FORMALDEHYDE AS A POISON FOR HOUSE-FLIES. 



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Percent,a^e strength in d/lutfon. 



, . _ . . Flies tasting fluid which remain to drink. 

 , — r^-r^ Percentages killed of those drinking. 



Relative effect of various strengths. 



Deterrent effect of formaldehyde vapour in enclosed spaces. 



Diagram 2. Showing the most effective strength of formaldehyde. 



The Optimum Concentration. 



The conclusion drawn from the experiments just described was checked by the 

 following series of further experiments. In each series concentrations of 10 per cent. , 

 7*5 per cent., 5"0 per cent., and 2 '5 per cent, formaldehyde, with 50 per cent, lime water 

 and 2*5 per cent, sugar, were placed in saucers with blotting paper covers and these 

 were exposed to flies in cylinders (10 in. diam., 8 in. high). Each experiment was 

 continued for twenty-four hours, and counts were made at intervals of the flies lying 

 inert. Seven series were completed, and about 100 flies were used in each cylinder. 

 The summary of the whole is given in Table XIV. In this are shown the average 

 number of flies in each cyHnder, and the average percentages inert at the end of 

 various intervals. These results are plotted in Diagram 3 ; the dot-and-dash line 

 represents 2 '5 per cent, formaldehyde; the continuous one 5 per cent. ; the hne of 

 dashes 7 '5 per cent.; the dotted line 10 per cent. Although recoveries are only 

 (681) e2 



