58 



CAPT. LL. LLOYD. 



compared to those which tasted ; and the percentages of those temporarily and 

 permanently incapacitated compared with those which drank. The calculation of the 

 numbers of those which died against those which drank is more satisfactory than 

 calculating them against those which only tasted. The numbers thus obtained are 

 frequently above 100, as at a strength of 5 per cent, and upwards the amount taken 

 by a fly in its taste is often enough to kill it. Keference to the table will show that 

 not only the averages, but also the maxima and minima, run in series corresponding 

 to the strength of the poison, allowance being made for occasional variation due to 

 experimental error. The percentage of those which died compared to those which 

 drank gives the factor of toxicity to each strength of formaldehyde. These factors 

 are plotted in Diagram 2 and are represented by the Hne of dashes. The toxicity 

 increases rapidly from 15 percent. upwards, and falls rapidly from 5 percent. down- 

 wards, the rise from 5 per cent, to 15 per cent, being gradual. The dot and dash curve 

 is taken from Diagram 1, with the addition of the factors for 15 per cent, and 20 per 

 cent, strengths which are taken from Table XIII. This curve represents the per- 

 centage of tasting flies which remain to drink a solution of formaldehyde containing 

 50 per cent. Ume water and 2 '5 per cent, sugar. It is the curve of deterrence. By 

 combining these two curves a hypothetical curve is constructed which is represented 

 in the diagram by the continuous Hne. For example with a strength of 10 per cent, 

 formaldehyde exposed in the open, 47 per cent, of the flies that taste it will remain to 

 drink. Some however will absorb enough poison in the taste to kill them, and the 

 number which finally die is not 47 per cent, of the tasting flies but 133 per cent, of 47, 

 that is, 62'5 per cent, of the flies that taste die. The relative effects of different 

 strengths of formaldehyde, as represented by the curve, were thus found. It is a natural 

 curve with its maximum at 5*0-6'0 per cent, formaldehyde, and this is therefore the 

 optimum strength to use in practise. The straight dotted line in the same diagram 

 represents, as explained above, the deterrence of the vapour at different strengths in 

 enclosed spaces. Its tendency is to recommend the use of as weak a solution as- 

 possible, but the dechne in the effect below 5 per cent, is so rapid that this factor could 

 play no part in the decision as to which strength to use. 



Table XIII. 

 Showing the relative Toxicity of various strengths of Formaldehyde. 



Solution 



Tests 

 no. 



Total 

 flies 



Av. 



no. 

 flies 



% of flies 

 tasting 



%-y of tasting 



flies which 



drank 



% to 



drinking flies 



50% lime 



water, 2-5% 



sugar 



Down in 

 1 hour 



Dead after 



9 liom s 





Av. Max.Min. 



Av. Max. Min. 



Av. Max. Min. 



Formald. 20% 



15% 

 10% 



7'50/o 



5-0% 

 2-5% 



i 

 4 ; 



3 ! 



4 ; 



4 

 4 



154 

 138 

 220 

 182 

 219 

 174 



39 

 44 

 55 

 45 

 55 

 43 



43 67 30 

 41 53 35 



53 55 45 1 

 59 69 49 

 61 65 53 i 

 66 80 54 i 



20 25 15 

 30 36 25 

 54 67 43 i 

 68 76 63 ! 

 86 93 79 ! 

 91 96 87 1 



229 

 146 i 

 136 ! 

 Ill 

 106 

 51 



219 300 125 

 143 150 140 

 133 156 110 

 112 136 • 87 

 103 121 86 

 39 62 23 



