200 
TENTH REPORT. 
tions of oil have been separated to give, in the one case, that part having 
the highest germicidal value and in the other, the one having highest insecti- 
cidal value. The former is made so that dilutions of 2%, or greater in water 
make clear solutions, while the latter contains only sufficient soap to make a 
satisfactory emulsion; the amount necessary varies with different oils, but 
approximates a mixture containing 20 to 25%. 
An ideal preparation for general purposes would be a mixture of the two, 
but unfortunately, all experiments to date indicate that such a mixture 
produces an insecticide no better in value than that of the weaker member; 
while one would naturally expect it to be lowered only to the extent of the 
dilution. This would indicate that the presence of certain constituents in 
an insecticide must be avoided in order to obtain one of the highest efficiency; 
to be specific, the insecticidal properties appear to be neutralized by the pre- 
sence of Carbolic Acid, or any member of the phenol series; and the value 
is lowered to that of the particular phenols which may be present, although 
they may constitute less than 10% of the whole. An oil containing 8% 
of phenols and whose greatest insecticidal value is 5 times that of Carbolic 
Acid may have that value increased to 125 by the removal of the phenols. 
This shows that, beyond a certain point, the phenols alone are of very 
ittle value; so little, in fact, that Carbolic Acid will not kill the test insects 
in the strongest possible aqueous solution with one minute’s immersion. 
This dilution and time, however, have been taken as the unit for the sake of 
using a standard the same substance as the unit for each of the three properties 
— toxicity, germicide and insecticide. 
By comparing, in the charts, the insecticidal values of Carbolic Acid in 
•aqueous solution, a solution of soap, and a mixture of the two, one may con- 
clude that whatever insecticidal value Carbolic Acid appears to have when 
in contact with the insect for one minute is due entirely to the soap, as it 
alone would have been the more efficient. 
The use of a gaseous insecticide has an advantage which is in many cases 
not possible to obtai.i with liquids, in particular as a means of eradicating 
household pests — the bed bug, cockroach, ant, fly, mosquito, and moth. 
But the diffusibility of gases while being their chief advantage, is a weighty 
objection to their use, because many rooms and houses are not sufficiently 
tight to retain the gas. The very poisonous or irritating gases like hydro- 
cyanic acid or sulphur dioxide may be effective while the less poisonous or 
more diffusible gases never become sufficiently concentrated to accomplish 
much. 
The method adopted for testing gaseous insecticides is as follows: The 
gas or vapor is generated at the bottom of a loosely covered glass cylinder 
4 inches in diameter, and 12 inches high. This is placed in a hood with 
closely fitting glass doors and with an easily regulated exhaust pipe for 
removing the vapors at the end of the experiment, also with means for heating, 
either gas or steam, controlled from the outside. The insects are placed in 
the same tubes used for dipping and are suspended in the upper end of the 
cvlinder. This is for intimate contact with the almost undiluted vapor. 
Then two open vessels with insects are placed, one on the floor and one near 
the top to determine the effect of the more diffused gas or vapor. The great 
difference between results of action from direct contact and from the more 
diffused gases is striking, showing that it is a method upon which no reliance 
can be placed. Bed bugs recovered in nearly every case when exposed to 
diffused coal gas, formaldehyde, carbon dioxide, sulphur dioxide, camphor 
vapors and vapors of many volatile oils including coal tar oils, pine oil, oil of 
