177 
tent, and then filling the latter with the gas. It must be apparent to 
all that an insecticide in the form of a gas would reach not only those 
insects on the inclosed tree that would naturally have been reached 
by aspray, but also those that might be protected from the spray, since 
the gas would penetrate every nook and crevice on the tree; in short, 
would go wherever the air could go. But the gas also posseses another 
decided advantage over the spray. The application of the latter even 
when the most. approved methods are employed is a laborious task, 
and the operators, becoming wearied with their work, are almost cer- 
tain to slight it. To remedy this, a method is desired that operates 
on the principle of “ You press the button—we do the rest.” And 
this we have in the gas treatment. After the tent is on the tree to be 
treated, the button act consists of simply turning the chemicals into 
the generator—the chemicals do the rest. 
Hydrocyanic acid gas was the first one that I experimented with, 
and although I have since tested a large number of other gases, inelud- 
ing arseniuretted hydrogen, sulphuretted hydrogen, sulphurous gas, 
carbon bisulphide, nitric and nitrous oxide, ammonia, chloroform, car- 
bonic acid gas, and carbon monoxide, yet none of them gave as good 
results as the one first mentioned. Some of the gases were much 
slower in becoming diffused throughout the space inclosed by the tent, 
others were more severe in their effects upon the tree, while several, 
which were commonly supposed to be very deadly to animal life of 
every description, produced very little impression upon the insects 
against which they were employed. 
A long line of experiments which the writer carried, out under the 
auspices of our national Division of Entomology at Washington, 
establishes the fact that the best results will be obtained by generat- 
ing the gas from undiluted commercial sulphuric acid and undissolved 
fused potassium cyanide of about fifty-eight per cent purity, the pro. 
portions being: one fluid ounce of the acid, one ounce by weight of 
the cyanide, and three fluid ounces of cold water. For a generator, 
almost any open earthen vessel will answer, its size depending upon 
the size of the tree or plant to be treated. The necessary quantity of 
water is first poured into the generator, followed by the acid, after 
which the generator is placed on the ground under the tent and the 
eyanide added. Experience has shown that the best time for treating 
the trees is at night, or during cool, cloudy days; at such times the 
trees are more or less in a state of rest, and therefore less liable to be 
injured by the gas than if subjected to it during bright, sunny days. 
Moreover, as is well known, this gas is not very stable even under the 
most favorable conditions, but it is less liable to decompose when kept 
at a low temperature than if subjected to any considerable degree of 
heat. For these reasons the best results will be obtained by using it 
only during cool weather, and experience has shown that the various 
