4 BULLETIN 750, U. S. DEPARTMENT OF AGRICULTURE. 
possible probably would be preferable. On first thought, such a pro- 
cess would be the direct slaking of the stone lime with the requisite 
strength arsenic acid solution. But such a procedure is not prac- 
ticable. Owing to the slow surface action, the As 2 3 is locally 
always in excess, resulting in the formation of dicalcium or even 
monocalcium arsenate (if the H 3 As0 4 is strong enough), which, 
while it may be changed later by the excess of lime, renders the 
product lumpy or granular. Even the use of strong arsenic acid 
with a concentrated lime paste suffers from the same drawbacks — 
the addition of the acid causes a further thickening of the paste, 
and thorough and rapid mixing becomes impossible. 
SLAKING THE LIME. 
Only more or less dilute lime paste, made by the slaking of quick- 
lime, 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 determine the best procedure to follow in slaking 
so that the paste would be smooth and free from lumps were made 
first. It was soon found that the use of 3 to 3y 2 times as much 
water (by weight) as lime gave a paste which was entirely satisfac- 
tory, if the lime employed was sufficiently pure. This produces a 
paste rather thick, but not thick enough to interfere with good mix- 
ing when working with small quantities. Best results were ob- 
tained by using boiling water for slaking, although in bulk the 
natural heat of reaction would be sufficient. 
PROPER PROPORTION OF ARSENIC OXID AND LIME. 
As previously noted, it is essential that a compound for spraying 
purposes shall be free, or nearly so, from arsenic in water-soluble 
forms. As seen by reference to the description of practically pure 
crystalline tricalcium arsenate, that material is appreciably acted 
upon by water. Other work done in the Bureau of Chemistry on 
this subject has shown that all of the calcium arsenates more acid 
than this are likewise somewhat soluble (or decomposable), the 
action increasing until the monoarsenate, which is very readily 
soluble in water, is reached. Evidently, then, it is necessary to 
look toward the basic side. It has long been known that the addi- 
tion of an excess of lime to a spraying compound renders the arsenic 
insoluble. The object here was to find the right proportions to 
produce a material combining high arsenic oxid content with suf- 
ficiently low water-soluble As 2 5 to prevent burning or reduce it to 
the lowest possible limit. Two series of experiments were made 
