Insecticides and Related Substances. 



297 



tions in the color of samples in bulk, especially an abnormally 

 pale shade, and a tendency to dampness or lumping, indicate 

 almost certain adulteration. 



Microscopic examination offers the most certain and satisfac- 

 tory of simple methods for testing the purity of Paris green. 

 The sample is prepared for this test as in the " glass test" just 

 described and the dust is then examined under a medium power 

 objective. The Paris green will be seen in the form of clean 



On the right pure Paris-green; on the left adulterated Paris-green. 



round balls ; and in perfectly pure samples these are all that can 

 be seen. Impure samples will exhibit also a considerable quan- 

 tity of material of crystalline or irregular shapes, and usually 

 white in color. Excess of free arsenious oxide is not so readily 

 distinguished by this test. When mixed with the prepared Paris 

 green it is as easily recognized by the microscope as is any other 

 form of adulterant, but when added in the process of making, 

 it adheres firmly to the particles of true green and causes them 

 to stick together in clusters. 



Chemical analysis is the only absolute means of determining 

 the purity of this insecticide. One of the most important of the 

 chemical determinations, is that for estimating the soluble ar- 

 senic in Paris green and other insecticides. Two procedures are 



