468 SCIENCE PROGRESS 



ten inches, then our pint of larva poison would be " watered down " to a 

 proportion of i in 15,000, and the requisite strength of i in 1,500 could 

 only be secured by using ten pints of the poison instead of one. 



Quite generally, if d be the depth of the water in inches, n the number 

 of parts in which one part of the given larva poison is effective, and c 

 the ratio of its price to that of paraffin, then, if the cost of treating a given 

 area by paraffining is taken as unity, the cost of treating the same area by 

 the poison is given by the expression 



1500 X c X d 

 n 



As an illustration of the use of this formula, we may take the case of 

 cyllin, the only larva poison recommended in a recent Government publica- 

 tion, the degree of dilution specified being a teaspoonful to a gallon, a pro- 

 portion which works out at i part in 1,280. The price of cyllin being about 

 six times that of paraffin, the relative cost of employing it in the strength 

 given would be 



1500 X 6 

 -128^ ^ ^' 



or almost exactly 7 x d times that of treating the same area with paraffin. 



If, for example, the depth of the water were 15 inches (a very usual 

 figure) the cost of the cyllin required would be more than 100 times that of 

 the necessary paraffin. 



For the treatment of water-places to which, owing to inaccessibility or 

 any other reason, frequent visits of inspection are inconvenient, the adop- 

 tion of the poisoning method, which produces a more lasting effect than the 

 other, is sometimes advisable. In cases, too, where excessive growths of 

 water-weeds prevent the creation or maintenance of an effective oil film, 

 larva poisons are often indispensable ; and it is not difficult to imagine 

 other situations in which the advantages gained by their use may amply 

 justify the additional expense. 



John F. Marshall. 



