35 



between the stalks and leaves of water-weeds. The dura- 

 tion of the film depends upon the amount of oil used and 

 its rapidity of evaporation. Heavy crude petroleum gener- 

 ally makes a slow film, but one which may last for weeks ; 

 while the more refined lamp-petroleums, such as kerosene, 

 spread quickly but evaporate sooner. Crude creosote 

 spreads quickly and lasts long, but does not seem to be 

 very lethal ; blast furnace oil spreads and lasts well, and is 

 also very lethal. I speak from experience of the samples 

 tried by me, and cannot answer for other samples. The 

 crude oils are, of course, much cheaper than the refined 

 ones. 



Where the water to be treated is waste water, crude 

 oils making a durable film are the best to use ; but when 

 we are called to deal with drinking or other water in which 

 it is desired to destroy larvae without pollution of the 

 water, evaporating oils are indicated. Thus, fine kerosene 

 or eucalyptus oil will kill the larvae in wells or cisterns of 

 drinking water without imparting a disagreeable flavour. 



The mode of application is often of importance. Oil 

 poured in slowly often does not spread as well as when it 

 is sprinkled violently, or " painted " over the water by 

 means of a rag tied to the end of a stick. 



Numerous similar details are referred to by writers ; 

 but it is scarcely worth while discussing them here, as any 

 man of sense can find out such " wrinkles " for himself. 

 The chief thing to do is to use the oil in a manner to kill 

 the larvae, and not merely to make a film which looks as if 

 it should kill them. It is not unusual, on returning to a 

 pool some hours after an application of oil has been made, 

 to find the larvae just as numerous and happy as they were 

 before. 



