CALCIUM ARSENATE. 7 



It will be noted that in every case series B gave lower soluble 

 arsenic figures than series A, and also that only when the ratio 

 equals 4 or more does the soluble arsenic decrease to about 0.2 per 

 cent. Consequently, all commercial calcium arsenates 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 probably 

 crystallization (about 14 per cent), might be reduced by heating to a 

 sufficiently high temperature if this were found to be desirable from 

 a commercial standpoint. Probably next in importance to the ques- 

 tion of soluble arsenic comes the question of lightness of material, 

 that quality which governs the suspension in water. This is con- 

 trolled 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 mix- 

 ing, the temperature, the concentration, etc. In the experiments 

 reported in Table 1, the arsenic acid was added quite 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 mix- 

 ing 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 this quality of lightness. In all 

 the experiments the quantity of lime and acid used was the same — 

 11 grams of lime (93 per cent CaO) and 10 grams of As 2 5 . In 

 every case the lime was slaked with 35 cubic centimeters of boiling 

 Avater. 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 fast as possible 

 into the lime. In the more concentrated mixtures, especially when 

 warm, reaction took place rapidly, phenolphthalein being reddened 

 immediately. But as dilution increased and the temperature fell, 

 action was delayed, the solution remaining acid for perhaps 10 

 minutes. This is no doubt due to the fact that the acid does not 

 penetrate instantaneously into the small particles of lime in sus- 

 pension. When both solutions are hot the reaction is violent, caus- 

 ing rapid boiling, while when both solutions are cold, only a mod- 



