THE NATIONAL NURSERYMAN 
.S07 
is to the part of tlie ars(Miical \\ liioli is S()liil>l(‘ in wa- 
h'r and ^\ lii(*li may tliiis (Mil(‘r lh(‘ lissiu' of llio |)laiil to 
(list orb its functions. 
Arsenic has Jjccn combined witli many substances to 
(ind a combination wbicli is the least soluble and the U'ast 
alfecicd by weather conditions, and to find a combina¬ 
tion that is the most suitable for use in other ways. Of 
all the combinations thus fai‘ tric'd, a chemical comhina- 
tion of arsenic with lead seems to most fully meet tin* 
above conditions. At present, there are two types of 
lead arsenate uj)on the market, each having its own 
place in spray (cractice. 
Types of Commercial Lead Arsenate 
Acid Lead Arsenate; Lead Hydrogen Arsenate (often 
Lareled “Standard”.) —In an investigation to find the 
most suitable form in wliich an arsenical could lie ajiplied 
to foliage, Mr. F. C. Moulton, chemist for the Mass¬ 
achusetts Gypsy Moth Commission, selected lead arsenate 
as the combination most suitable for use in his work. 
This arsenical offered so many advantages over other 
arsenicals in use previous to his work, that it is rapidly 
sup[)lanting all other forms of arsenicals for use on 
foliage. This arsenical may be prepared by mixing in 
proper proportions a soluble salt of lead, usually lead 
nitrate or lead acetate, and a soluble salt of arsenic acid, 
usually sodium arsenate. The process has been per¬ 
fected and cheapened by commercial manufacturers so 
that the use of tliese soluble salts, as raw materials, has 
been largely discontinued. At present, many of the 
manufacturers prepare commercial lead arsenate paste 
from lead oxide (litharge) and arsenic acid. This pro¬ 
cess greatly cheapens and simplifies the manufacture of 
the paste in ways which need not be discussed in this 
paper. As ordinarily made, there results a compound 
which may be spoken of in chemical language as an acid 
lead arsenate or possibly a mixture of this and neutral or 
basic lead arsenate. It was found that lead arsenate 
produced in this way gave very uniform and satisfactory 
results in most cases for the control of leaf-eating insects. 
Basic Lead Arsen.4te (usually Labeled “Tri- 
Pluairic” or “Neutral”). —The lead arsenate produced 
in the usual way, however, was found to produce very 
s(*rious foliage injury under certain climatic conditions 
which prevail in the Pajaro Valley, the prineijial ap])le 
growing section of California. In the s])ring of 1903, 
held and laboratory work was commenced by the Lntom- 
ological Division of the Univ(‘rsity ol (ailitornia to find a 
more suitable arsenical or to modify the known methods 
of ])re[)aration of lead arsenate so that it could be used 
without injury under the conditions ])revailing in tin' val- 
h'v. The held work was conducted by Mr. W . 11. \ olck 
and Mr. F. E. Luther, students in the College of Agricul¬ 
ture. After numerous experiments, tin' pi'oci'ss of ])r('i)- 
aration was so modified that a new type ol h'ad ai'si'nalc 
w as produced w hich could be us(‘d w ith safi'ty upon the 
apple trees in that section. The modified ])rocess and 
some theories of foliage injury are discussed in the ])ub- 
lication previously referred to. This new^ jirocess lead 
arsenate was at hrst lielieved to be what may be teimed 
a neutral lead arsenate. Our knowledge of the chem¬ 
istry of lead arsenates is still very im])(‘rfecl. but later in¬ 
vestigations seem to indicate that the material produced 
in lh(^ manner (h'sci'ibi'd by Volck may inoi’i' j)ropcrly be 
ri'fern'd to as basic lead arsi'iiatc. 
CoMi'AitisON OF THE Two Types.-- To take uj) a full <lis- 
cussion of th(‘ composition of the two types mi'idioncd 
would he of loo technical a nature for pn'si'ulation in r. 
paper of this kind. The acid typi^ is very susci'ptiblc In 
the action of other chemicals and is more or less dissolved 
by chemicals of an alkaline nature w Inch are commoidy 
found in many sjiray materials. This |)i'opci-ty distin¬ 
guishes the acid type of lead arsenate from the basic. Tin* 
latter is not easily affected by alkaline chemicals and is 
a much more stable chemical com|)ound umhu’ ncuti’al oi‘ 
alkaline conditions. It is not casilv made soluble bv or- 
dinary influences. A theory w as advanced hy Voh'k that 
the acid type of lead arsenate was (h'composi'd by the 
small amounts of ammonia (alkaline) which are sonu'- 
times present in the atmosphere, jiroducing a soluhlc 
form of arsenic. Under the conditions prevailing in the 
Pajaro Valley, there was present on the foliage enough 
moisture to dissolve the soluble arsenic, thus formed, and 
make possible its absorption by the foliage. The basic 
lead arsenate, on the other hand, is absolutely insolubh* in 
ammonia and soluble arsenic cannot lie formed in this 
manner. Whether this theory is correct or not has not 
been fully confirmed by other experimenters, but the fact 
remains, however, that the basic type is the safer arsen¬ 
ical to use upon foliage in the humid coast regions. The 
basic type is jiractically the only arsenical which may be 
safely mixed wdth any of the other spray materials as .i 
combination spray. This has been fully tested out in 
the Insecticide and Fungicide Laboratory and has been 
found to be much more suitable w hen alkalies of any sort 
are mixed w ith it. The acid type is a stronger poison, 
however, and its action upon the insects is much more' 
rapid. The basic type contains mush less arsenic to tin* 
pound and a much longer time is required to poison 
insects. 
The stronger and (luicker acting acid lead arsenate is 
therefore the one to be naturally selected for use ujion fo¬ 
liage which is not peculiarly susceptible to spi’ay injury 
and where weather conditions ari' not apt to causi' its (h'- 
composition, that is, in the more arid regions away from 
lh(' coast. Tin* basic ty])e should lu' used wlu'iH'M'r an 
arsenical is to be mixed w ith any other mati'rial (with 
one or two exceptions') as a combination sjiray. The 
basic lij])e is the one to be recommended for use in the 
humid coast regions and upon all sloae frails or any 
others whiidi art' especially susceplibh' to sju'ay injury. 
SuSGEBTIBILITY' OF StONE FrUITS TO Sl'llAY In.ILDV 
It is a well-known fact that the foliagi' of all sloiu' 
fruits is jx'culiai'ly susceptibh' to injury fi’om many kinds 
of sprays. This laid has Ix'i'ii nol(‘d in r(*sp(*el to I If* 
use of arsenicals and the Califoi'iiia Agricultural Expeii- 
uH'ul Station has advisc'd that tin' basic type ol h'ad ar- 
simali' should hi' tin' only arsi'iiical applif'd to slime I mils. 
The w isdom of this ad\ici' has bi'i'ii douhli'd hy a gri'al 
many and tlu' causi' of the doubt is not dillii'ull to find. 
If the weather is favorable, tlu' acid ty|)e ol h'ad arsi'nalf' 
usually moy be ajiplied wu’th imjmnity. In fact, this 
has been done in the Santa Cdara A alley lor a number of 
vears w ith onlv occasional bad effi'cts. This yi'ar, how- 
ever (1915), has showm that this proci'dure is not a safe 
one and some other way must be chosen if the growers 
