532 Journal of Agricultural Research voi. xxiv, no. & 
EFFECTS OF SHAKING BRANCHES AFTER SPRAYING 
Both in the practical orchard and field spraying and in experimental 
work there is often more or less wind that shakes some of the liquid 
from the leaves. It is important to know, therefore, the extent to which 
this will decrease the amount of arsenical injury to the foliage. In all 
the experimental work of this investigation the spraying was done when 
there was little or no wind, but this w^as probably an unnecessary pre¬ 
caution. On several occasions apple limbs were sprayed in duplicate, one 
being shaken vigorously immediately afterwards and the other hanging 
quietly until the spray mixture had dried on. Similar experiments were 
conducted with potted plants in the greenhouse. In no c^e was there 
a marked reduction in the amount of burning and usually no difference 
could be detected. As this shaking removed the excess liquid from the 
leaves so much more quickly and thoroughly than an ordinary wind, it 
is fair to conclude that any wind not strong enough to practically pre¬ 
vent spraying operations will have no material effect on the amount of 
arsenical injury. 
EFFECTS OF LIGHT AND DARKNESS 
Three experiments were conducted to determine if light increases or 
decreases the amount of injury caused by arsenical spraying. 
Potted tomato plants were placed in a dark room in the evening. 
Early the next morning two were sprayed with calcium arsenite and put 
under bell jars in dark boxes wrapped with black cloth; two were 
sprayed and put under bell jars not in dark boxes, and unsprayed con¬ 
trols were kept under both these conditions. All boxes and bell jars 
were kept in a strong diffused light for two days and then the plants 
were removed and placed on a bench in the greenhouse. In two of the 
experiments the injury to the sprayed plants was equally bad in the dark 
boxes and in the light. In the third, all the sprayed plants were killed, 
making comparisons impossible. The unsprayed plants remained in 
healthy condition. 
SIGNIFICANCE OF THE RESULTS 
The place of arsenical compounds in the category of insecticides is so 
important that we can not at present expect their replacement by any 
other group of chemicals. They have their faults, however, and among 
these is the marked tendency to injury of the crop they are intended to 
protect. It is therefore of prime importance to learn the factors that 
determine this injury and with this knowledge to reduce it to a mini¬ 
mum. Some of the existing beliefs regarding this injury are founded on 
fact, the difficulty being to know the extent of the tendency. Other 
beliefs appear to the based on a wrong conception. 
It has been found quite difficult to make exact statements concerning 
the toxicity of the different arsenicals because of the variability of their 
composition. This is especially true of the lead arsenates. Theoretically 
one might expect ortho, meta and 'pyro arsenates each in the form of 
monoplumbic, diplumbic, and triplumbic salts, a total of 9 possibilities. 
Actually the number that form is fewer, but, on the other hand, many 
commercial lead arsenates are not single salts of lead and an arsenic acid 
but a mixture of different lead arsenates. Furthermore, the product of 
a company may change from time to time, and even within a short 
