FORMALDEHYDE AS A POISON FOR HOUSE-FLIES. 



51 



It is therefore advisable always to add as much alkali to the formaldehyde as 

 possible, without deterring the flies, to neutralise any formic acid which may be 

 present, or any which may be formed by oxidation during the exposure of the solu- 

 tion. Table VII shows the benefit of the procedure. 



Table VII. 

 Showing the Advantages of adding certain Alkalis to Acid Formaldehyde. 



Solution, 10% iu each case 



Percentages killed (actual 

 numbers in biackets) 



Control formaldehyde (125 grms. HCOOH per lit.) 

 ,, +50% lime water in dilution 



,, +K2CO3, 15 grms. per. lit, 



(equiv.). 

 „ +(NH4).2C03, 10 grms. per lit. 



(equiv,). 



100 in each case 



283 (155), 241 (82) 



300 (36), 268 (147), 270 (92) 



68 (37), 82 (28) 



Control formaldehyde (1-8 grms. H'CO'OH per lit.) 



+ 100% lime water in dilution 

 + 50% 

 + 

 + 1% NH4OH 



Control fo 



1% K,C03 



maldehyde (1-1 grms. H'CO-OH per lit.) 

 , +50% lime watei in dilution 



> I ■^" /O >' " '^ " 



100 in each case 



76 (47), 90 (923) 

 114 (2(59), 104 (312), 142 (57) 

 125 (50) 



50 (20) 



100 in each case 

 131 (297), 124 (31) 

 110 (28) 



It will be seen that the addition of the alkali improved the effect in every case 

 except when the whole fluid of dilution was lime water, which made it too strongly 

 alkaline, and when ammonia was used, methylamine being formed in those cases. 

 Lime water is recommended in practice in preference to other alkalis because it is 

 a weak solution of definite strength. It is prepared by allowing cold water to stand 

 over lime, slaked or unslaked, for a few hours and then filtering the solution before 

 use. Such a solution contains 0*14 per cent. Ca(0H).2. If made with hot water 

 the solution is weaker. The maximum amount, half the total fluid of dilution, 

 should always be used to counteract the formic acid which would otherwise be 

 formed after dilution, during exposure to the air, or by the bodies of dead flies 

 falUng in. In a trap which will be presently described, but an imperfect model, 

 which was exposed in the fly room, this was very clearly shown. The fluid in the 

 trap was 7 '5 per cent, formaldehyde, 35 per cent, lime water (equivalent to the acid 

 present), and 5 per cent, sugar. On the first three days this trap killed 1,800 flies, 

 and then became ineffective. Four flies had crept into the poison, and when this 

 was titrated it was found to have become +2 acid. An equivalent of KOH was 

 added and it became again effective. 



4 



Humidity of the Atmosphere. 



A test was made at a temperature of 96° F., and the poison was found to be 

 effective. There is no reason apparent why it should not be effective in a tropical 

 country. The humidity of the air, however, plays a considerable part, as indeed 



