April, '12] COO LEY: ORTHO ARSENITE OF ZINC 143 



A further understanding of the physical quaUties of this insecticide 

 may be obtained by referring to the accompanying photographs of 

 glass plates, which had been sprayed with arsenate of lead (pi. 2, fig. 

 1), arsenite of zinc (pi. 2, fig. 2) and Paris green (pi. 2, fig. 3). All are 

 magnified 8 diameters. It will be observed that arsenate of lead and 

 arsenite of zinc spread equally well, adhering to the glass as many 

 exceedingly fine particles, while the coarser granules of Paris green 

 have settled to the edges of the droplets and could be easily removed 

 by rain. 



The manufacturers claim that the product contains "over 40 per 

 cent of arsenious oxide," which would be about four-fifths as strong 

 in this form of arsenic as is Paris green. Compared with arsenate of 

 lead, it contains 40 per cent of arsenious oxide against twelve and 

 one-half per cent of arsenic oxide (As2 O5) required in the arsenate. 



An analysis by Professor Burke of the Montana Experiment Station 

 showed the sample to contain 0.67 of one per cent of water-soluble 

 arsenic against 3.5 per cent allowed in Paris green and 0.75 per cent 

 of arsenic oxid (Aso O5) allowed in arsenate of lead. Thus, this insec- 

 ticide is intermediate in strength between our two standard arsenicals, 

 and with respect to the water-soluble forms present is better than 

 either. We have not made analysis to determine the total arsenic 

 present. 



Suspension Qualities 



Experiments conducted by Mr. J. R. Parker and reported in bulletin 

 No. 86 of the Montana Experiment Station (1911), have shown that 

 the addition of soap to water mixtures of arsenate of lead and arsenite 

 of zinc has a marked influence on the rapidity of settling of these com- 

 pounds. You are referred to the original record for the details of the 

 experiments, but we wish here to compare the results obtained. Pre- 

 vious te.sts had shown that the suspension of Paris green is unaffected 

 by the addition of soap to the water. 



The method followed was to thoroughly mix known quantities of 

 the poison in 600 cc. of water in glass cylinders. Two grams of soap 

 in solution were added to one cylinder, while the check was left with- 

 out soap. After being left for definite periods, 500 cc. were siphoned 

 off from the top, the 100 cc. in the bottom removed to an evaporating 

 dish, dried, and weighed. Before any settling began the bottom 100 

 cc. naturally contained one-sixth of all the poison placed in the total 

 600 cc. of water. Thus the difference between this one-sixth and the 

 amount found in the bottom 100 cc. after settling had taken place 

 showed how much had settled out of the 500 cc. at the top. Ten 

 grams, dry. of arsenite of zinc and 20 grams of arsenate of lead in 

 paste form were used in each test. 



