THE MONTHLY BULLETIN. 385 



be measured in the laboratory by means of fine-bore tubes. Professor Gray 

 has devised a method for rapidly testing the relative capillarity of oils. He 

 uses crayon sticks — the common chalk used for writing on blackboards — and by 

 noting the relative heights to which the nils climb and comparing with a 

 water standard he gets a basis for a practical comparison. 



So far factors have been considered that can be determined by a physical 

 analysis of the oil. In order to get the right proportion of light and heavy oil 

 it is necessary to resort to field experiments on the insect it is desired to kill. 

 To be accurate these experiments must be on a large scale. In the case of 

 orchard insects rows, plats or acres should be used to compare different 

 samples and not just units of trees or branches. It has been found by experience 

 that laboratory experiments are out of the question. 



If all oils were identical in composition there would be nothing further to 

 investigate. Unfortunately they are not and we have much more to learn 

 about their toxicology. Petroleum is a complex mixture of many compounds 

 and series of compounds. It has been found that the oils from different 

 fields and even wells in the same field differ radically from each other. It is 

 absolutely unknown which ingredient or ingredients of the oil contain the toxic 

 qualities. It is no doubt true that in many cases where oil sprays have failed 

 and meteorological conditions blamed that it was the lack of some essential 

 ingredient in the oil that was the real cause of failure. Some day when the 

 division of petroleum into its constituent compounds is practical on a large 

 scale, it will be possible, to find out which are the important killing agents. 

 That knowledge will eliminate this factor of doubt. 



In the meantime it is practical to find which groups of oils are most 

 effective. Out of many possibilities the following short list will give examples 

 of different oil types that can be compared. Some of these types will prove 

 uniformly more effective than others, thus evading in a practical way our lack 

 of knowledge of the real toxic elements in the oil. 



1. Pennsylvania paraffin base crude oil. 



2. California asphalt base crude oil. 



3. California paraffin-asphalt base crude oil (Coalinga). 



4. Pennsylvania kerosene. 



5. California kerosene. 



fi. Crude oil with sulphur compounds. 



7. Crude oil low in sulphur compounds. 



8. Crude oil with nitrogen compounds. 



9. Crude oil low in nitrogen compounds. 



10. Crude oil with unsaturated compounds present. 



11. Crude oil with the unsaturated compounds removed. 



12. Stove distillate with cracked products present. 



13. Stove distillate with cracked products removed. 



This list could be indefinitely extended by including oils from different 

 localities, other fractions of the oils, and also distinguishing between some of 

 the unsaturated compounds. Field experiments are practical for a comparison 

 of the effectiveness of these different types of oils. 



Any experiments on the effect of oils on insects must also consider the effect 

 of oils on the host plants on which these insects live. Plant physiologists tell 

 us that petroleum is more or less injurious to plants. Ultimately it may be 

 found that either the compounds in petroleum toxic to insects are the same as 

 those injurious to plants, or it may be found that they are totally different. 

 Whatever that ultimate discovery may be. at the present time it is necessary 

 when spraying plants to dilute the oil with water by some method. The con- 

 centration of oil in water must be the minimum that will kill the insect in 

 order that the injury to the plant may be as little as possible. This fact is 

 • iic of the axioms of insect control. 



This brings us to the subject of the application of the oil to the insect. The 

 problem is simply mechanical where plants are not concerned such as the 

 control of mosquito larvse, household pests, etc. As stated above where plants 

 are involved some method of dilution with water is necessary, and since oil 

 and water are not mutually soluble, it is necessary to resort to the use of 

 some form of emulsion. The simplest form of an emulsion is the mechanical 

 mixture of water and oil. This involves the use of a machine so constructed 



