6 BULLETIN 750, U. S. DEPARTMENT OF AGRICULTURE. 
In every case Series B gave lower soluble arsenic figures than 
Series A. Only when the ratio CaO to As 2 5 is 4 or more does the 
soluble arsenic decrease to about 0.2 per cent. All commercial 
calcium arsenates therefore should be at least as basic as 4CaO.As 2 5 , 
upon which ratio all later experiments were based. The content of 
water of constitution, and that probably of crystallization (about 14 
per cent) , might be reduced by heating to a sufficiently high tempera- 
ture if this proved desirable from a commercial standpoint. 
Probably next in importance to the question of soluble arsenic 
comes the question of lightness of material, that quality which 
governs suspension in water. This is controlled by two things, 
actual density and size of particles. All the calcium arsenates have 
densities around 3, approximately half that of the corresponding 
lead compounds. This is a characteristic property of each compound, 
practically independent of conditions. The size of particles, however, 
is influenced by the manner of mixing, the temperature, the con- 
centration, etc. In the experiments reported in Table 1, the arsenic 
acid was added slowly to the lime in the belief that it was best to 
prevent even the slightest local excess of acid. It was found later, 
however, that rapid mixing gave a product which was much more 
easily powdered. 
Table 1 also shows that temperature has a marked effect, a more 
bulky product being formed when the solutions are mixed in the cold. 
EFFECT OF DILUTION AND TEMPERATURE UPON THE COMPOUND. 
A group of 16 experiments was designed to show the effect of 
dilution and temperature upon " lightness." In all the experiments 
the quantity of lime and acid used was the same — 1 1 grams of lime 
(93 per cent calcium oxid) and 10 grams of arsenic oxid. In every 
case the lime was slaked with 35 cubic centimeters of boiling water. 
Four temperature combinations were used: Both hot; lime hot, 
acid cold; acid hot, lime cold; both cold. Four dilutions were used. 
In every case the acid was dumped as quickly as possible into the 
lime. 
In the more concentrated mixtures, especially when warm, reaction 
took place rapidly, phenolphthalein being reddened immediately. 
But as the dilution increased and the temperature fell, action was 
delayed, the solution remaining acid for perhaps 10 minutes. No 
doubt this is due to the fact that the acid does not penetrate in- 
stantaneously into the small particles of lime in suspension. When 
both solutions are hot the reaction is violent, causing rapid boiling, 
while when both solutions are cold, only a moderate rise in tempera- 
ture occurs. For instance, the temperature rose from 32° C. to 
65° C. in 4 A, and from 32° C. to 40° C. in 4 D. 
