May 12, 1923 lujuTy to FoUagc by Arsenical Spray Mixtures 
517 
wood, lilac (purple), lilac (white), maple (Norway), Siberian pea tree, 
willow (golden). Repeated tests showed that in all species tested the 
upper epidermis is decidedly more protective against arsenical injury 
than the lower. 
RE^LATIVE INJURIOUSNESS OF ARSENICAL CHEMICALS 
When a number of arsenical compounds are sprayed upon plants the 
relative injury done by them may be influenced by two factors, (i) their 
relative solubility and (2) their relative toxicity to the plants treated. 
It so happens tiat with reference to solubility, arsenical compounds 
fall into two fairly well defined groups, those that dissolve readily in a 
relatively small amount of cold water and those that are only slightly 
soluble in cold water. The former we will for convenience designate as 
the soluble arsenical compounds and the latter as the insoluble arsenical 
compounds, it being fully recognized that a small amount goes into solu¬ 
tion while the greater part remains undissolved when used in spraying 
work. 
READILY soluble ARSENICAL COMPOUNDS 
Compounds of this class can not be used for spraying plants to con¬ 
trol insect pests owing to the injury done when enough is applied to 
kill the insects. Their practical significance in this connection lies in 
their utility as weed killers and in the possibility of their presence as 
impurities in the so-called insoluble arsenical insecticides. Their tox¬ 
icity may be compared on either of two bases, (i) using equal parts by 
weight of the different chemicals or (2) such amounts of them as will 
contain equal amounts of arsenic. Our experiments included tests on 
both these bases. 
The results shown in Tables VI and VII correspond very well with 
those that were usually found in the many tests of these compounds. 
It will be noted that on the basis of toxicity whether equal weights of 
the chemicals be used per liter or amounts to contain equal weights of 
arsenic, these compounds may be divided into three groups; cacodylic 
acid and the cacodylates are most toxic, the arsenites and arsenic acid 
next, and the arsenates least. Ammonium arsenate constitutes one ex¬ 
ception, as it causes injury equal to the arsenites. Ammonium arsenite 
was used a little, but is unsatisfactory owing to its instability. 
Table VI .—Apple sprayed with soluble arsenical compounds using the same number of 
grams for 2 liters 
Chemical. 
Injtfry. 
Concentration 
I to 10,000. 
Concentration i to 
5.000. 
Concentration 
I to 2,000. 
AriH ars<»Tiif* . 
None. 
Slight. 
Bad. 
. . 
Moderate... 
Moderate. 
Very bad. 
AmiTinTiiiiTn . 
None. 
.do. 
Bad. 
Potassitun arsciiate. 
...do. 
None. 
Moderate. 
PotA-*iSilitn arsenite . . 
., .do. 
Very slight. 
Bad. 
Potassiuin cacodvlatc ... 
.. .do. 
Moderate. 
Very bad. 
3 odin^ arsenate . 
.,.do. 
None. 
Slight. 
fiftHiiim arsenite. . 
...do. 
Very slight. 
Bad. 
Sodium cacodylate . 
...do. 
Moderate. 
Very bad. 
