52 



green, the free arsenic being- determined by dissolving- it from the 

 Paris green by means of distilled water. 



What I desire to call attention to here is that there may be three 

 conditions giving rise to a scorching of the foliage by the use of Paris 

 green: 



(1) As I have already said, there may be a certain amount of arseni- 

 ous oxid in the Paris green over and above that combined with the 

 other constituents. This is called "free" arsenious oxid, and has 

 until recently been considered as the only cause for the scorching of 

 foliage by Paris green. 



(2) A number of Paris greens now on the market are poorly made, 

 and when brought in contact with water, especially water charged with 

 carbon dioxid, readily break up in the course of a short time, arseni- 

 ous oxid being set free in the process. This is true to some extent 

 even of the best Paris greens, but the poorly made ones break up much 

 more quickly and readily. Let us see, then, what the effect of such 

 greens would be upon the foliage. They would, in most cases, be 

 sprayed upon the leaves while suspended in water. The water, along 

 with the carbon dioxid of the air, would immediately commence to set 

 arsenious oxid free. Not only would this go on with the original water, 

 but the rain and dew that collect upon the plant Avould carry the 

 process still further, and in the case of a poorly made article there 

 would soon be enough free arsenious oxid to seriously scorch the 

 foliage. Of course this change would go on in the case of well-made 

 Paris greens, but so slowly that unless the climatic conditions were 

 very adverse not enough arsenious oxid would be present at any one 

 time to do any serious damage. 



(3) It is a well-known fact that even the best Paris greens, when 

 ground to a very fine powder and applied to the foliage, will scorch. 

 This seems to be due to the fact that the line grinding exposes more 

 surface to the action of water (and water charged with carbon dioxid), 

 and that consequently the decomposing action of the water on the 

 Paris green is accelerated and enough free arsenious oxid soon gathers 

 to do serious damage. 



It appears, then, that there are three conditions of Paris green that 

 may give rise to free arsenious oxid and consequent scorching, and 

 what confronts the chemist is the discovery of a method or methods 

 by which he can tell how much free arsenious oxid is actually present 

 in a sample of Paris green, or how much, because of its condition, 

 may be set free in a short length of time. 



This is rather a hard problem and, from the nature of the case, only 

 an arbitrary method can now be proposed or expected. 



Manifestly it would not do to extract the sample for twenty-four 

 hours with water since, in this length of time, even where free arse- 

 nious oxid is present as such, it does not all go into solution nor does 



