38 
ciently fatal effects were secured to warrant a more thorough determina- 
tion of the time of exposure and quantities of material which would 
produce the best results. Numerous experiments were carried on for 
this purpose, and it was shown that even small amounts were effective. 
It was also shown that even in these small quantities an injurious effect 
upon the foliage was produced. In the beginning of the experiments, 
" mining cyanide " of potassium was used. It is a very impure material 
and contains along with the cyanide a considerable amount of carbonate 
of potassium. For this reason many of the first treatments were prac- 
tically ineffective. 
Later treatments with pure cyanide were more successful in destroy- 
ing the insects, but the foliage was proportionally injured. Treatments 
varying in dose from 4 to 12 ounces of cyanide, and in time from 15 to 
GO minutes, showed that the effect produced on the foliage by longer 
treatment was not proportionally greater than that produced by short 
treatment. Neither was the effect of longer treatments proportionally 
more fatal to the insects. It was thus clearly shown that the gas mixt- 
ure should be of considerable strength iu order to insure rapid action. 
The effect of the gas was so disastrous to the foi iage that it became 
necessary to find some means of remedying this trouble. This was 
sought in applying a second gas, which might preserve the foliage. 
Sulphureted hydrogen was therefore injected into the tent, together 
with the cyanide gas, both from the same generator; a portion of the 
sulphureted hydrogen beiug introduced before the cyanide was gen- 
erated. It was found that the insects appeared stupefied when the tent 
was raised, but large numbers revived in a few hours. The effect of 
the cyanide seemed therefore to have been decreased by the sulphu- 
reted hydrogen. The foliage was nut preserved, although not so badly 
affected as by treatments with cyanide alone. 
Carbonic acid gas was next tried. Trees were treated with larger 
doses of cyanide than heretofore used, and the carbonic acid from li 
pounds of carbonate of soda was at the same time introduced with these 
doses. The insects were killed and the foliage of a 12-foot tree re- 
mained unharmed, while that of a 14-foot tree with the same amount of 
carbonic acid was slightly injured. Thus it was shown that it would 
require li pounds of bicarbonate of soda to preserve tree tops 12 feet 
iu diameter, and that with tins protection the deadly cyanide could be 
successfully used. 
The regulation of the doses for the different sized trees so as to pro- 
duce uniform treatments is calculated on the basis of the results of the 
experiments which determined the amount of each constituent for a 12- 
foot tree. The following table indicates the amounts for trees of differ- 
ent dimensions of top, based upon the rates of cubical contents : 
