August, '17] GRAY: ARSENATES AND STONE FRUITS 387 



previous to his work, that it is rapidly supplanting all other forms of 

 arsenicals for use on foliage. This arsenical may be prepared by 

 mixing in proper proportions a soluble salt of lead, usually lead nitrate 

 or lead acetate, and a soluble salt of arsenic acid, usually sodium 

 arsenate. The process has been perfected and cheapened by com- 

 mercial manufacturers so that the use of these soluble salts, as raw 

 materials, has been largely discontinued. At present, many of the 

 manufacturers prepare commercial lead arsenate paste from lead 

 oxide (litharge) and arsenic acid. This process greatly cheapens and 

 simplifies the manufacture of the paste in ways which need not be 

 discussed in this paper. As ordinarily made, there results a compound 

 which may be spoken of in chemical language as an acid lead arsenate 

 or possibly a mixture of this and neutral or basic lead arsenate. It was 

 found that lead arsenate produced in this way gave very uniform and 

 satisfactory results in most cases for the control of leaf -eating insects. 



Basic Lead Arsenate (usually Labeled "Tri-Plumbic" or 

 "Neutral"). — The lead arsenate produced in the usual way, however, 

 was found to produce very serious foliage injury under certain cKmatic 

 conditions which prevail in the Pajaro Valley, the principal apple 

 growing section of California. In the spring of 1903, field and labora- 

 tory work was commenced by the Entomological Division of the 

 University of California to find a more suitable arsenical or to modify 

 the known methods of preparation of lead arsenate so that it could be 

 used without injury under the conditions prevailing in the valley.^ 

 The field work was conducted by Mr. W. H. Volck and IMr. E. E. 

 Luther, students in the College of Agriculture. After numerous 

 experiments, the process of preparation was so modified that a new 

 tj^pe of lead arsenate was produced which could be used with safety 

 upon the apple trees in that section. The modified process and some 

 theories of foliage injury are discussed in the publication previously 

 referred to. This new process lead arsenate was at first believed to 

 be what may be termed a neutral lead arsenate. Our knowledge 

 of the chemistry of lead arsenates is still very imperfect, but later 

 investigations seem to indicate that the material produced in the 

 manner described by Volck- may more properly be referred to as basic 

 lead arsenate. 



Comparison or the Two Types. — -To take up a full discussion of 

 the composition of the two types mentioned would be of too technical 

 a nature for presentation in a paper of this kind. The acid type is 

 very susceptible to the action of other chemicals and is more or less 



1 Volck, W. H., Science, N. S., vol. XXXIII, No. 857, pp. 866-870 (1911). 

 Wp. cit. 



