16 INSECTICIDES AND FUNGICIDES. 



the tree. This might be expected from the work done by Avery and 

 Beans, which shows that for any particular green the finer the green 

 the more arsenious oxid goes into solution in water in a given length 

 of time. It is, therefore, apparent that the water-extraction method 

 for determining free arsenious oxid does not distinguish between the 

 arsenious oxid set free from a badly made sample of Paris Green and 

 the arsenious oxid set free from a very fine sample. Too much arsen- 

 ious oxid from cither of these causes, however, is objectionable, 

 although, perhaps, not equally so. The finely made greens stay in 

 suspension in water much better than the coarser greens, and are, 

 therefore, better adapted for spiking purposes, although, of course, 

 there is a safety limit, above which the arsenious oxid extracted by 

 water should not run for these as well as for the coarser greens. What 

 constitutes the best Paris Green, then, for spraying purposes is a very 

 fine sample which will stay in suspension in water but will not break 

 up to a great extent upon standing in contact with it. Such greens 

 as are mentioned can be made, since a number of those above analyzed 

 were very fine, yet gave a very low figure for soluble arsenious oxid 

 by the water-extraction method. 



The amount of free arsenious oxid allowable in Paris greens varies 

 somewhat in different climates, but in the East, generally, and in Cali- 

 fornia 1 per cent is considered the maximum amount allowable. In 

 Idaho" 6 per cent has been adopted as the maximum amount. In all 

 cases the 7-day or 10-day water-extraction method is the one to 

 be employed in making determinations of this constituent. In this 

 connection it may be stated that 4: per cent was adopted in the East as 

 the maximum amount of free arsenious oxid allowable in Paris Green 

 when the extraction with water was limited to 2-i to 48 hours, but 

 recent work has shown that all of the free arsenious oxid does not go 

 into solution in this time, and that very likely where the old method 

 gave from 3 to 1 per cent the new method would give at least 5 to 6 

 per cent. It is the opinion of the writer, then, that more work should 

 be done upon this subject, using more modern methods of analysis, 

 and that until such work is done the 6 per cent adopted by the Idaho 

 station gives more closely the safety limit of the green (when a 10-day 

 extraction is used for analysis and when lime is mixed with the green 

 before using) than does the 4 per cent used in California and the East. 

 The climatic conditions in different parts of the United States would 

 most likely change this figure somewhat for different localities. 



Again, while considering this point it seems, at any rate from a 

 theoretical standpoint, that in preventing the scorching of foliage by 

 adding lime to Paris Green it would be much better to mix the green, 

 in suspension in water, with the lime about J.O days before use and stir 

 the mixture occasionally, since the free arsenious oxid does not go 



a See Bui. No. 25, Idaho Agricultural Experiment Station, 



