LECTUKE3 AND ESSAYS READ AT INSTITUTES. 24T 



The consideration of the above facts led me to the conclusion that it was 

 desirable to make careful experiments to determine definitely, if may be, just 

 the extent of the danger involved in the practices referred to above, that all 

 might be advised of their safety or warned of their danger. 



I first experimented to find the amount of the poisons necessary to destroy 

 the insects. I found that potato beetles, currant slugs, and cabbage caterpil- 

 lars would all be destroyed by use of London purple or Paris green — I have 

 failed to detect that one of these arsenites is superior to the other — mixed with 

 plaster in the proportion of one part in bulk of the poison to 100 parts of the 

 adulterant, and dusted on to the leaves of the plant just so it can be easily 

 seen. I also found that a heaping spoonful of the arsenites — 14 grammes or 

 ^ ounce of London purple, or 16 grammes, a little more than ^ ounce, of Paris 

 green — to the gallons of water, when first applied to the foliage, which must 

 be thoroughly wet, was quick death to the insects. I found that even a more 

 dilute mixture — one pound of the arsenites to 100 gallons of water — was 

 effective if forced on with the fountain pump, which throws the liquid with 

 considerable force. We see by the above that it takes very little of the 

 poisons to destroy insect life. 



I next sought to determine how long after the poison was applied before it 

 would cease to be effective. In one case, the insects were placed on the plants 

 immediately after the application of the poisonous mixture, and in every case 

 were killed. A brisk shower, even if it occurred immediately after the appli- 

 cation, rendered the poison inoperative. In all such cases, the insects ate the 

 foliage without harm. Plants out of doors, entirely sheltered from the rains, 

 were eaten by the insects ten days after the application of the insecticides with 

 but little apparent harm to the eaters. In all the cases that I tried, the 

 insects seemed to suffer no harm from eating the foliage twenty days after the 

 poison was applied. The same might be said with very slight exception for 

 fifteen days. The plants were all sheltered from the rain. I next treated the 

 plants to the poisons and kept them indoors, where no wind could strike them. 

 I then found that the insects were not poisoned as speedily as at first, yet I 

 found that even after twenty days the poison did destroy some of them. 



I next sought to find the reason that the insects were not poisoned by eating 

 the foliage fifteen or twenty days after the arsenites were applied. This was 

 not difficult, as an examination with the compound microscope quickly 

 revealed it. Examination of the leaves that had been washed by the rains 

 showed liardly a trace of the poisons. The faintest trace, owing to the 

 decided color, was quickly detected. Leaves that had been freely exposed to 

 the winds showed no green or purple, except in places where the cuticle of the 

 leaves had been destroyed, or where some o^her depression had formed a lodg- 

 ment for the poisons. I need say no more to show how slight this amount 

 was than to call attention to the fact of the very slight amount of the poison 

 necessary to destroy the insects, and to the further fact that in this case there 

 was not sufficient to injure the insects at all. The plants that are kept out of 

 the wind, as in close rooms, show much of the poisons even after twenty days. 

 The amount, however, is much less than when first put on. I found that by 

 blowing on the plants, or jarring them, I could remove nearly all of the 

 poisons. I could accomplish the same result readily by sprinkling the plants. 



The necessary conclusion, then, is that the poison is all removed by 

 mechanical means. The elaborate analyses by my colleague, Dr. E. C. 

 Kedzie, some years since, showed conclusively that the poisons are not 

 absorbed by the plants. 



