this purpose. In spite of this favorable report the poison was not in demand, because of its caustic properties.

After the Paris green and London purple had become well known they were both considered safer insecticides than white arsenic, and the first was acknowledged to be the best of the three. Riley, however, did not entirely accept this opinion, but thought that London purple is not more injurious upon cotton foliage than Paris green is.1 Cook was the first to make a careful study of this point. He made applications to the foliage of plum, cherry, apple, pear, peach, willow, elm, and maple trees, to determine the comparative degree in which the three arsenites mentioned above are injurious. His first conclusion is as follows: "London purple is more injurious to the foliage than is Paris green; and white arsenic- arsenious acid - is more harmful than is either London purple or Paris green." Later experiments have confirmed this result, and the truth of its general application is accepted.

The milk of lime was first used in connection with the arsenites to overcome their caustic properties, by Gillette, in the fall of 1889.3 The results were so encouraging that extensive experiments were carried on the following year, and many valuable conclusions were reached in consequence of the careful and extended observations made during that season. It was plainly shown that "lime added to London purple or Paris green in water greatly lessens the injury that these poisons would otherwise do to the foliage.' 4 Another interesting result obtained in these experiments was that "lime added to a mixture of white arsenic in water will greatly increase the injury that this poison would otherwise do to foliage. If the arsenic is all in solution, the lime will then lessen the injury, as in the case of London purple and Paris green."

The next step in this series of advancements was taken by Kilgore. During 1890 his investigations were made in lines almost identical with those followed by Gillette, and many of the latter's conclusions were verified. In addition to these,

1 Riley, U. S. Ent. Com. 1880, Bull. 3, 62.

2 Mich. Agric. Exp. Sta. 1889, Aug. Bull. 53.

3 Iowa Agric. Exp. Sta. 1890, Aug. Bull. 10, 410.
4 Ibid. 419, 420.

5 N. C. Agric. Exp. Sta. 1891, July, Bull. 77 b, 7.

he also gave directions for the manufacture of an insecticide, in which white arsenic entered as one of the principal ingrediIt was made "by boiling together for one-half hour in two to five gallons of water

ents.

White arsenic.
Lime.

1 pound,
2 pounds,

and dilute to required volume, say one hundred gallons. . . . It is desirable that the lime should be present in the boiling solution of white arsenic, since it renders the latter insoluble as fast as it goes into solution, thus reducing the volume of water and shortening the time for obtaining the arsenite."

Two other compounds of arsenic have been used for the destruction of insects, the first trial being made upon the Gipsy moth, in Massachusetts.1 One of these, the arsenate of soda, "has been recommended by various parties as an insecticide," but the results of the experiments show that it injures foliage before an efficient quantity for the destruction of the caterpillars can be applied. The other, however, the arsenate of lead, is promising. It was proposed as an insecticide in 1892, by F. C. Moulton, a chemist in the employ of the Gipsy Moth Commission. It was first tested against tent caterpillars in 1893, with the following results: 2 "The smaller proportions, as pound or less to 150 gallons of water, do not kill the caterpillars [Clisiocampa Americana] as quickly as is desirable. . . The larger proportions seem unnecessary and would, of course, be rather expensive for general field work, but some such proportions as 1, 11, or 2 pounds to 150 gallons of water would prove entirely satisfactory so far as we can judge from these experiments." When used as strong as 24 pounds in 150 gallons of water, no injury to apple foliage resulted, which is indeed remarkable when the small amount necessary to destroy the insects is considered.

Caustic and Non-poisonous Insecticides.

Having thus traced the introduction of the various compounds of arsenic, and their gradual adoption by agriculturists for the destruction of chewing insects, there still remains the consider

1 Fernald, Mass. Hatch Agric. Exp. Sta. 1894, April, Bull. 24.

2 Ibid. 5.

ation of insecticides which destroy the organism, not by entering its body with the food, but by penetrating the outer coverings directly, and causing the death of the insect only after the material has come in contact with some vital part. There are several materials which possess this power, and some of them have long been in use. Strong alkalies, such as potash or soda, were among the first substances employed for this purpose. The insectidal value of soap is largely due to alkalies which have commonly been applied in this form. The intrinsic value of all soaps has caused them to be used as the foundation for many mixtures. Quassia wood contains an alkaloid which is fatal to insect life, and decoctions of the "chips" have been recommended since the early part of this century. Pyrethrum, kerosene, and resin can also be added to the list, although their value has not been known, in all cases, so long as that of the materials mentioned above.

