W. F. COOI'KR AND W. 11. NUTTALL 277 



poly-sulphides is probably largely responsible. But the necessary 

 quantity of carbon-dioxide to effect the precipitation of sulphur from 

 commercial potassium sulphide is very large ; consequently the process 

 is slow, and probably some weeks elapse before the beneficial effects of 

 precipitated sulphur come into play. It is, however, not a difficult 

 matter to prepare solutions from which sulphur can be precipitated in 

 an exceedingly finely divided state by a mere trace of carbon-dioxide, 

 and there is little doubt that these would be found more effective than 

 ordinary sulphide preparations. 



The efficiency of many insecticides is entirely dependent on the 

 fact that they suft'ocate the insect by blocking up its breathing orifices. 

 Of this type, viscose suggests itself to the chemist as a most excellent 

 material. Viscose is a derivative of cellulose, which is extensively 

 employed in the manufacture of artificial silk. It is characterised by 

 the fact that even in dilute aqueous solution it forms an extremely 

 viscous liquid, which by exposure in thin layers gives a tenacious and 

 impermeable film. Obviously, viscose preparations could not be used 

 for spraying the entire foliage of trees, but as a winter wash or as a 

 local application, in the case of scale, such are certainly worthy of 

 trial. 



So much then for the directions in which a search for new and 

 improved insecticides might be made. Many more could doubtless 

 be suggested, but the foregoing will suffice to indicate the wide field 

 awaiting inquiry. 



Quite apart from the quest for new substances, there is another 

 point, which we take this opportunity of emphasizing, because it is one 

 which many economic biologists are apt to lose sight of, in the selection 

 of remedies for the eradication of insect and fungoid pests. This is 

 the extraordinary importance of the physical condition of the insecticide 

 in its influence on the efficiency. Attention has already been drawn to 

 the fact that a chemical assay of a carbolic disinfectant is really no 

 measure of its bactericidal power, because it is based on the false assump- 

 tion that only phenolic compounds have bactericidal properties. But 

 the chemical assay of a disinfectant fails for another reason, and that 

 is because it takes no account of the physical condition of the active 

 compounds. It has long been known that, of two preparations con- 

 taining equal percentages of the same tar acid, emulsified in the one 

 case, and in a state of simple solution in the other, the emulsified 

 preparation possesses a much higher bactericidal power than that 

 containing the tar acid in solution. Chick and Martin {J own. of Hygiene, 



19—2 



