August, ’18] HOLLAND: CALCIUM ARSENITE AND ARSENATE 355 
trade name, its solubility precludes its direct use as a spray but it may 
serve for the preparation of calcium arsenite by mixing with milk of 
lime or Bordeaux as directed by the venders. Its use by inexperienced 
persons cannot be recommended. 
Calcium arsenate is manufactured by a number of firms and some 
experiments have been reported relative to its efficiency. In order to 
secure more definite information on the subject the Department of 
Chemistry undertook the preparation of one or more pure salts of 
calcium and arsenic acid for experimental use by the Department of 
Entomology. By analogy one might assume that three salts of cal¬ 
cium and of arsenic acid were possible,—the tricalcium, the dicalcium 
and the monocalcium arsenates as in the case of the phosphates; or at 
least two salts,—the so-called neutral (tricalcium) and acid (dicalcium) 
arsenates as in case of lead and arsenic acid. The acid lead arsenate 
having proved very satisfactory, attention was directed chiefly to the 
preparation of a similar calcium salt. Precipitation of calcium arsenate 
from soluble salts of calcium and of arsenic promised the most definite 
and uniform product as the resulting compound is not recrystallizable. 
Calcium chloride, neutral and free from other bases forming insoluble 
arsenates, proved a satisfactory source of calcium (preferable to the 
acetate), and disodium arsenate, free from arsenite, carbonate and 
sulfate, a reliable source of arsenic. Previous experience with calcium 
arsenite indicated that the degree of concentration of the solutions 
would be a factor in the preparation of the arsenate; unnecessary dilu¬ 
tion tends to make difficult precipitation with considerable loss of 
salt, and too great concentration an unwieldy precipitate with greater 
occlusion. One-half molecular (M/2) and molecular (M) strength 
solutions appeared feasible considering the solubility and amount of 
active agent in each salt, but after considerable preliminary work 
molecular solutions were found preferable. 
Acid calcium arsenate precipitated in the cold from the chloride 
by an excess (10 per cent) of sodium arsenate generally carries an ex¬ 
cess of calcium probably due to the alkalinity of the sodium arsenate 
and to the failure of the arsenic acid to readily unite with the lime. 
Heat facilitates the interchange of bases, and arsenic in neutral form is 
preferable for the excess. Monosodium arsenate is neutral and can be 
easily prepared from a solution of the disodium by adding the calcu¬ 
lated amount of hydrochloric acid. Under these conditions the fol¬ 
lowing reaction may be said to take place: 
CaCl 2 . n H 2 0 + Na 2 HAs0 4 .7H 2 0-j-NaH 2 As0 4 = CaHAs 04 . H 2 0 + 
2NaCl+n H 2 0 equal volumes of M solutions 10% excess 
The mixed solutions were heated slowly to 95° C. with agitation, al- 
