FORMALDEHYDE AS A POISON FOR HOUSE-FLIES. 



49 



The results of the tests are summarised in the next table. In nearly every case 

 the purer sample proved the more effective. No suggestion can be given for the 

 purification of formaldehyde of this nature, and it should be avoided if any other 

 is obtainable. 



Table III. 

 Co7nparing a Yellow Formaldehyde with Fishy Odour (Methylamine) with a Clean 



Sample. 



Solution 



Percentages of flies 

 killed 



Average 



o 

 /o 



Total flics 

 kiUed 



Formaldehyde control, 

 ,, impure, 



lOo^ 



100 in each case 

 134, 67, 96, 89, 230, 36, 

 17,68,58. 



100 

 89 



1509 

 1118 



Formic acid, H'CO'OH. 



Formaldehyde is prepared by the dehydrogenation of methyl alcohol : CH3OH + 

 = CHoO + HgO. Theoretically in the process there is no formic acid formed. The 

 samples of formaldehyde examined, however, were all found to be acid. Three 

 which were titrated were found to be +24, +39, and +60 acid (Eyre) respectively. 

 (A fluid " +x acid " requires x ccs. of normal alkali to make one htre neutral,) 

 The acid present was proved to be formic by a test suggested by Dr. A. M. Whiteley. 

 A few CCS. of the formaldehyde are shaken up with lead carbonate and quickly 

 filtered. If formic acid is present crystals of the sparingly soluble lead formate 

 separate out in the filtrate after a short interval. The crystals are minute, very 

 characteristic needles. On the assumption that formic was the only acid present, 

 these samples contained 1*1, 1'8, and 2*76 grms. of formic acid per litre respec- 

 tively. When the dilute poison 5-10 per cent, formaldehyde is made up the acid 

 would be present in amounts varying from '0055 to *0276 per cent. Even these 

 small quantities are detected and disliked by flies. 



Table IV. 

 Showing the Deterrent Effect of Formic Acid in Water. 



Solution 



No. of Av. flies 

 tests tasting 



Av. flies 

 drinking 



Av. percentage 

 which drank 



Tap water control 



Formic acid -1% 

 •Of^o/ 



•02 -04% 

 •01 -005% 



2 

 1 

 3 

 4 

 3 



35 

 33 

 32 

 36 

 36 



24 



3 



15 



2!0 



18 



69 

 10 

 47 

 54 

 50 



Temperature 77-9° F. ; relative humidity 57%. 



Testing saucers, two inches in diameter, were filled with solutions of formic acid 

 in water and covered with discs of blotting paper to soak up the fluid. These 

 were then exposed one at a time in the fly room and watched for five minutes, a count 



