•27-t InsecticldeH from a Chemical Stand2)oiiit 



be taken, of ascertaining what particular characteristic of that com- 

 pound is primarily responsible for the results obtained. For example, 

 in the absence of proof to the contrary, it is not wise to assume that, 

 because tar substances possess insecticidal properties, the latter are 

 wholly dependent upon the presence of phenolic compounds. In the 

 case of coal-tar disinfectants it was the practice until recent times to 

 determine the germicidal value by a chemical estimation of the phenolic 

 content alone, whilst no account was taken of the non-phenolic con- 

 stituents. It has since been shown, by actual tests on living cultures 

 of bacteria, that some of the non-phenolic constituents may possess 

 an even higher germicidal value than the phenols themselves, and, 

 in consequence, germicidal values of coal-tar disinfectants, based on 

 chemical determinations, have fallen into disrepute. This would have 

 been avoided had a careful examination of the factors which influence 

 the germicidal action of a coal-tar disinfectant been made. 



It follows, therefore, that until many, if not all, of the factors which 

 determine the efficiency of an insecticide are known, the observer is 

 liable to draw similarly erroneous conclusions. 



Hitherto the economic entomologist in his search for suitable 

 insecticides has confined himself to those compounds which to him 

 appeared to be cheap and easily obtained, but it seems to us that a 

 wide field still remains unexploited, in which, nevertheless, these 

 essential conditions obtain. 



There are many organic compounds of probably high insecticidal 

 value, which are readily prepared in the laboratory, and which, if a 

 suitable demand arose, would be commercially possible. The choice 

 of these for experimental trials should not be made in a haphazard 

 manner, but should be preceded by a systematic investigation, so 

 arranged as to determine what particular characteristic conferred the 

 insecticidal property. Two illustrations will suffice to make our meaning 

 clear. 



It has been found that xylene is the most effective of the tar hydro- 

 carbons in destroying wood-boring beetles. To what particular pro- 

 perty is this superiority due? Does some special grouping of the atoms 

 in the molecule play a part, or is it a purely physical question ? If it is 

 a special grouping of the atoms which endows xylene with such a degree 

 of insecticidal efficiency, then it is reasonable to suppose that other 

 compounds with a similar molecular constitution would be equally 

 effective. If, on the other hand, the insecticidal characters of xylene 

 are dependent on its pliysical j)roperties, then other fluids with icliMiticnl 



