Arsenic. — Known to chemists as arsenious acid, or white oxide of arsenic. It is considered an unsafe insecticide, as its color allows it to be mistaken for other substances ; but in its various compounds it forms our best insecticides. From one to two grains, or less, usually prove fatal to an adult; 30 grains will usually kill a horse, 10 grains a cow ; and 1 grain, or less, is usually fatal to a dog. In case of poisoning, while awaiting the arrival of a physician, give emetics, and, after free vomiting, give milk and eggs. Sugar and magnesia in milk is useful.

A cheap and effective insecticide may be prepared from white arsenic by the following methods: —

For use with bordeaux mixture only. Sal soda, 2 pounds; water, 1 gallon; arsenic, 1 pound. Mix the white arsenic into a paste, and then add the sal soda and water, and boil until dissolved. Add water to replace any that has boiled away, so that one gallon of stock solution is the result. Use one quart of this stock solution to 50 gallons of bordeaux mixture for fruit-trees. Make sure there is enough lime in the bordeaux mixture to prevent the caustic action of the arsenic.

For use without bordeaux mixture. Sal soda, 1 pound; water, 1 gallon; white arsenic, 1 pound; quicklime, 2 pounds. Dissolve the white arsenic with the water and sal soda as above, and use this solution while hot to slake the 2 pounds of lime. Add enough water to make 2 gallons. Use 2 quarts of this stock solution in 50 gallons of water. Arsenicals. - A term popularly used for compounds of arsenic. The leading arsenicals used in destroying insects are arsenate of lead, paris green, and london purple.

Arsenate of Lead. - This can be applied in a stronger mixture than other arsenical poisons, without injuring the foliage. It is, therefore, much used against beetles and other insects that are hard to poison. It comes in the form of a paste or powder, and should be mixed thoroughly with a small amount of water before placing in the sprayer, else the nozzles will clog. Arsenate of lead may be used with either bordeaux mixture or lime-sulfur without lessening the value of either. It is used in strengths varying from 4 to 10 pounds per 100 gallons, depending on the kind of insect to be killed.

London Purple. - An arsenite of lime, obtained as a by-product in the manufacture of aniline dyes. The composition is variable. The amount of arsenic varies from 30 to 50 per cent. The two following analyses show its composition : (1) Arsenic, 43.65 per cent; rose aniline, 12.46; lime, 12.82; insoluble residue, 14.57; iron oxide, 1.16; and water, 2.27. (2) Arsenic, 55.35 per cent; lime, 26.23; sulfuric acid, 0.22; carbonic acid, 0.27; moisture, 5.29. It is a finer powder than paris green, and therefore remains longer in suspension in water. It is used in the same manner as paris green, but is sometimes found to be more caustic on foliage. This injury is due to the presence of much soluble arsenic; but it can be averted by the use of lime, as advised under paris green.

Paris Green.—An aceto-arsenite of copper. When pure it contains about 58 per cent of arsenic. By the provisions of the federal insecticide act of 1910, paris green must contain at least 50 per cent of arsenious oxide, and must not contain arsenic in water-soluble form equivalent to more than 31/2 per cent of arsenious oxide. It is applied in either a wet or dry condition ; but in any case, it must be much diluted. For making a dry mixture, plaster, flour, air-slaked lime, road dust, or sifted wood ashes may be used. The strength of the mixture depends upon the plants and insects to which it is to be applied. The strongest dry mixture now recommended is one part of poison to fifty of the diluent; but if the mixing is very thoroughly done, 1 part to 100, or even 200, is sufficient.

Paris green is practically insoluble in water. When mixed with water, the mixture must be kept in a constant state of agitation, else the poison will settle, and the liquid from the bottom of the cask will be so strong as to do serious damage, while that from the top will be useless. For potatoes, apple-trees, and most species of shade trees, 1 pound of poison to 200 or 250 gallons of water is a good mixture. Paris green is very likely to burn the foliage of stone fruits, especially peaches and Japanese plums, and has been generally replaced by arsenate of lead for such purposes. In all cases, the liquid should be applied with force, in a very fine spray. At some seasons of the year foliage is more liable to injury than at others. The addition of a little lime (twice the bulk of lime as of paris green) to the mixture will tend to prevent any caustic injury upon the foliage.