32 REPORTS OF OBSERVATIONS AND EXPERIMENTS. 



minutes, after which it was removed from the furnace and allowed to 

 cool. The resulting arsenide was very brittle, and pulverized readily. 



Even when the arsenide was used in such large quantities that its 

 cost was three times as great as that of potassium cyanide used in 

 sufficient quantity to destroy all the insects on the tree, it proved fatal 

 to only about two-thirds of the scale-insects (Lecanium liesperidum and 

 L. olece) on the lower portion of the tree, while those on the upper por- 

 tion escaped unharmed; the tree operated upon was less than five 

 feet in height, and was not appreciably affected by the gas. The latter 

 was confined upon the tree for a space of a full hour, and yet, owing 

 to the great specific gravity of the gas, it did not become thoroughly dif- 

 fused throughout the space inclosed by the tent. Owiug to this char- 

 acteristic of arseniureted hydrogen, and the further factjof its being 

 more expensive than hydrocyauic-acid gas, there is nothing to recom- 

 mend it over the last-named gas. 



These experiments indicate that for the destruction of insects on 

 trees hydrocyanic-acid gas is much cheaper than either arseniureted 

 hydrogen or sulphureted hydrogen. 



