45 
ordinary practice. The smaller the insect, of course the more 
quickly it may be killed by the hot blast; and the better the living 
tissue of the plant is protected by a lifeless cuticle or a layer of bark, 
the longer it may be exposed to this blast without being heated to 
the point of injury. Bark-lice on trees and shrubs are thus favor- 
able objects for experiment; but where thick-bodied insects, like 
caterpillars and large beetles or bugs, themselves covered with a 
dense crust of lifeless cuticle, are feeding on the young green leaf, 
the margin of safety is greatly narrowed and may wholly disap- 
pear. The practical utility of this method of destroying insects in 
any case, evidently depends on the existence and extent of this mar- 
gin of safety. 
The gasoline blast may, in fact, be used to kill any insect on any 
plant. The time and method of use necessary to kill the insect will 
vary widely for different kinds of insects and for the different states 
and stages of each kind ; and the time and method of use sufficient 
to injure various kinds of plants will likewise differ widely accord- 
ing to the kinds and condition of the plants themselves. The actual 
effect of the blast on either insect or plant will also vary enormously 
according to small details of the method by which it is applied. 
They will vary, first, with variations in the pressure, which deter- 
mines the extent and heat of the flame; second, with the distance 
from the object at which the torch is held; third, with the rate of 
movement at which the flame is passed over the surface treated; 
fourth, with the temperature at the time and the amount and direc- 
tion of the wind ; fifth, with the direction of the blast, whether per- 
pendicular or oblique to the surface; and sixth (not to specify 
further), with the steadiness with which the flame is applied to a 
given surface, whether held at one point for a definite time or 
swayed back and forth over a considerable surface for a variable 
number of times. 
To determine the effect of all these different classes of variations 
with sufficient exactness for practical guidance, and then to combine 
all the various results of this inquiry with each other in a way 
to form a system of practice which can be accurately described and 
safely recommended for general use, is a task which no intelligent 
investigator would enter upon lightly, or without such preliminary 
tests as would enable him to judge whether any important result 
was likely to come from more exact and extensive experiments. 
The observations here reported are all in the nature of such pre- 
liminary tests, made by different persons, each for his own satisfac- 
tion, on different objects and in different parts of the country. They 
are practical tests rather than complete scientific experiments, and 
