1912.] PUBLIC DOCUMENT — No. 31. 207 



There is evidently a difference in stability between acid and 

 neutral lead arsenates as measured by boiling ammoniacal solu- 

 tions, but, contrary to general belief, it is apparently only a 

 matter of degree. Both salts are decomposable, yielding soluble 

 arsenic acid. 



Neutral arsenate, sample 31, page 201, after being twice 

 heated with ammonia and washed, gave a residue which was 

 practically stable and tested as follows : — 



Per Cent. 



Water, 44 



Arsenic pentoxide, ......... 23.84 



Lead oxide, 75.02 



Occluded water, . .64 



99.94 



Stability was apparently the result of a reversible reaction, 

 ammonia setting arsenic acid free, and lead hydroxide, when 

 present in sufficient excess (10 per cent.), completely reprecip- 

 itating it. Similar results were obtained by adding freshly 

 precipitated lead hydroxide, litharge and lime to neutral arse- 

 nate, the excess base preventing the separation of arsenic acid. 



If properly made, neutral and acid arsenates of lead are 

 smooth, white pastes of very fine particles, low specific gravity, 

 excellent suspension and exceptional adhesiveness. The power 

 of suspension is injured by drying. The readings reported 

 for sample C^ are not the maximum, but were taken when no 

 movement of particles was perceptible, although the mixture 

 continued milky for a considerable period thereafter. 



Both acid and neutral lead arsenates are slow-acting poisons 

 of low arsenic content, and that in the form of pentoxide. They 

 are practically insoluble in water and fairly stable. The fine- 

 ness of the particles and low specific gravity insure a high 

 power of suspension and uniform distribution. The white 

 mixture readily indicates the leaf surface covered and dries to 

 a film which adheres with great persistence. 



