92 
Summary of Spraying Results. 
Finally, to sum up in a word the most important practical re- 
sults of the orchard experiment with arsenate of lead, we may say 
that four sprayings of apple-trees of late varieties, exposed to a 
very heavy attack by the plum-curculio, the first spraying applied in 
early May just as the trees are coming into bloom, and the others at 
intervals of lo days thereafter, the whole operation costing 17 cents 
per tree, may be expected to increase the yield of the orchard about 
one half, to increase the average size of the fruit by about a fifth, 
and so to improve the quality of the apples that they should be worth 
from 2^ to 3 times as much as if the orchard had not been sprayed. 
Poison Tests oe Sprayed Apples. 
The arsenate of lead used in experiments described in this paper 
is an unusually adhesive insecticide, remaining upon trees in visible 
quantity weeks and even months after it has been applied. Evident 
traces of the mixture were still visible in October on leaves which 
had fallen from trees sprayed with the arsenate of lead July 4. In 
view of this fact is seemed to me important that experiments ^liould 
be made to determine the amount of poison carried by apples 
treated with this insecticide within a reasonable limit of time after 
the spray was applied. I consequently directed, in 1905, the spray- 
ing of apple-trees with various arsenical compounds, and among 
others with the home-made arsenate of lead, and obtained analyses 
of the peelings of apples so sprayed taken from 2 trees within a day 
of the application of the insecticide, and from a third tree 2 months 
thereafter. 
The first of these trees, of the Duchess variety, received 2 spray- 
ings with the arsenate of lead — the first June 9 and the second 
July 4 — not having been previously sprayed at all that year. In 
both these sprayings the insecticide was used 4 times the common 
strength — 12^ ounces of acetate of lead and 5 ounces of arsenate 
of soda to 12^ gallons of water instead of the usual 50 gallons. 
The next day, July 5, six apples, averaging two and a fourth 
inches in diameter, were carefully picked from this tree by the stem 
and peeled at once, the apple being held upon the point of one knife 
while it was peeled with another, to avoid removing any part of the 
poison unnecessarily. The peelings were then dropped into a dry, 
clean, new Mason jar and submitted for chemical analysis to Dr. 
Wm. M. Dehn, of the Chemical Department of the University of 
Illinois. It should be said that the weather in the interval between 
spraying and collection of the apples had been dry. According to 
Dr. Dehn's report, these apple peelings thus collected yielded 36.6 
parts per million of arsenious acid, enough to equal .2562 grains of 
