THE LEAD ARSENATES. 19 



tors are more powerful at medium values, or when both act in medium 

 amounts, than where either one is low, even though the other be high. 



7. In the case of the plum the elbow is between H70 and H80, while with 

 the peach it tends to move back toward lower humidities; or, in other 

 words, the plum seems to be more sensitive to higher humidities than the 

 peach, and also at the extremes of the safety lines to both T and H. 



8. From the tests with lead arsenate, the peach and the Bradshaw plum 

 at least appear to have about the same degree of resistance to arsenical 

 sprays. 



From the evidence at hand it would seem that, with reliable arsenicals 

 properly made, mixed and applied, injury results from the combination of 

 temperature, humidity and light factors. A high value for either of the 

 first two factors, provided the other is low, indicates probable safety, par- 

 ticularly on sunny days. 



Why divergence from these requirements should cause burning has not 

 been brought out by this work. It may be that, as the injury generally 

 appeared only after a week or more, there was some chemical factor at 

 work. With some carbonic acid in the air and heavy dews at night it 

 might be possible that a slow decomposition of the arsenate on the leaves 

 took place, gradually liberating the arsenic and resulting after a time in 

 injury. If this were correct, however, it would seem as though the de- 

 composition of the arsenate would take place when sprays were applied at 

 T and H combinations below the safety line, and cause burning in those 

 cases also. Possibly the leaf differs in its physiological activities under 

 different conditions of light, temperature and humidity, and under some 

 of these is susceptible to influences not effective under others. 



The most that can now be said is that this work has failed to answer the 

 question why arsenical sprays sometimes injure foliage, though it has 

 sho\\ii that of the four explanations given at the beginning, the first, 

 second and fourth can be rejected, and that the problem is apparently one 

 for the plant physiologist, the chemist, or both working together, to solve. 

 The demonstration of safety limits for spraying can hold good, however, 

 even though the question of why they are located where they are remains 

 unanswered. 



