4 BULLETIX 750, U. S. DEPARTMENT OF AGRICULTURE. 
SLAKING THE LIME. 
Only dilute lime paste, made by slaking quicklime, was used in the 
experiments here reported. It is self-evident that the ease with 
which such a paste will combine with acid is directly dependent on 
the smoothness of the paste. Consequently, experiments to deter- 
mine the best procedure to follow in slaking, so that the paste would 
be smooth and free from lumps, were made first. 
The use of 3 to 3J times as much water (by weight) as lime gave a 
paste which was entirely satisfactory, if the lime employed was suffi- 
ciently pure. This produced a paste rather thick, but not thick 
enough to interfere with good mixing when working with small quan- 
tities. Best results were obtained by using boiling water for slaking, 
although in bulk the natural heat of reaction would be sufficient. 
PROPORTIONS OF ARSENIC OXID AND LIME. 
For spraying purposes a compound must be free, or nearly so, 
from arsenic in water-soluble forms. Practically pure crystalline 
tricalcium arsenate is appreciably affected by water. All of the cal- 
cium arsenates more acid than this are likewise somewhat soluble 
(or decomposable), the action increasing until the monoarsenate, 
which is readily soluble in water, is reached. Evidently, then, it is 
necessary to look toward the basic side. 
It has long been known that the addition of an excess of lime to a 
spraying compound makes the arsenic insoluble. The object here 
was to find the right proportions to produce a material combining 
high arsenic oxid content with sufficiently low water-soluble arsenic 
oxid to prevent burning or reduce it to the lowest possible limit. 
Two series of experiments were made along this line. Each series 
included eight experiments, the molecular ratio, CaO to As 2 5 , being 
varied from 3 to 5 by appropriate steps. The quantity of arsenic 
oxid used was kept constant, being 25 cubic centimeters of a solution 
containing 0.4 gram of arsenic oxid per cubic centimeter (in other 
words, a total of 10 grams of arsenic). The weighed quantities of 
lime were slaked in beakers with three times their weight of boiling 
water, and the cold arsenic solution was added slowly, from a pipette, 
with constant stirring. In one series (^L) the acid was added to the 
lime while still hot from slaking; in the other series (B) the lime paste 
was permitted to cool to room temperature before mixing. Each 
mixture was then divided into two portions. One portion was per- 
mitted to stand over night and was then decanted. The other was 
filtered and sucked as dry as possible on a Buchner funnel. Water was 
determined in portions of each paste by drying in the oven at 105° C. 
The dried portions were used for chemical analysis. The water- 
soluble arsenic was determined on the dry material, using 1.2 grams 
in 500 cubic centimeters (equivalent to 1 pound in 50 gallons) . The 
results of these experiments are summarized in Table 1. 
