HOME-MADE LEAD AKSENATE. 23 



facts into consideration, it would appear from our knowledge at the 

 present time that the product prepared from lead nitrate is slightly 

 more desirable. 



PHYSICAL PROPERTIES OF LEAD ARSENATE. 



The physical properties or characteristics of all insecticides which 

 are to be applied as a spray are very important. Freshly precipitated 

 lead arsenate is a white, very light, flocculent compound, and it is 

 hard to conceive of an insecticide possessing more desirable physical 

 properties. When sprayed on foliage it forms a thin film over the 

 leaf, and after once having been dried thereon it is with difficulty 

 washed off by ordinary rains, and therefore need not be applied so fre- 

 quently as some other insecticides. This is quite an important con- 

 sideration, particularly as the greatest expense connected with spray- 

 ing is the cost of applying the mixture. 



Another important point is the ease with which it may be kept in 

 suspension in water. Such materials as Paris green, Scheele's green, 

 and others which have a high specific gravity are with difficulty kept 

 in suspension during spraying, and there is always great danger from 

 the material becoming too concentrated in the bottom of the spray 

 tank, thus causing too strong an application and resulting in the 

 scorching of the foliage. Paris green is particularly objectionable in 

 this regard, as it settles very rapidly unless thoroughly and constantly 

 agitated. Lead arsenate shows considerable variation in the time of 

 settling, depending upon the w r ay in which it has been treated and also 

 the chemicals from which it has been made. If it has once been dried, 

 on mixing with water again it settles out much more readily than if it 

 has never been dried. It is for this reason that is is generally put on 

 the market in the form of a paste. There is also a difference between 

 that prepared from lead nitrate and that prepared from lead acetate. 

 The former is more bulky and remains in suspension much longer. 

 After drying there is very little difference in rapidity of settling 

 between the products made from the different lead salts. Plate I 

 shows graphically the variation in settling observed among prepara- 

 tions of lead arsenate which have received different treatments. As 

 stated in the legend, tube a is lead arsenate prepared from sodium 

 arsenate and lead acetate ; in tube b lead nitrate was used instead of 

 the acetate ; tubes c and d are the same as tubes a and 6, respectively, 

 except that they have been dried out and then mixed with water 

 again. All of the samples represent the same amount of actual lead 

 arsenate and the column of water in each case is 12 inches high. All 

 were thoroughly shaken and then photographed, fig. 1 after they had 

 stood two minutes, and fig. 2 after they had stood fifty minutes. It 

 will be noticed that after two minutes tube b had settled but very 

 little, tube a about one-third of the way down, tube c nearly to the 



