SOCIETY OF NORTHERN ILLINOIS. 311 



or dissolved arsenic on a tree ; to observe at the end of the season 

 that the fruit was less wormy than on trees not so treated ; and 

 to report the result in this vague, general way, seemed to satisfy 

 the workers of that day. Parallel with my own first work, how- 

 ever, a similar, but smaller series of precise experiments was 

 made during the same season at the New York Experiment 

 Station, the general result in both cases going to prove that at 

 least seventy per cent, of the loss commonly suffered by the 

 fruit grower from the ravages of the codling moth, or apple 

 worm, might be prevented, at a nominal expense, by thoroughly 

 applying Paris Green in a spray with water, once or twice in 

 early spring, as soon as the fruit is fairly set. 



Since the conclusions of my report were published, as Bul- 

 letin No. 1 of the office, and also as an article in my fourth 

 report as State Entomologist, a number of Agricultural and 

 Horticultural Experiment Stations — organized under the opera- 

 tion of the Hatch Act — have taken up the subject of the appli- 

 cation and utility of the arsenical insecticides; have experi- 

 mented carefully in the scientific spirit; and have reported the 

 results of their work with a fullness and system which enables 

 us to discuss them in comparison with each other, and with 

 those of earlier work. As the usefulness of these insecticides is 

 undoubtedly the most important, purely practical subject now 

 before you, I have thought I could do you no better service at 

 the present time than to summarize for you the results of this 

 more recent work, so that all might see just what has been ac- 

 complished, and what remains to be learned hereafter. 



THE COMPOSITION OF THE ARSENITES. 



The poisonous nature of these substances makes it important 

 that all should know just what they are. Two recent analyses 

 made in the Minnesota and Vermont Stations give the composi- 

 tion of unadulterated Paris Green and London Purple. London 

 Purple is essentially composed of arsenic and lime, and Paris 

 Green of arsenic and copper oxide. In the former, we may say 

 in general terms, that the arsenic ranges from forty to forty-five 

 per cent., and in the Paris Green from fifty-five to sixty; while 

 the lime in the London Purple varies from twenty to twenty-five 

 per cent., and the copper oxide in the Paris Green averages 

 about thirty per cent. The other ingredients of these sub- 

 stances are of no horticultural interest. Of the various adver- 

 tised insecticides, slug shot and the so-called peroxide of 

 silicates are perhaps the most widely known. These have been 

 shown by the above analyses to depend for their efficiency 

 almost entirely on about one and one-half per cent, of arsenic, 

 mixed with plaster of Paris in one case, and with land plaster in 

 the other, the arsenic being sometimes present as such, and 



