COMMERCIAL CALCIUM ARSENATE. 3 
gration of the lumps. One preparation, dried at about 70° C. on the 
steam bath, had the following composition: 
Per cent. 
Calcium oxid (CaO) 32.00 
Arsenic oxid ( As 2 5 ) 42. 96 
Water (by difference) (small quantity of C0 2 ) . 25. 04 
100. 00 
The molecular ratio of CaO to As 2 5 is here 3.05 to 1, showing the 
compound to be tricalcium arsenate. Its formula approximates most 
closely to Ca 3 (As0 4 ) 2 .8H 2 0, a compound which has been shown to 
exist by the Department of Agriculture. Drying at 110° C. results 
in the loss of about seven of the eight molecules of water of crystalliza- 
tion. The lightness of this material may be judged from the fact that 
2 ounces of it filled a graduated cylinder to the 500 cubic centimeter 
mark, which is equivalent to 240 cubic inches per pound, this, of 
course, without any attempt at packing. If jarred for a sufficient 
time the space occupied will be reduced to about 60 per cent of this 
figure. 
The compound thus formed yields appreciable quantities of soluble 
arsenic when treated with water. In one experiment 1.8 grams was 
treated with 1 liter of water (equivalent in arsenic content to 1 
pound of dilead arsenate in 50 gallons) for 24 hours, and 55 milli- 
grams, or 3.04 per cent, of arsenic oxid was rendered soluble. R,. H. 
Robinson 5 found 79 milligrams of arsenic oxid per liter for somewhat 
similar material. This, of course, is excessive, and would make the 
material unfit for use alone on any but the hardiest foliage, such as 
will stand applications of Paris green. 
If the manufacture of calcium arsenate by this process were at- 
tempted commercially, the liquid left from the settling of one batch 
of arsenate could be used again for the preparation of more lime- 
water, and thus the consumption of water might be greatly reduced. 
However, methods requiring the handling of as little material as 
possible probably would be preferable. On first thought, such a 
process would be the direct slaking of the stone lime with an arsenic 
acid solution of the requisite strength. But such a procedure is not 
practicable. Owing to the slow surface action, the arsenic oxid is 
locally always in excess, resulting in the formation of dicalcium or 
even monocalcium arsenate (if the arsenic acid 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. 
5 Oregon Agr. Exp. Sta. Bui. 131, p. 7. 
