cost of the green and should be classed as a mere "make weight." If 

 care is used in the manufacture, there is no reason for it being present in 

 more than very small amounts, say one-half of one per cent. 



Free or Soluble Arsenious Oxide in Paris Green. It is on account 

 of the presence of this substance in Paris green that we sometimes find 

 that after spraying, the leaves of the plant treated turn black, having the 

 appearance of being burnt, or even, in more extreme cases, drop off 

 altogether, leaving the plant defoliated. This, of course, is very objec- 

 tionable, since the physiological functions of the plant are thus severely 

 checked, a case where the cure is as bad, or worse than, the evil. 



To account for the occurrence of this scorching, J. K. Haywood, of 

 Washington, D.C. , states three causes.* 



(i) There may be a certain amount of arsenious oxide over and above 

 that combined with the other constituents. This is "free" arsenious 

 oxide and until recently it has been considered the only cause of the 

 scorching of the foliage by Paris green. 



(2) The greens may be poorly made, so that the constituents are very 

 loosely held together. When such greens are brought in contact with 

 water, especially water containing carbon dioxide, they soon break up 

 and arsenious oxide is set free. Between the water of the spray, and the 

 action of dew and rain, enough oxide may be liberated to severely scorch 

 the foliage. 



(3) The green may be extremely fine. The best greens when ground 

 to a fine powder and applied to foliage will scorch. This is doubtless due 

 10 the fact that more surface is exposed to the action of water which, 

 containing carbon dioxide, would soon set enough arsenious oxide free 

 to cause serious damage. 



Following up these statements, however, Mr. Haywood says: "It 

 is a very common occurrence to secure a commercial Paris green that 

 scalds because of one of the first two causes, but the writer has never 

 found a commercial sample of green that scorched because it was in too 

 fine a condition." 



As to the breaking up of Paris green when in contact with water, 

 with the liberation of free arsenious oxide, Colby, of California,! ex- 

 presses some doubt, since, as he says, "aceto-arsenite of copper, as 

 manufactured to-day, is instantaneously precipitated from complex solu- 

 tions containing alkali and often excessive quantities of various acids." 

 However, as this may be, we do know that Paris green often destroys 

 foliage and that it is due to free arsenious oxide. There is no sure and 

 ready method by which the free arsenic content of Paris green can be 

 ascertained. Reagents, such as ammonia, which dissolve Paris green 

 also dissolve the oxide almost or quite as readily. The microscope has 

 been highly recommended, especially for the detection of "white arsenic" 

 which has been added as an adult erant, but not for that which has been 



* U. S. Dept. Agri., Bureau of Chemistry, Bull. 82, pp. 5-6. 

 t College of Agriculture, Bull. 151. p, 19. 



