Material. Paris green, powdered arsenate of lead, paste arsenate of lead.

Apparatus. Quart glass jars, stirring-rod.

Insecticides differ from fungicides in that they are chemicals used for the destruction of insects that prey upon cultivated plants. There are two kinds of insecticides: namely, stomach poisons, or insecticides that are eaten by the insects and kill by being taken into the stomach; and contact insecticides, or those which are sprayed on the insects and kill by coming in contact with their bodies.

Paris Green

Paris green, one of our important insecticides, is an aceto-arsenite of copper, and Ehrmann has given the composition of pure Paris green as follows:

Copper oxide

.............31.29%

Arsenious acid

.............58.65%

Acetic acid

.............10.06%

However, most samples show some variation from the above. The free arsenic in the Paris green is dangerous to foliage, and when Paris green is used alone, lime should be added to avoid burning the leaves.

The usual formula for spraying is

Paris green

...........l lb.

Water

...........100 to 300 gal.

Laboratory formula. Weigh out one fourth of an ounce of Paris green and place it in a quart fruit jar of water. Study carefully the action of the poison when placed in the water. Describe what takes place. Now with a stirring-rod vigorously agitate the water. Does it mix with the water? If not, why? Describe. Is Paris green in water a solution or a mixture?

Arsenate Of Lead

Arsenate of lead is another of our important insecticides and to-day probably ranks first. It can be purchased in two forms - as a powder or as a paste. The paste form is the most widely used. The usual formula for arsenate of lead is from two to three pounds to fifty gallons of water, but the proportion varies with the crop to be sprayed.

Laboratory formula. Weigh out one-half ounce each of the powdered and the paste form of arsenate of lead, and dissolve each in a separate quart glass fruit jar of water. Study carefully what takes place. Now place all three of the jars containing the insecticides where the light will pass through them. Stir each vigorously for a minute and note the results. Watch the solutions for fifteen minutes, noting what takes place in each case. Are the insecticides completely dissolved, forming a chemical union, or are they held in suspension, with only a slight chemical action taking place? Discuss. Note accurately the length of time required for each solution to settle. Discuss in detail why it is absolutely necessary to have the spray mixture thoroughly agitated during the whole time of spraying.