PARIS (SREEN. 113 



The burning effect of a Paris green upon foliage seems to 

 depend upon the amount of water soluble arsenic it carries. The 

 amount of arsenic that goes into solution varies with the time 

 to which it is exposed to the water and to the fineness of the 

 green, and with the care that is used in the manufacture of the 

 green. The Connecticut Experiment Station determined the 

 amount of arsenious oxide soluble in water when the green was 

 treated one day and when treated 10 days and found that there 

 was from 2 to 4 times as much arsenious oxide dissolved in 10 

 days as in one day. Investigations by the Bureau of Chemistry 

 of the U. S. Department of Agriculture* have shown that with 

 the water extraction method, some of the combined arsenious 

 oxide and copper go into solution. This, however, was found 

 to be due to a breaking down of the Paris green and in the case 

 of well made Paris greens there was very much less of the com- 

 bined arsenious oxide that went into solution, and it is evident 

 from their examinations that it is possible to make Paris greens 

 which can be ground very fine and will give a low figure for 

 water soluble arsenious oxide and it is this class of greens that 

 are desirable as insecticides. 



The last column of the table on page no gives the amount of 

 arsenious oxide which the copper present in the samples required. 

 In 3 or 4 instances it will be noted that this amount is larger 

 than the arsenious oxide actually found in the sample, showing 

 that there was an excess of copper oxide present. As a rule, 

 however, it is the arsenious oxide that is present in excess; so 

 much so that in 3 of the samples which were examined under 

 the microscope, a large amount of arsenious oxide was seen 

 present in white crystals. See table on page 114. 



FINENESS OF PARIS GREEN. 



In the case of most insecticides, the smallest possible particle 

 of poison would be a fatal dose to an insect. It is" obvious then 

 that the fineness of the Paris green is very important because 

 not only the finer the green the better it will stay in suspension, 

 but also the finer the green, the greater is the number of particles 

 and hence, theoretically, the larger number of insects that may 

 be killed with a definite weight of green. In some of the early 



* Bui. 68, Bureau of Chem., U. S. Dept. Agr. 



