COKN OIL 383 



has been used, in order to commingle detached surface sheets of oil. In some of 

 his early work on Staten Island, Dr. Doty, the Health Officer of 'New York, used 

 a pump with a submerged discharge, throwing the oil out at the bottom of a pool 

 and allowing it to rise to the surface. It seems that the idea was to destroy the 

 insects feeding at the bottom more quickly, but as most mosquito larvae rise 

 to the top to breathe, there is practically nothing to be gained by such a method. 

 The use of larvicides in tropical regions introduces certain new features which 

 complicate the problem of mosquito destruction to a certain extent. Colonel 

 Gorgas, and his corps of workers at Panama, have been using petroleum very 

 extensively. Just as they did at Havana. They find, however, that at Panama the 

 rapid growth of vegetation prevents the oil from spreading uniformly and that 

 it can not make a thin uniform film over the surface of water in which vegeta- 

 tion grows. They find also that algae on the surface of the waters form with 

 the oil a dark scum, which collects at the bottom of shallow pools. This scum 

 later breaks up and floats about on the surface, rendering succeeding oilings 

 less efficacious, and necessitating the use of larger quantities of oil. They also 

 find that where vegetable debris collects in a large body of water it will be blown 

 about as a mass, its location changing with the wind. Mosquito larvae also hide 

 in this vegetation, which protects them from fish. The wind blows the oil to 

 one side of the surface and, of course, where the film is not perfect the larvae find 

 free places to breathe. It evaporates very rapidly in the tropics. During the 

 rainy season it is washed away very rapidly before it destroys all of the larvae. 

 The bulk of the oil and the cost of transportation in rough territory for work 

 on a large scale are disadvantages. In such work they found that they must 

 constantly occupy themselves in removing vegetation before oil is applied, in 

 order to avoid using excessive amounts of oil. They found that new growths of 

 algse developed very rapidly after the oil had united with the previous crop and 

 sunk to the bottom. 



CORN OIL. 



Some effort has been made to find if there are any other oils that may be used 

 to better advantage than petroleum. A suggestion was once made by Mr. W. 

 J. Matheson that corn oil might be used. This is a substance which is made 

 rather extensively in certain parts of the country and which, considering the 

 enormous crops of com grown in western States, which in fact are so great that 

 sometimes in years of overproduction corn is burned as fuel, might reasonably 

 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 under- 

 taken in 1900 indicated that corn oil does not spread readily. It gathers to- 

 gether 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 can breathe 

 comfortably and live. 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 non-spreading qualities, however, as well as its price, removes it from prac- 

 tical consideration. 



