CALCIUM ARSENATE. 9 



The results show equal bulkiness for 2 B, 4 B, and 1 C, 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 tioo more volumes of hot 

 water, and then add the cold arsenic acid solution as rapidly as 

 possible (method 1 C). Methods 1 B, 2 B, and 1 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 

 the commercial dilead 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 conditions. 



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,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, CaHAsCV,.H 2 Q, which on continued boiling change to 

 CaHAsO,. In the presence of excess CaC0 3 , reaction continues 

 until practically all the As 2 5 has been precipitated as CaHAs0 4 . 

 Tli is, 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: 



