions, indicate that the second method of eliminating spray 

 injury — the one involving a stable arsenical of the highest 

 grade — is probably best. 



On present evidence, this arsenical is the neutral ortho- 

 arsenate of lead. As shown in plot 6 and column 3 of Table 

 II, this material in combination with lime-sulphur solution 

 gave practically no injury on peaches, when the ordinary 

 lead arsenate and arsenite of lime under similar conditions 

 were giving injuries of 12% and 18% respectively, besides 

 causing very distinct amounts of fruit and foliage drop. 

 The work of the single season, of course, does not warrant 

 one's advising even this combination as yet on peaches, 

 but it does indicate very strongly that much of the serious 

 injury on apples may be eliminated by the use of ortho- 

 arsenate. 



Chemically, the ortho-arsenate of lead differs from the 

 ordinary commercial lead arsenates in the fact that it con- 

 tains nothing but the tri-plumbic arsenate, Pb3(As04)2, 

 while the latter, as made in the East up to the present time, 

 generally contain in addition, more or less of the pyro and 

 acid arsenates, Pb2As207 and PbHAs04, respectively.*. 



The importance of this lies in the fact that these lat- 

 ter arsenates are only stable under acid conditions and they 

 are gradually transposed into the stable compound, the 

 ortho-arsenate, under neutral and alkalin conditions, such 

 as occur in most of our spray solutions. This transposi- 

 tion involves the liberation of arsenic acid, which must 

 cause injury, unless enough extra lead or other satisfac- 

 tory bases are present to re-precipitate it into forms that 

 are insoluble under the conditions involved. 



The above fact doubtless explains why more injury 

 does not occur from the sulphur-arsenical combination, 

 and it also suggests the third method of guarding against 

 it. This is to add extra lead, in a soluble form, to the 



* See article by Volck in Science, June 2, 1911, pp. S66-S70. Besides these 

 forms, some water soluble arsenic usually occurs. 



