22 BULLETIN 278, TJ. S. DEPARTMENT OF AGRICULTURE. 
Experiment XIII. 
ARSENATE OF CALCIUM VERSUS ARSENATE OF LEAD, ALONE AND COMBINED WITH KERO- 
SENE EMULSION AND WITH LIME-SULPHUR SOLUTION AGAINST LARVAE OF THE FALL 
WEBWORM. 
In this experiment a comparative test was made with homemade 
arsenate of calcium at several strengths, commercial arsenate of cal- 
cium, paste and powder, and other materials. 
The homemade arsenate of calcium used in this and succeeding 
experiments was prepared by adding sodium-arsenate crystals to 
stone lime during the course of slaking. 
Formula 1. — Stone lime, 1 pound; sodium arsenate, \ pound; water, \\ quarts. 
This formula, according to chemical analysis, gave a total arsenic oxid (Aso0 5 ) content 
of 4.19 per cent; no soluble arsenic oxid. 
Formula 2. — Stone lime, 3 pounds; sodium arsenate, 3 pounds; water, 4 quarts; 
analysis — total arsenic oxid (As20 5 ), 6.16 per cent, no soluble arsenic oxid. 
Formula 3. — Stone lime, 4 pounds; sodium arsenate, 2 pounds; water, 4 quarts; 
analysis — total arsenic oxid (As20 5 ), 3.93 per cent; no soluble arsenic oxid. 
Formula 4. — Stone lime, 4 pounds; sodium arsenate, 1 pound; water, 5 quarts; 
analysis — total arsenic oxid (As 2 5 ), 1.88 per cent; no soluble arsenic oxid. 
Formula 5. — Stone lime, 3 pounds; sodium arsenate, 1 pound; water, 3 quarts; 
analysis — total arsenic oxid (AS2O5), 2.92 per cent; no soluble arsenic oxid. 
Formula 6. — Stone lime, 4 pounds; sodium arsenate, 2 pounds; water, 4 quarts; 
slaking not vigorous; not analyzed. 
In all of the homemade formulas lime has been used in considerable 
excess, with a corresponding decrease of the arsenical content. The 
commercial arsenate of calcium, paste, showed an analysis of 18.82 
per cent total arsenic oxid — soluble arsenic oxid, a trace. 
With a view to making a combination spray for peaches and other 
stone fruits, arsenate of calcium was prepared in the same operation 
with the making of self -boiled lime-sulphur (8-8-50 formula). As 
soon as the lime started to slake, 2 pounds of sodium-arsenate crys- 
tals and then the sulphur were added and the mixture made up in 
the usual way. Arsenate of calcium made with self-boiled lime- 
sulphur may be of value as a spray for stone fruits, owing to the fact 
that the large excess of lime would tend to prevent burning. Arse- 
nate of calcium, alone, causes injury to peach foliage, unless there is an 
excess of lime. The calcium on exposure to the atmosphere gradually 
combines with the carbon dioxid of the air and becomes calcium car- 
bonate, thus releasing some soluble arsenic. For the results of this 
experiment, see Table XIII. 
Arsenate of lead alone killed all larvae in 8 days; combined with 
kerosene emulsion, 10 per cent, in 6 days (see Experiment XI) ; with 
lime-sulphur in 6 days, and with self-boiled lime-sulphur in 8 days. 
Commercial arsenate of calcium, powder, required 8 days to kill. 
Commercial arsenate of calcium, paste, required 27 days, but when 
combined with lime-sulphur solution, killed in 10 days. 
