juneio, i9i8 Physical Properties Governing Contact Insecticides 529 



materials. Second, emulsions made with Castile or soft soap adhered 

 to the insects and spread over the body, penetrated the tracheae, carrying 

 the emulsified oil with them. Third, emulsions made with using Ivory 

 soap adhered to the body of the insect; but, owing to the high cohesion 

 of the liquid, spread very slowly. In the third case it was noticed that 

 in a comparatively short time, depending upon the temperature and 

 humidity of the surrounding atmosphere, the water evaporated and the 

 emulsion broke down, after which the oil spread over the surface of the 

 body, penetrating the tracheae. The length of time required for the 

 breaking of the emulsion was from 5 to 30 minutes. If the oil was rather 

 volatile, it would evaporate before it succeeded in penetrating the 

 tracheae. By spraying a number of insects it was determined that oils 

 more volatile than xylene were too volatile to succeed in penetrating 

 the tracheae in large enough quantities to result in the death of the insect. 

 On the other hand, the oil may be so viscous that, even after the breaking 

 of the emulsion, it is unable to spread over the body and enter the 

 tracheae in a reasonable length of time. The viscosity of a number of 

 oils was determined in terms of water by measuring the length of time 

 required for 5 c. c. of the liquid to flow through a glass tube of small 

 diameter arranged in the form of a stalagmometer. The results of these 

 are shown in Table II. 



By placing a small drop of oil on the wing of a cochroach and watching 

 it under t^ie miscroscope it was possible to divide the oils into four classes: 

 (i) Those spreading rapidly, (2) those spreading slowly, (3) those spread- 

 ing very slowly, and (4) those spreading so slowly as to preclude any 

 possibility of their reaching the spiracles in a reasonable length of time. 



The oils in Table II are divided into these four groups. It is apparent 

 that oils with a viscosity as high or higher than castor oil are so viscous 

 that their value as contact insecticides is questionable. 



