22 MISSOURI BOTANICAL GARDEN BULLETIN 
effective, combining strong poisoning and sticking qualities. 
Hellebore is desirable for short duration use, losing its ef- 
fectiveness in 4 or 5 days. 
The sucking insects derive their food by inserting their 
beaks through the surface of the leaf, fruit, or bark, and 
sucking out the internal juices. It is useless to apply a 
stomach poison for this type, as up to the present time there 
has been no preparation devised which would penetrate the 
outer surface of the leaf without injuring the internal tis- 
sues. Therefore, the second class of insects must be killed 
by contact sprays, i. e., by chemicals applied directly to their 
bodies. The contact poisons include commercial lime- 
sulphur, kerosene emulsion, and tobacco extract. The kero- 
sene emulsion is prepared by dissolving 1% pound hard soap 
in hot water, adding 2 gallons kerosene, and later diluting 
in from 5 to 15 parts of water, depending upon the type of 
insect and the kind of foliage. The commercial tobacco 
extract contains nicotine sulphate. The usual dilution of 
this substance when sprayed is 1 to 500 parts of water or 
— weaker, depending upon the tenderness of the insect 
skin. 
Advantage has also been taken of the suffocating prop- 
erties of certain gases for destroying insects, particularly in 
stored products. These chemicals are known as fumigants. 
Carbon bisulphide (1 pound to each 100 cubic feet of space) 
is a fumigant very commonly used. The material to be 
treated is placed in a tight receptacle or room and the chem- 
ical poured into shallow dishes. As the carbon bisulphide 
is a heavier gas than air, the dishes should be located above 
the material to be fumigated. Sulphur mixed with lime 
and painted upon heating pipes also throws off fumes in- 
jurious to tender-bodied insects. The most active fumigative 
agent, however, is hydrocyanic gas, made by combining 
water, sulphuric acid, and potassium cyanide in the ratio of 
3 ounces water, 1 ounce sulphuric acid, and 1 ounce potas- 
sium cyanide or sodium cyanide, Fumigation with this gas 
should be done at night and all plant surfaces should be dry, 
failure to observe either of these precautions resulting in in- 
jury to the pane The gas is a deadly poison to human — 
eings as well as insects, so that extrao inary caution must 
be used in its application. Commonly, the water is placed 
in an earthen vessel and the sulphuric acid poured slowly 
into it. The Sonnet" cyanide is then wrapped in a piece 
of cloth and dropped into the vessel, the operator leaving 
the room quickly. Fumigation with tobacco is now done 
by the use of specially prepared papers saturated with a solu- 
