Porato SPRAYING IN MINNESOTA 27 
slide received as nearly as possible the same intensity of spray. The 
slides were washed for from two to six hundred seconds, dried and 
again weighed, cleaned and weighed a third time. From the figures 
thus obtained, the percentage of the material washed from the slides in a 
given time was computed and a curve constructed for each material. 
At least 5 points were determined on each curve and each point was 
the average of five slides washed for the same length of time. 
The results of this series of experiments showed that of the in- 
secticides used, paris green washed from the slides most readily, 
100 per cent being washed off in ten seconds. The other three materials 
resisted washing much better, zinc arsenite losing about 98 per cent, 
lead arsenate 91 per cent and calcium arsenate 88 per cent in 600 
seconds. 
These results showed that of the four materials used, calcium 
arsenate has the greatest power of adherence to glass. Whether or 
not the same ratio would hold good on leaves, was open to doubt. To 
check this point the second method was used. Geranium leaves were 
used in this series of experiments. 
The materials were mixed with water as before and placed by 
means of a medicine dropper upon the leaves. A measured amount of 
the liquid containing the arsenical was placed on each leaf, being care- 
ful that none ran off. In this way approximately 4 milligrams of 
the desired arsenical was deposited on each leaf. Ten leaves were 
used for each material. After drying, these leaves were washed for 
5 minutes under a uniform spray. Later they were digested with 
sulphuric acid and their arsenic content determined by the Gutzeit 
method.” The results are shown in Table 1, tabulated in terms of the 
arsenicals used. 
AWANIBILIE, Il 
Milligrams Percent Coefficient of 
left after washed adhesion to ge- 
washing oft ranium leaves 
Galciim»peansenate sce oe ae ee 3.0 25 74 
Weadmanrsenateme sees ce acer: 25 On, 62 
ATI CALS CMe sien ss Oo aero nis 2.00 50 50 
Paleise eaneeun ina peed Haat So cero 0.04 99 1 
The range of experimental error is greater in using leaves than in 
using glass slides, but the results from these two sets of experiments 
are so comparable that it is safe to assume that the same ratio of ad- 
hesion holds good in each case. 
2 Scott—Methods of Chemical Analysis. 
