COMPOSITION OF LEAD ARSENATES ON THE MARKET. 11 



DISCUSSION. 



Some of the samples examined had dried out considerably before 

 they were received, as was evident from their mechanical condition, 

 weight of package, etc. In such cases the per cents given on the origi- 

 nal sample are based on the goods as received and will not represent 

 the correct composition of the material as placed on the market, on 

 account of this decrease in the moisture content, making the per cent 

 of the other constituents as given higher than they were originally. 



In making lead arsenate from lead acetate and disodium arsenate 

 a certain amount of acetic acid is formed. This had not been com- 

 pletely washed out in all cases, as was shown by the fact that several 

 of the samples had a strong odor of acetic acid. No quantitative 

 determination was made of the amount, but as it would be driven 

 off at the temperature of drying, the term "moisture" not only 

 includes water, but any other material volatile at from 105 to 110 C. 



One of the samples examined was lead arsenite and another was 

 a mixture of the arsenate and arsenite in about equal proportions. 

 Several others contained small amounts of arsenic as arsenite, but 

 usually only traces were present. In such cases the water-soluble 

 arsenic reported as arsenic oxid (As 2 O 5 ) contained some arsenious 

 oxid. As soluble arsenic is injurious in either form, the two have 

 not been determined separately, except in the cases noted, where it 

 was present entirely as arsenite. 



On inspecting the analyses given in Table I, the first striking fact 

 that will be observed is the great variation in the composition of the 

 different samples. The content of arsenic oxid ranges from 6.03 to 

 43.81 per cent (the latter as As 2 O 3 ); lead oxid varies from 23.06 to 

 72.57 per cent; moisture from 0.41 to 61.84 per cent; water-soluble 

 arsenic from 0.02 to 5.45 per cent (As 2 O 3 ) ; and water-soluble impur- 

 ities from 0.08 to 7.54 per cent. 



In order to secure a more uniform basis for comparison all of the 

 determinations have been calculated to moisture-free material. A 

 much greater uniformity is shown when this is done, but there is 

 still a considerable variation. Arsenic oxid ranges from 10.30 to 

 44.70 per cent (the latter As 2 O 3 ); lead oxid from 49.58 to 77.93 per 

 cent; water-soluble arsenic oxid from 0.03 to 5.56 per cent (As 2 O 3 ); 

 and water-soluble impurities from 0.14 to 13.86 per cent. 



Evidently in some cases the salts formed as by-products in the 

 manufacture have not been washed out, or at most the material 

 has simply been run through the filter press to remove the super- 

 fluous liquid, as is shown by the high per cent of water-soluble mate- 

 rial in a number of samples. 



Lead arsenate is recommended for spraying purposes mixed with 

 water in various proportions. A standard formula and one fre- 

 quently recommended is 2 pounds to 50 gallons. It is easy to see 



