New York Agricultural Experiment Station. 251 



began to drip slightly. By so doing all of the emnlsion that 

 would adhere was applied to each tree. It is apparent that if 

 less were applied it would be very difficult to tell whether the 

 work had been thorough or not. On the other hand prolonged 

 spraying and consequent over drenching would be merely a 

 waste of material and would have practically no effect upon the 

 quantity of petroleum that finally clings to the tree because the 

 excess of emulsion does not adhere to it. Also if the machine 

 emulsifies the oil and water as thoroughly as in our own experi- 

 ence there will be no danger from free oil as the excess of emul- 

 sion will drain off before separation can take place. Hence a 

 tree sprayed to a moderate excess with an emulsion containing 

 say 25 per ct. of petroleum would finally retain no more petro- 

 leum than if sprayed merely to the dripping point. If, there- 

 fore, the spray is directed upon each tree until its full capacity 

 to hold the emulsion is reached, that is until it begins to drip, 

 an amount of oil in proportion to the percentage indicated is 

 practically maintained. However, it is to be noted that in the 

 case of unusually rough or cracked bark or open wounds that 

 would hold the emulsion the accumulation of oil may cause 

 injury. It is also possible that, in case of over drenching, the oil 

 that would soak into the ground might cause injury to the roots. 



character of crude petroleum. 



Crude petroleum is an oily, inflammable liquid varying in color 

 from very dark brown to greenish tints. By refining it yields 

 a number of valuable products including paraffin, lubricating 

 and illuminating oils and a series of highly volatile oils. It is 

 the heavier oils that make it especially valuable as an insecti- 

 cide. Crude petroleum varies in appearance and composition 

 according to the locality from which it was taken. The eastern 

 oils are said to vary greatly from the western and most foreign 

 oils, the former having a paralfin and the latter an asphalt 

 base. The true indication of the safety of petroleum as an 

 insecticide evidently depends upon its specific gravity; as it has 

 been found that petroleum having a specific gravity of 43° or 