Pyrethrum first attracted the attention of Europeans early in this century. It had long been sold by the people living south of the Caucasus Mountains in southeastern Asia, the plant being a native of the district. An Armenian named Jumtikoff learned that the powder was obtained from the flower heads of certain species of pyrethrum, and in 1828 his son began to manufacture the powder on a larger scale. It was exported, and at present this industry brings large revenues into the countries in which it is carried on. The species which furnishes the best powder is Pyrethrum roseum (properly Chrysanthemum coccineum); it is not cultivated in Asia, but the flowers of the wild plants are gathered. The Dalmatian powder is produced from Pyrethrum (or Chrysanthemum) cinerariafolium, a closely related species. About 1850, pyrethrum powder was introduced into France for the destruction of insects in houses. In 1856 good seeds were obtained from the Caucasus, and these having been planted, a crop of home-grown seed was secured two years later. Plants of Pyrethrum roseum were grown in America as an ornamental plant at least as early as 1870 ; but the Dalmatian form has been grown for the purpose of producing the highly prized powder. The first to engage in

1 See U. S. Patent Office Rept. Agric. 1857, 129; Ibid. 1861, 223; and Ann, Rept. U. S. Com. of Agric. 1881-82, 76.

this industry was G. N. Milco, a native of Dalmatia, who successfully cultivated Pyrethrum cinerariæ folium near Stockton, Cal. The powder which he made has been sold under the name "Buhach." It is in every respect apparently as good as the imported article, and is even superior to the latter in regard to strength. It is probably the best form to use in this country. Plants of both species were grown in Washington, D.C., in 1881, with satisfactory results. In the spring of that year Riley distributed seeds of the two forms to growers in various parts of the country. In this way the plants have become fairly well known, and the use of the powder has rapidly increased. On account of the cost, it has been used principally in dwellings and in greenhouses. It is most frequently applied in the dry form, but during the past few years it has given good results either when mixed with water, or when the essential oil has been applied after being extracted by alcohol; an infusion of the entire flowers is also effective.

It is difficult to say when kerosene oil began to be valued for the destruction of insects. The oil was undoubtedly used before any records of its insecticidal value were published; and one might suppose, from its nature, that it would possess energetic properties of this character. Turpentine mixed with earth and water was successfully used to destroy worms on trees as early as 1835,1 and it is but a step to pass from this liquid to the use of kerosene. The latter was recommended for the destruction of scale on orange trees in 1865,2 and was also successfully applied to oleander, sago-palm, acacia, and lemon trees. The oil was poured into a saucer and applied by means of a feather. In June of the following year, the Gardener's Monthly recommended this oil for destroying all insect life; but in an issue of the next month, the statement was modified by saying that the vegetable oils were safer. Many others probably had the same experience, for if not applied very carefully, much injury to the foliage may result. It has been the practice, both in Europe and this country, to apply kerosene with a certain amount of water, having one part of oil to twenty-five

1 The Cultivator, 1835, 176, cites M. D. Thosse in Silliman's Journal.

2 Gardener's Monthly, 1865, Dec. 364.

3 Ibid. 1866, June, 176, and July, 208.

of water, more or less. This was applied by means of a hand syringe, and a fairly uniform mixture was obtained by dashing the contents of the filled syringe back into the vessel holding the liquids. Rapid work was the price of a good mixture. This practice is still followed to a certain extent in England, but is rapidly giving way to more desirable methods.

Soap, water, and kerosene can be so thoroughly mixed together that a permanent emulsion will be formed. Although the product may be a comparatively new one, the idea which led to its manufacture is not so recent. As has already been said, soap and water formed the basis of many mixtures. Records can be found showing that nearly all insecticides, especially if they possess much value, have at one time or another been used in connection with soapy solutions. It is simply carrying out the idea that if a certain remedy is effective, its value will be increased if another substance also possessing value be added to it. Thus we find that a correspondent of the Gardener's Monthly says he had used soap water and crysylic acid together, first mixing them thoroughly; and carbolic acid was applied in the same manner.1 The insecticidal value of kerosene once being known, it was very natural that the oil and soap should be used together. The first record that I have found of such a mixture appeared in February, 1875.2 George Cruickshank, of Whitinsville, Mass., here says that he had been fighting the currant worm since 1866, but at first with unsatisfactory results. "In May, 1870, I began using kerosene with whale-oil soap, increasing the kerosene until it would kill the worm and not injure the foliage of the plant. I used 5 pounds of whale-oil soap, and 1 wine quart of kerosene to 25 gallons of soft water to mix. Stir the soap and kerosene together till thoroughly mixed; add two pails of hot water, stir till the soap is dissolved, then add the balance of cold water and it is ready for use. Apply with a syringe with force, in bright sunshine. Where the kerosene and soap was used, I had no worms after two years. In 1873 I had a barrel of the liquid all mixed, and ready for use by the usual time the worm makes his appearance, but could find no worms to use it on." In June of the same year a similar note ap

1T. A." in Gardener's Monthly, 1868, Jan. 11.

2 Ibid. 1875, Feb. 45.