Apr. 29, i9i8 The Calcium Arsenates 293 



cient base to combine with the quantity of arsenic pentoxid present, and 

 further examination showed the latter was there as actual arsenic pen- 

 toxid uncombined. It is probable that the manufacturer attempted to 

 prepare the calcium arsenate from arsenic pentoxid and an impure lime 

 which was chiefly calcium carbonate. The latter would react very 

 slowly, if at all, and the excess arsenic pentoxid over the amount required 

 to combine with the calcium hydroxid would remain unchanged as noted 

 above. If the manufacturer would exercise a more complete control of 

 conditions and know definitel}- what materials are being used, a spray 

 material sufficiently pure for practical use could probably be obtained. 



Thus far our investigation has showTi the possibility of the calcium 

 arsenates' being a practical, economical, and evidently satisfactory sub- 

 stitute for the more expensive lead arsenate. The pure salts contain 

 57.8 and 63.9 per cent of arsenic calculated as arsenic pentoxid for the 

 tricalcium arsenate and calcium hydrogen arsenate, respectively, while 

 the hydrogen and basic lead arsenate contains only 33.1 and 25.5 per 

 cent of arsenic pentoxid, respectively; in other words, the calcium arse- 

 nates contain more than twice as much arsenic, the active killing agent 

 of the insecticide. For practical spraying purposes, therefore, only one- 

 half of the quantity of calcium arsenate should be used, compared with 

 lead arsenate. This point must, however, be more definitely determined 

 by field experiments, since there is a bare possibility that the lead has 

 toxic values that the calcium does not have. Relative toxic values and 

 killing efficiency has been recently shown in laboratory experiments with 

 the "common tent caterpillar" by Lovett and Robinson* and with the 

 "fall webworm" by Scott and Siegler,^ indicating equal killing efficiency 

 of calcium arsenate compared with lead arsenate. 



Field spraying experiments have been carried on by several investi- 

 gators with more or less successful results. This may be due to the use 

 of an unreliable calcium arsenate similar to the one cited above. Sanders,^ 

 however, reports very favorable results during the past two seasons. 

 Likewise, Scott and Siegler * obtained encouraging results.^ 



In cooperation with Prof. A. L. Lovett, Entomologist at this Station, 

 preliminary field experiments were tried with the pure salts prepared in 

 the laboratory. Favorable results, especially with tricalcium arsenate 

 plus calcium oxid were obtained, but owing to insufficient time and late- 



1 Lovett, A. L., and Robinson, R. H. toxic values and killing efficiency of the arsenates. 

 In Jour. Agr. Research, v. lo, no. 4, p. 199-207. 1917. 



* Scott, E. W., and Siegler, E. H. miscell.vneous insectiode investigations. U. S. Dept. Agr. 



Bui. 278, 47 p. I9IS- 



' Sanders, G. E. arsenate of le.\d vs. arsenate of lime. In Proc. Ent. Soc. Nova Scotia, igib, 

 p. 40-45. 1917. 



* Scott, E. W., and Siegler, E. H. Op. cit. 



^ Scott and Siegler gave the analysis of the calcium arsenate used as showing only 0.04 per cent of soluble 

 arsenic oxid. A possible explanation is that the spray material was prepared from stone lime (containing 

 80 per cent of calcium oxid), sodium arsenate, and water. A very slight excess of the stone lime would 

 prevent any arsenic from becoming soluble, as shown in our experiments reported above. 



