37 
It is upon record, however, that in cases where the vapor has not been 
thoroughly diffused, but was allowed to flow down from an open vessel 
placed in the top of the tent, serious injury was done to the foliage at 
points where the undulated vapor flowed down. 
Sulphureted Hydrogen. — Several treatments with this gas were made 
on a small sc ale, the application lasting from five to thirty -five minutes. 
The effects produced either with diluted or concentrated gas were simi- 
lar to those produced by chlorine, except that even the concentrated 
sulphureted hydrogen did not injuriously affect the foliage. An ex- 
periment in which a whole tree was treated in the tent for forty-five 
minutes with quite concentrated sulphureted hydrogen gas, showed 
clearly that the effect was far from being satisfactory ; the insects for 
the moment were stupefied, but in the course of an hour and a half the 
majority of them were again moving about. 
Ammonia. — The vapor from one pound and a half of strong ammonia 
water was applied to an lL-foot lime tree for 30 minutes. The results 
were disastrous to the foliage ; the leaves were all scalded, and in a few 
days all dropped from the tree, and even the newer growth of wood was 
injured. The insects, however, were not perceptib^ harmed. 
Carbon Monoxide. — Very strong hopes have been entertained by many 
for the successful application of this gas. Its apparent cheapness and 
easy production, when the necessary plant is once erected, would rec- 
ommend it. Unfortunately our experiments show that it is not suffi- 
ciently effective to warrant its use. The gas was obtained by forcing 
air through a small furnace filled with red-hot charcoal, care being taken 
to cool and to measure the gas before applying it. No appreciable ef- 
fect was noticeable after 40 minutes. In a duplicate experiment, in 
which the charcoal was more strongly ignited and continuously intro- 
duced into the barrel for 30 minutes, only slightly better results were 
obtained. 
Oxalic Acid. — It was thought that the production of carbon monoxide 
by decomposition of oxalic acid by heat might be substituted for the 
previous method of generating this gas. One-quarter of a pound of ox- 
alic acid was ignited, and the gases applied in a manner similar to that 
of the preceding experiment. Neither the insects nor the foliage were 
harmed in the least. This experiment has incidentally shown that the 
vapor of formic and oxalic acids, also produced during the heating of 
the latter, is likewise ineffective , 
Carbolic Acid. — It had been suggested that carbolic acid vaporized 
by heat would prove fatal to the insect. A dose of half a pound of 
liquid acid was volatilized in the furnace, and the vapor blown in the 
vessel containing the infected branch. At the end of 20 minutes all the 
old insects were still alive, and some of the young ones, just molted, 
were moving about. An hour later the foliage appeared as if scalded. 
Hydrocyanic Acid. — It was only with hydrocyanic or prussic acid (gen- 
erated by the action of sulphuric acid on potassium cyanide) tbat sum- 
