78 PREVENTIVE AND REMEDIAL WORK AGAINST MOSQUITOES. 



tages over crude oil : It acts as a poison and kills the larvae very rap- 

 idly. It brings the larvae out of their hiding places at once and is 

 useful as an aid to the detection of the presence of mosquito larvae. 

 It is found also that in continuous heavy rains the larvae are killed 

 by the phinotas oil before the rain dilutes the treated water to any 

 great extent. They find, however, that phinotas oil has certain dis- 

 advantages: It kills fish in a solution of 1 to 5,000, and it loses its 

 efficiency very soon after application, so that eggs are laid upon the 

 treated water quickly and the larvae develop. Doctor Gorgas points 

 out that there is considerable variation in the quality of this substance 

 as shipped to the Isthmus. Some barrels will kill larvae quickly in a 

 solution of 1 to 3,000 parts of water, while other lots require for the 

 same results 1 part to 1,000. Doctor Gorgas has recently published 

 a list of the desiderata for the perfect larvacide for use in the Tropics, 

 agreeing with the opinion expressed by Konald Ross when he re- 

 turned from his first visit to Africa that nothing as yet known is per- 

 fectly satisfactory : 



(a) Low ultimate cost. 



(b) Ability to affect and kill mosquito larvae promptly, the more rapidly the better. 

 It must be effective in moving water as well as in still water. 



(c) Ability to form a solution with water and to thoroughly diffuse and mix with all 

 the water of a small pond if applied only to one part thereof. Also the substance must 

 not lose its larvicidal properties for a week or more after its application. The longer 

 it will retain its larvicidal properties after it has been placed in the body of water the 

 more valuable it will be. 



(d) Ability to diffuse in water and through all parts of a body of water such as in a 

 pond containing grass, water lilies, other aquatic vegetation, and vegetable debris. 



(e) Ability to kill green algae promptly. 



(f) A concentrated larvicide is necessary so that one part of it to five thousand or 

 more parts of water will promptly kill mosquito larvae and pupae. 



(g) Nonpoisonous to human life or animals when taken in a strength of 1 to 1,000 

 and accidentally used as drinking water. 



(h) That it have the property of discoloring the water to which it is applied, or of 

 giving off sufficient odor to induce persons not to use water containing it in solution 

 for drinking purposes. 



(i) That the odor, if present, be not so obnoxious as to make its presence in water in 

 ponds or streams near habitations undesirable. 



(j) That it shall have a safe flash test and be nonexplosive. 



(k) That it shall be sufficiently stable so that it may be kept "standardized." 



Decoctions and emulsions of Derris uliginosa have been recom- 

 mended for larvicidal use, but experiments conducted at the Well- 

 come Research Laboratories at Khartoum show that while it has con- 

 siderable potency it also kills fish, and that even in regions where 

 these plants are native the different species of Derris have only a 

 limited use as insecticides. 



During the 1905 outbreak of yellow fever in New Orleans an at- 

 tempt was made to destroy mosquito larvae in the open gutters of 

 the city by the use of common salt. Dr. H. A. Veazie wrote us that 



