ESSAYS 467 



ordinary paraffin is probably the best oil to use ; this, when sprayed on the 

 water surface, forms a uniform and sufficiently lasting film.^ 



As regards larvicides of the " poisoning " class, a large number of substances 

 have been recommended (chiefly by American writers), but in the majority 

 of cases definite figures are difficult to obtain. In some instances where 

 full particulars have been given experiments carried out at Hayling have 

 demonstrated that either the advocates of the larva poisons in question are 

 unduly optimistic, or that the resisting power of the average English larva is 

 unusually high. As an example of this, a mixture of carbolic acid, caustic 

 soda, and resin, compounded according to an American recipe, and alleged to 

 be effective in the proportion of i in 10,000, was on one occasion added to the 

 water of a larva-infested tank up to a strength of i in 4,000, with the result 

 that the majority, if not all, of the larvae were as active as ever eighteen hours 

 later. Again, the addition of one part of sulphate of copper to 5,000 parts of 

 water has been extensively recommended, but experiment shows that, when 

 used in this proportion, it has no larvicidal effect whatever. 



The use of larva poisons is obviously inadmissible whenever any likeli- 

 hood exists of the water so treated being drunk by men or animals ; and in all 

 cases in which expense has to be considered, the superiority of the paraffining 

 method is overwhelming, as the following considerations will show. 



Owing to the scarcity of information regarding the approximate amount 

 of oil required to treat a given area of water surface, experiments were made 

 at Hayling early in 192 1 to ascertain the expenditure of paraffin (as well as 

 of time) involved in " filming " measured stretches of a larva-infested ditch 

 of considerable length and of an average width of three feet. An ordinary 

 pneumatic sprayer of the " one-handed " type was used, and the result 

 of several trials showed, in round figures, that one pint of paraffin was suffi- 

 cient to treat forty yards run of the ditch [i.e. to cover forty square yards of 

 surface) in the average time of two minutes. These figures were confirmed 

 on subsequent occasions, total destruction of the larvae being observed in 

 each case. 



Now, since a pint is equal to 34*659 cubic inches, the result of distributing 

 this amount of paraffin over a surface of 40 square yards is to produce a film 

 having a thickness of 



— - inches, 



40 X 9 X 144 ' 



which reduces to almost exactly tz\^ of an inch. 



Still considering our forty square yards of surface, let us now suppose our 

 pint of paraffin to be replaced by a pint of some " larva poison," and let us 

 furthermore suppose the depth of the water to be one inch. Under these 

 circumstances it is obvious that the poison would be mixed with the water 

 in the proportion of i in 1,500, and if we suppose, for the sake of argument, 

 that this were the correct dilution for the particular poison concerned, the 

 larvae would be destroyed just as well (though not necessarily at the same 

 cost) as they would have been by the paraffin film. If, however, we were to 

 repeat the process at a place where the depth, instead of being one inch, were 



1 The addition to the oil of small quantities of other liquids to increase 

 its " spreading power " has been advocated, amyl alcohol and asphalte 

 varnish being among a number of liquids recommended. Experiments 

 made by the writer with the latter liquid showed, however, that its addition 

 to paraffin in the proportions specified actually decreased the spreading 

 power. By kind permission of the Headmaster of one of our leading schools, 

 a series of experiments relating to the spreading qualities of different oils 

 and to various other matters connected with anti-mosquito work are now 

 being carried out in the school laboratory by some of the students. 



