96 MASS. EXPERIMENT STATION BULLETIN 210. 



at this end of the figure, are less resistant than the peach (6), though at 

 high T the latter is considerably less resistant than the other two. 



In cloudy weather (Fig. 14) the pear, elm and apple show about the 

 relative relations to each other that would be expected from the studies 

 on lead arsenates, while the cherry, plum and peach are almost identical 

 for the high H limit of safety, and nearly so at the other ends of their 

 safety lines. Such differences as thej^ do show might easily disappear 

 were more tests available, though, on the other hand, such tests might 

 have led to greater differences. 



Finally, it is evident that there is a wide difference in the safety lines, 

 and that the spraying of different kinds of trees cannot always be done 

 with safety on the same day. It may be perfectly safe to spray apples 

 on a day when spraying plums, peaches or even cherries might prove 

 disastrous. 



Calcium Metarsenite. — The two samples of this substance described 

 above, produced injury on the foHage of all the kinds of trees tested, 

 within two or three days after the appHcation, the injury increasing until 

 the leaves were practically destroyed and dropped off. Though the addi- 

 tion of milk of lime appeared to bring down the solubility of the arsenic 

 within reasonable safety limits in laboratory tests, this did not appear 

 to hold under field conditions, even when the milk of lime was increased 

 to 3 per cent, so further investigation of this material was given up. 



III. NOTES ON OTHER ARSENICALS. 



Magnesium Arsenate. — This substance, sent in by an insecticide manu- 

 facturing company for trial, was tested on the same basis as the other 

 materials. Two hundred and eight clear-weather tests were made at 

 temperatures and humidities ranging from T92 H54 through T86'H70 

 and T80 H80 to T77 H81, for the high hmits, and as low as T78 H55 and 

 T67.5 H 69. In every case, no matter how low T and H were, injury 

 developed on all the trees except the pear and one or two tests on the elm. 

 Apparently, spraying with magnesium arsenate is unsafe at almost any 

 combinations of T and H, except on the pear, where the higher combina- 

 tions become unsafe, and possibly on the elm, where at low combinations 

 only traces of injury were evident. 



In cloudy weather 108 tests were made at combinations of T and H as 

 low as T73 H76 and T67 H72, and as high as T82 H74, T78 H84 and 

 T68 H90. In every test injury, often very serious, followed, except in 

 two instances on the pear. 



As a general conclusion from these tests, therefore, magnesium arsenate 

 is not a safe material for spraying under any conditions. 



Zinc Arsenite. — Two samples of this material, received from different 

 manufacturers, were tested in 1913. Both were finely divided, bulky 

 powders, hght and "fluffy." They were applied, at the rates of 1 pound 

 and 1| pounds in 50 gallons of water, to the same kinds of trees as were 



