278 Insecticides from a Chemical Standpoint 



VIII, 5, pp. 698-703) have shown this to be due to the fact that the 

 bacteria are adsorbed on to the fine emulsified particles of the tar acid 

 and are thus brought into direct contact with the germicidal agent in 

 a highly concentrated form. Now, whatever may be the cause, there 

 is little doubt that, of liquid insecticides, an emulsified one is far more 

 efficient than a non-emulsified one. Whether Chick and Martin's 

 explanation holds good in this case also, is questionable ; possibly, with 

 very minute insects, adsorption may play some little part. Where, 

 however, an emulsified insecticide is undoubtedly vastly superior to a 

 non-emulsified one is in its wetting-power, and with insecticides this is 

 a point of paramount importance. It is needless, here, to point out the 

 extreme difficulty of wetting the chitinous integument of an insect, or 

 the protective woolly secretion of the aphis of American blight ; nor is 

 it necessary to dwell upon the fact that an insecticide cannot do its 

 work unless it actually comes in contact with the insect : such are self- 

 evident to all who have to deal with entomological pests. Perhaps 

 what is not so generally recognised is the fact that different liquids have 

 very different wetting powers. A simple experiment serves to illustrate 

 this. The American cloth back of an exercise book may represent, 

 sufficiently well for the purpose, the chitinous integument of many 

 insects. When water is poured on to the back of such a book it runs 

 into droplets and flows off without wetting it, precisely as water would 

 run off the integument of the insect. If, however, an emulsion is 

 poured on to the book, the whole surface is completely wetted, and the 

 liquid forms a continuous film. If, now, the emulsion is washed off by 

 a stream of water, the unwettable character of the surface again becomes 

 evident, the water flowing into droplets and running off. A liquid 

 insecticide, in order to be efficient, must be capable of thoroughly 

 wetting the insect and its surroundings. It must "rwn" easily and be 

 able to penetrate the interstices of the buds, the folds and crinkles of 

 the leaves, waxy secretions, hairy or woolly surfaces, the minute orifices 

 of the insects themselves, etc. ; and experience has shown that liquids 

 of low surface tension, e.g. emulsions, are best suited to this end. 



There is an additional advantage in the use of an emulsion, which 

 was brought out in a pertinent manner in some work carried out by one 

 of us (W. F. C.) in South Africa four or five years ago. We were then 

 concerned with the eradication of the Bont Tick, which infests cattle 

 and transmits the fatal disease — heart -ivater. Sodium arsenite had 

 long been used as a means of destroying these ticks, but it was found 

 that, at the strength necessary to kill the ticks, the cattle themselves were 



