460 journal of economic entomology [vol. 10 



Combinations 



This test has been applied successfully to aqueous solutions of 

 "free" nicotine, nicotine sulphate solutions, nicotine-soap solutions, 

 nicotine-arsenate of lead, and nicotine-Bordeaux. It has not been 

 found effective in testing nicotine-lime-sulphur, as the presence of 

 colloidal sulphur derived from the polysulphides seems to interfere with 

 the test. 



Discussion 



The boihng point of nicotine is 247° C. (447.8° Fahr.). This is 

 particularly significant in view of the popular belief that nicotine 

 evaporates much more rapidly than water and that when the spray 

 has dried and is no longer visible on the plants, the nicotine has by that 

 time also disappeared. 



When nicotine solutions are used for fumigating greenhouses, there 

 are generally two or three more ox less distinct periods of boiling. 

 When aqueous solutions of nicotine are used, there are two distinct 

 periods, in which the water boils off first and later the nicotine. When 

 alcoholic solutions are used, there are three more or less distinct periods 

 of boiling: First alcohol, then water, then nicotine. 



It is true that upon evaporation, under ordinary temperatures, 

 concentrated solutions of nicotine become stronger. A sample of 

 nicotine sulphate containing 40 per cent nicotine kept in a tumbler 

 exposed at ordinary room temperatures for about three months showed 

 at the end of that time a nicotine content of 49.46 per cent. A small 

 sample of "free" nicotine left in a shallow dish at ordinary room 

 temperature for two weeks increased in nicotine content from 40.71 

 per cent to 94.82 per cent. 



In evaporation of dilute solutions under ordinary temperatures, 

 probably the same general condition exists; namely, that the water 

 evaporates much more rapidly than the nicotine, resulting in a con- 

 tinually increasing concentration of the nicotine film on the sprayed 

 parts of the plant, until finally a very highly concentrated though 

 invisible film of nicotine remains. The actual amount of nicotine 

 left may be so small as to defy any attempt to determine it quanti- 

 tatively and yet may show quite distinctly in the qualitative test. 



How this film would work as an insecticide is as yet a matter of 

 conjecture. The general opinion has been that the film may act as a 

 stomach poison in being eaten by chewing insects. On the other 

 hand, some believe the "odor" or fumigation would have some effect. 

 All of these are possible, but it is also quite possible that this film is 

 effective as a direct contact insecticide, on larvae as well as other 

 stages. As the film is very much more highly concentrated than the 



