66 THE GYPSY MOTH APPENDIX. 



Freshly prepared arsenate of lead consists of very finely 

 divided particles, slightly aggregated into a flocculent, curdy 

 white precipitate, having a specific gravity of about 1.00668, 

 and remaining in suspension for a remarkably long time. 

 The flocculent character of the precipitate facilitates its uni- 

 form application to the foliage, and when applied as a fine 

 spray it soon dries to a tenacious and permanent film. In 

 drying in mass this salt becomes compact and increases in 

 specific gravity. Analyses of two air-dried samples of ar- 

 senate of lead, prepared respectively from the acetate and 

 nitrate of lead, gave the following results : 



TaUc No. 5. 



In the preceding table sample No. 1 was prepared from 

 "white granulated acetate of lead" and "50 per cent, 

 arsenate of soda ; " No. 2 was prepared from commercial lead 

 nitrate and " 65 per cent, arsenate of soda." 



Where arsenate of lead is to be made on a large scale, 

 after determining the relative amounts of the salts to be 

 used it is advisable to test the formula by preparing a trial 

 quantity, and examining the supernatant liquid for soluble 

 lead and arsenic. For this purpose it is necessary to filter 

 a portion of the supernatant liquid. If an excess of lead 

 is present, by adding a few drops of potassium neutral or 

 bi-chromate solution a beautiful chrome yellow precipitate is 

 formed. A simple test for soluble arsenic acid may be made 

 by adding to a portion of the filtered solution a few drops 



