CALCIUM ARSENATE. 9 
The results show equal bulkiness for 2 B, 4 B, and 1 0, and all of 
these, especially 1 C, were easily powdered. Based on this ivork, the 
authors are of the opinion that the most advantageous method to 
produce a reasonably light 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 solution 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 men- 
tioned in the beginning of this article, is quite as bulky as some of 
1he commercial clileacl arsenates, some of which run as high as 140 
cubic inches per pound when measured in the manner prescribed 
here. However, this calcium arsenate, as aforesaid, yields an ex- 
cessive amount of water-soluble arsenic, and is prepared by a 
method that is hardly practicable under manufacturing condition?. 
EXPERIMENTS WITH LIMESTONE. 
It was at first thought that calcium arsenate might be made di- 
rectly from limestone, instead of from lime, thus saving the expense 
of burning. A few experiments along this line showed that calcium 
carbonate reacts quite readily with arsenic acid at room tempera- 
ture, and continues to do so until the solution is about neutral to 
methyl orange. At this point, the solution contains CaO and As 2 0- 
in the proportions to form calcium monoarsenate, CaH 4 (As0 4 ) 2 . If 
this solution is heated by itself, it deposits crystals of dicalcium 
arsenate, CaHAs0 4 .H 2 0, which on continued boiling change to 
CaHAs0 4 . In the presence of excess CaC0 3 , reaction continues 
until practically all the As 2 5 has been precipitated 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 tri- 
calcium arsenate stage. This dicalcium arsenate is fairly crystal- 
line 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, owing to its granular nature, re- 
action would be slow and when sprayed the lime might be com- 
pletely 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 acid may be summarized as follows : 
