May 12,19*3 Injury to Foliage by Arsenical Spray Mixtures 
533 
period different lots from the same factory may vary. It is encouraging 
that the product of certain companies is becoming stabilized, and it is to 
be hoped that others will follow their example. At the present time, 
however, we can not make definite statements about lead arsenate 
without specifying the brand and lot. These remarks apply in some de¬ 
gree to zinc arsenite and calcium arsenite, while Paris green and arsenic 
trioxid are much more uniform in composition. 
It is rather surprising that calcium arsenate should ever have been 
used as extensively as it was some 20 years ago when one observes its 
tendency to bum foliage. To be sure, the competition with other 
arsenicals, e. g., lead arsenate, did not then exist, and in the less frequent 
injury by these compounds we find one of the chief causes of its pro¬ 
gressive abandonment. 
The most striking example of a poorly founded prejudice is the quite 
general belief that arsenic trioxid (common white arsenic) will bum 
foliage even when occurring as a very small impurity in other arsenicals. 
We are not able to explain fully this belief. Probably it lies partly in 
the fact that chemists expressed their findings of soluble arsenic in in¬ 
secticides in terms of “arsenious acid,” the reasons for which are not 
always clear to others. Probably also, the letting of the spray mixture 
stand for hours or days before applying increases the danger from arsenic 
trioxid more than from other arsenical compounds, and has been a factor. 
In any event, we feel that we have established quite conclusively that a 
freshly prepared suspension of arsenic trioxid is not particularly harmful 
to foliage, and we entertain a hope that this relatively cheap form of 
arsenic may come into general use for the control of certain insects. 
Of course its efficacy as an insecticide has yet to be determined. 
The hope expressed by Gillette (jj) that arsenic trisulphid wouid 
prove less injurious than the arsenical insecticides in common use, seems 
unlikely of realization, for, while this compound has a low solubility In 
pure water, it nevertheless causes considerable burning when sprayed 
upon foliage. 
In the selection of an arsenical insecticide for any crop to be sprayed, 
the entomologist will do well to consult the table of susceptibility on 
page 512, for on some plants a mixture maybe used that would be quite 
destructive to others. 
The practice of adding to an arsenical spray mixture some other 
material to act as a supplementary insecticide or fungicide, or as a 
“sticker’' or “spreader,” or to decrease the tendency to injury, has been 
the subject of considerable controversy. Conspicuous among these 
materials is soap, which, in addition to being a spreader, has been shown by 
Parker {22) to be of considerable value iu keeping lead arsenate in 
suspension. That soap increases the tendency to burning by nearly all 
“insoluble” arsenicals is clearly established (p. 523), but it is safe to say 
that the increase in burning tendency with lead arsenate is not sufficient 
to discourage its use on the more resistant plants such as cabbage, sugar 
beets, potatoes, and apple. It is indeed a fortunate circumstance that 
cabbage and sugar beets, on which a spreader is a necessity, are resistant 
enough to arsenical injury to make use of soap permissible. It is 
likewise fortunate that soap has been found to actually decrease the 
injurious properties of Paris green, for gardeners may now rest assured 
that the addition of soap to it is not only permissible but desirable. 
Lime sulphur increases tlie injury by nearly all the arsenical insecticides 
in about the same degree that soap does, and must not be used in com- 
