Journal of Agricultural Research 
Vol. XXIV, No. 6 
528 
strongly confirms this observation. Quite uniformly the second applica¬ 
tion did much more damage than the first, and indeed a strength of 3.2 
grams per liter, applied twice at intervals of about three weeks, did 
more harm than 6.4 grams applied once on either date. With calcium 
arsenite this tendency was noticeable but not nearly so pronounced as 
with zinc arsenite, while with lead arsenate and with copper aceto- 
arsenite there was but little more injury after the second spray than 
after the first. 
EFFECTS OF HUMIDITY 
That humidity is one of the great contributing factors in arsenical 
spray injury is one of the most striking facts constantly in evidence 
throughout this investigation. This observation is by no means new 
and had already been noted by Gillette (u) and others; and certain 
manufacturers of lead arsenates have come to recognize the necessity 
of using safer mixtures on the Pacific coast and oQier humid regions 
than are required elsewhere. 
Even with these facts in mind, however, we were almost astonished 
at the difference in results on leaves kept in moist and dry air. To 
demonstrate if possible the effects of humidity, an extensive series of 
spray mixtures was applied about simultaneously in different parts of 
the State, including an apple orchard near Flathead Lake (Mont.), where 
the humidity is generally relatively high. As it happened, however, this 
section was unusually dry during the course of the experiment and 
much more so than in the other localities where tests were made. Not 
a drop of rain fell for weeks and the dew point was not reached on a 
single night during the experiment. The days were rather hot, and 
wet and dry bulb thermometer readings made three times daily indicated 
a very low humidity every day and nearly every night. As a result 
almost none of the spray mixtiues used caused any injury whatsoever. 
Even calcium arsenite (2 gm. in 2 liters of water) caused only slight 
injury. In the majority of cases, including all localities and dates, 
this compound in half this strength caused very serious injiny to apple 
foliage, and sometimes defoliation. 
A still more striking demonstration of the effects of humidity in extreme 
cases was made repeatedly by spraying tomato and other plants in pots 
and covering certain ones with bell jars and keeping duplicates outside. 
Under these conditions the moisture transpired by the leaves kept the 
air under the bell jars saturated. 
The plants were all kept in diffused light in a basement laboratory to 
prevent rise of temperature under the bell jars as would happen if kept in 
direct sunshine. The plants were kept in this condition for usually one 
or two days and aerated twice daily. The bell jars were then removed and 
all plants placed under like conditions in the greenhouse until the notes 
were taken. Table XVII is representative of the results under these 
conditions. 
Probably nothing in the whole series of experiments on arsenical 
spraying is more stAing than the contrast of injury in dry air and in the 
saturated atmosphere under the bell jars, especially when soap is added. 
It might seem at first that the bell jar makes an extreme condition of 
humidity, but a saturated atmosphere out of doors is by no means unu¬ 
sual, especially near large bodies of water, as on the Pacific coast and in 
the Great Lakes region. 
