538 
JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 16 
phates of iron and copper, when freshly prepared, act as hydrated colloids, 
and fulfill the requirements of a good emulsifying agent, as given by 
Holmes. Pickering obtained good emulsions with solid emulsifiers 
only when they were freshly prepared and in the medium in which they 
had been precipitated. He failed to get satisfactory emulsification with 
previously dried materials, even when they were very finely divided. 
In the light of the modern conception of the colloid condition as a state 
in which any substance may exist, it seems unnecessary to make an 
exception of Pickering’s emulsions. 
It has been stated 9 that oil-soap emulsions should not be used with 
Bordeaux, due to change of phase and liberation of free oil. We have 
seen no signs of such liberation of oil when the soap emulsions are added 
to freshly prepared Bordeaux. A p 2 -J^-50 Bordeaux mixture has 
been widely used as a stabilizer when oil-soap emulsions are to be used 
with hard water. Other substances, such as starch, flour, glue, casein, 
and calcium caseinate, which are used to stabilize oil-soap emulsions in 
the presence of the calcium ion, can be used as emulsifying agents. 
It seems probable that the so-called stabilizing material forms a film 
around any unprotected oil particle present. These oil particles may 
be gotten into the presence of the hydrophile colloid chemically, by 
destruction of the soap film, or mechanically, by breaking up by means 
of a spray. 
The reason why only freshly prepared Bordeaux mixture is efficient 
as an emulsifying agent is somewhat obscure. This property is prob¬ 
ably dependant on the degree of dispersion of the basic copper sulphate. 
Coagulation goes on rapidly in Bordeaux, due to the presence of two 
flocculating agents, lime and gypsum. If Pickering’s views are correct, 
it may be the increased size of the particles which prevents their acting 
as an emulsifying agent. If we assume that the Bordeaux acts as a 
solvated colloid, the decrease in hydration due to coagulation may prevent 
emulsification. A parallel to the latter hypothesis exists in the case of 
skim milk. Fresh skim milk makes good emulsions for a time, but 
clabbered skim milk, where the casein has coagulated and become less 
hydrated, is not a satisfactory emulsifying agent. 
It is not within the scope of this paper to discuss the advisability of 
replacing lime-sulphur with engine oil emulsion as a scale spray. It 
may be said, however, that in the Middle West 2% engine oil emulsified 
according to the Government Formula, is considered to be an efficient 
9 Mahin, E. G. and Carr, R. H. Quantitative Agricultural Analysis, New York, 
923. 
