DESTRUCTION OF LARV^. 75 



be supposed to be a cheap oil. This, however, is not the case, and 

 its price is prohibitive as compared with ordinary grades of kerosene. 

 Experiments undertaken in 1900 indicated that corn oil does not 

 spread readily. It gathers together in large patches on the surface 

 of the water, and mosquito larvae rising to the surface and finding 

 themselves under a patch of oil will simply wriggle violently until they 

 find the spaces between the patches where they breathe comfortably 

 and live for several days. In this experiment the object was not 

 only to secure a cheap and efficient oil, but to secure a persistent oil 

 which will not evaporate and which will remain for at least several 

 weeks over the surface of the water. Its nonspreading qualities, 

 however, as well as its price remove it from practical consideration. 



To sum up the whole question of larvicides, nothing has been 

 found more satisfactory as regards efficiency and price than common 

 kerosene of low grade, or better still, that grade known as fuel oil. 

 This conclusion has not only been arrived at in the United States, 

 but elsewhere, although petroleum has been more extensively used 

 in the United States than elsewhere, and it is better understood in 

 this country. In choosing the grade of the oil, two factors are to be 

 considered. First, it should spread rapidly; second, it should not 

 evaporate too rapidly. The heavier grades of oil will not spread 

 readily over the surface of the water, but will cling together in spots 

 and the coating will be unnecessarily thick, as in the case of the corn 

 oil just mentioned. The rapidity of spread of film is also important. 

 Ronald Ross, in his ^'Mosquito Brigades," pages 34 to 35, makes the 

 following statement : 



Mr. Hankins of Agra informs me that the addition of amyl alcohol greatly expedites 

 the formation of the film; and it is very necessary to obtain a film which makes its 

 way between the stalks and leaves of water weeds. 



Earl}^ in the course of antimosquito work in the United States 

 careful experiments were made by Mr. W. C. Kerr, in the work of 

 the Richmond County Club, on Staten Island, to which we have 

 referred before. He tried several grades of oil and found a low grade 

 of oil known as ''fuel oil" to be best adapted to the work. Of the 

 oils which he tried, some contained too much residuum of a thick 

 nature, which appeared as a precipitate and could scarcely be 

 pumped; some were too thick in July weather and could not be 

 pumped at all, while some were limpid, easily handled, made a good 

 uniform coating on the ponds, and were very effective. So long as 

 oil flows readily and is cheap enough the end is gained, provided it 

 is not too light, and does not evaporate too rapidly. The grade 

 known as light fuel oil was recommended by the writer to the United 

 States army workers in Cuba at the close of the Spanish war and was 

 found to be effective. The price of oil of this kind has varied from 

 $2.25 per barrel to $3 per barrel, f. o. b. Philadelphia. 



