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The acid salt has a general resemblance to wheat flour ; both salts are 

 amorphous under ordinary conditions, and their ultimate particles, 

 under the usual conditions of preparation, are about the same size, 

 but when suspended in water the particles of the basic salt have a 

 tendency to collect together into larger particles and thus to settle 

 more rapidly, and it is possibly this property which prevents it 

 from spreadmg so evenly over foliage as does the acid salt. 

 The fruit-grower, in handling materials of this kind, requires 

 fine subdivision and low gravity, so that they will remain a 

 long time in suspension. The specific gravity of both was 

 very carefully determined by the authors, and they found that 

 of the acid salt to be 5*786 and that of the basic 7105. Suspension 

 and setthng experiments were made with both and a photograph is 

 reproduced showing the amount of settlement in 2 minutes and 

 1 hour respectively ; in the former case the basic salt had practically 

 all settled and the acid salt only slightly, in the latter the basic salt 

 had completely settled, leaving the supernatant hquid quite clear, 

 while the acid salt still showed particles in suspension. The question 

 of solubility is of great importance to users, as a very small amount 

 of soluble arsenic compomids is sufficient to cause injury to foliage. 

 It has also been fully demonstrated that insoluble arsenical compounds 

 are effective as stomach poisons, the arsenic being probably set 

 free by the solvent action of the juices of the digestive tract of the 

 insect. The tendency of manufacturers is to supply an insoluble 

 form, and this has been stimulated by the passing of State and national 

 laws restricting the amount of soluble arsenic permissible in commercial 

 lead arsenates. A difficult chemical question is involved in determining 

 the solubility of these salts, as water under certain conditions reacts 

 chemically with them, producing other soluble bodies. The authors 

 feel warranted in asserting that, with the pure acid salt, this hydrolytic 

 action is so small as to be negligible, and the same appears to be true 

 of the pure basic salt. The commercial article is however not pure ; 

 the nature of the impurities varies, and with it the possibihty of 

 reactions occurring, which will set free soluble arsenic. The result of 

 spraying experiments with each of the arsenates (presumably laboratory 

 made and pure, though this is not definitely stated) at strengths of 

 2, 4 and 8 lb. in 100 U.S. gallons (83 Imperial) of water is given and 

 in no case did scorching occur. Lead hydrogen arsenate, like most 

 acid salts, reacts readily with many substances, and sodium chloride, 

 carbonate and sulphate were found to liberate arsenic from it in 

 considerable quantity ; alkalies gave the same result ; the basic salt 

 is relatively inert, but nevertheless reacts to some extent with the 

 salts above mentioned. Experiments were made with the two arsenates 

 at the usual spray strengths on tent caterpillars [Malacosoma spp.] 

 and controls were sprayed with suspensions which contained equal 

 amounts of arsenic in the form of the different arsenates. The acid 

 arsenate was found to act more quickly than the basic, though the 

 latter finally killed the caterpillars. One result of these experiments, 

 which the authors intend to repeat, seemed to be that in ordinary 

 spraying practice the arsenates of lead are perhaps used in greater 

 quantity than is necessary. The chemistry of the assaying of 

 lead arsenate by various methods is discussed, and the results show 

 that the conditions laid down in the Federal Insecticide Law are, in 



