106 
pile, the deviation is less in proportion to the bad conducting 
power of the interposed substance. Now the conducting power 
of olive oil is, according to Rosseau, 675 times worse than other 
vegetable oils ; but the addition of two drops of poppy or beech 
nut oil to 154/^ grains of olive oil is sufficient to quadruple the 
conducting power of the latter. The fourth method is by nitrate 
of mercury. If recently made nitrate of mercury (prepared by 
dissolving six parts of mercury in 7’5 parts of nitric acid, sp. gr. 
1’36) be mixed with twelve times its weight of pure olive oil, and 
the mixture strongly agitated, the whole mass becomes solid in 
the course of a few hours. With poppy or other oils, the nitrate 
of mercury does not form a solid compound, and therefore when 
they are mixed with olive oil, we judge of their presence and 
quantity by the degree and quickness of the solidification of the 
suspected oil. I have already discussed the theory of the pro- 
cess. In this country the employment of olive oil for the table 
is limited, when we compare it with the extensive and important 
uses made of it in some other more southern countries. Thus in 
Spain it is consumed as a substitute for butter. 
143 Insects, destruction of. At the last meeting of the Horti- 
cultural Society, a communication was read from Air. IngTam, 
of Southampton, on a simple an efficacious method of destroy- 
ing the red spider, green fly, thrip, and scale, and other insects 
obnoxious to vegetation, without any injury to the plant. It 
consisted merely in placing the plants within a frame well closed, 
and putting between the pots laurel leaves well bruised. After 
remaining in this state for about an hour it will be found that 
all the two former insects are destroyed, and the plants must 
then be removed to a warm place, but a long exposure to the 
vapour of the leaves for about eight hours is found necessary 
for the removal of the thrip and scale. For a house, 20 feet 
long by 12 feet wide, two bushels of leaves are found sufficient, 
which may be bruised inside ; the roof and sides are to be kept 
close with matting, and the night time is considered best for the 
purpose of the experiment The effects are to be attributed to 
the hydrocyanic acid evolved from the leaves, which agrees with 
a recommendation made by Mr. Waterhouse, at a late meeting 
of the Entomological Society, for the destruction of insects in 
the canvas of pictures or wood of the frames, and in old books, 
&c., similar to the plan he employs in destroying insects in spe- 
143, Athenaeum, No. 510. 
