Circular 21 - Insecticides and Subsidiary Materials 33 
^iVhen a commercial fly spray is to be 
purchased, it would be advisable to specify 
clearly that the requirements of the Commer- 
cial Standards CS 72-3B of the National Bureau 
of Standards are to be met. For general use 
Grade B is quite satisfactory. Sprays of this 
grade designation may be procured from most 
local oil companies and filling stations or 
from the firms that are listed in the Bureau 
of Standards Publication No. CS 72-38 referred 
to above. Orders may be placed through local 
representatives of these fiims. 
A satisfactory fly spray may also be 
obtained under (jlass 51 of the General 
Schedule of Supplies, Procurement Division, 
Treasury Department. 
FOmiALIN 
Commercial formalin diluted in milk or 
milk and water with a small quantity of brown 
sugar added, is a safe and effective poison 
for killing house flies that may occasionally 
gain entrance to mess halls, barracks, and 
other buildings. The poison is conveniently 
exposed to the flies by inverting a partially 
filled drinking glass of it as a reservoir on 
a saucer or plate lined with blotting paper. 
A match stick should be placed under the 
edge of the glass. 
Formalin as ordinarily obtained is an 
iqueous solution of foriualdehyde gas (CH2O). 
The commercial solution contains 40 percent 
of fonnaldehyde and is colorless, but on 
standing it may become cloudy. The odor of 
this substance is sharp, choking, irritating 
to the eyes, and penetrating. It is miscible 
