U. S. DEPARTMENT OF AGRICULTURE. 
material, which at the same time is easily pulverized, is to slake the lime, 
add two more volumes of hot water, and then add the cold arsenic acid solu- 
tion as rapidly as possible (method 1 C) . Methods 4 B,2 B, and 4 A 
also, however, give a product which is reasonably light and easily 
pulverized. 
None of these products, except the tricalcium arsenate mentioned 
on page 3, is as bulky as some of the commercial dilead arsenates 
which run as high as 140 cubic inches per pound when measured in 
the manner prescribed here. This calcium arsenate, however, yields 
an excessive quantity of water-soluble arsenic, and is prepared by a 
method that is hardly practicable under manufacturing conditions. 
EXPERIMENTS WITH LIMESTONE. 
It was at first thought that calcium arsenate might be made directly 
from limestone, instead of from lime, thus saving the expense of 
burning. A few experiments showed that calcium carbonate reacts 
readily with arsenic acid at room temperature, and continues to do 
so until the solution is about neutral to methyl orange. At this point 
the solution contains calcium oxid and arsenic oxid in the proportions 
to form calcium monoarsenate (CaH 4 (AsOJ 2 ) . If this solution is 
heated by itself, it deposits crystals of dicalcium arsenate (CaHAs 
4 .H 2 0) , which on continued boiling lose their water of crystallization 
(CaHAs0 4 ). In the presence of excess calcium carbonate, reaction 
continues until practically all the arsenic oxid has been precipi- 
tated as CaHAs0 4 . This, however, takes place readily only if the 
carbonate is in a fine state of division, and hardly at all if it is in pea- 
sized lumps. Calcium carbonate is not alkaline enough to carry the 
conversion to the tricalcium arsenate stage. This dicalcium arsenate 
is fairly crystalline and granular, and, owing to its solubility and other 
physical properties, is not well suited for use as a spray. It could, 
of course, be mixed with excess lime, but, because of its granular 
nature, reaction would be slow, and when sprayed the lime might be 
completely carbonated before some of the crystals were changed, thus 
giving a chance for injury by the soluble arsenic. All things con- 
sidered, it appears that there is no advantage in attempting to sub- 
stitute limestone for lime. 
SUMMARY. 
The most desirable procedure for making calcium arsenate from 
lime and arsenic is as follows : 
1. Use a good grade of lime, containing a high percentage of cal- 
cium oxid. 
2. Slake the lime to as smooth a paste as possible, for upon this 
depends the smoothness of the final product, as well as the readi- 
ness with which the lime and acid react. Use from 3 to 3£ times as 
