256 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 
will be required to generate gas of sufficient strength to kill the insects. 
Double this strength, or the same amount of materials to 150 cubic feet 
enclosed, may be used upon woody plants without danger of seriously 
injuring them. The greater strength should be employed whenever 
practicable, as it will ensure the death of the eggs as well as the active 
insects.' 
" Imported plants are usually in a more or less dormant condition, 
which lessens danger of injury. Mr. Lounsbury writes, in his Report of 
June, 1897: 'Injury to the tips of new growth generally results. This 
injury is in no wise serious, and is quickly outgrown. The operators 
consider it a favourable indication, as when such injury results it is quite 
certain that the gas has been present in sufficient strength to destroy all 
the insects.' 
" With respect to fruit, I again quote from Mr. Lounsbury "s letter : 
' I had had Lemons and Oranges analysed after treatment, and found 
that after a few hours not more than a trace of the gas remained in the 
rind. There is much more natural cyanogen in a single seed (so the 
analyst told me) than what remains in the fi'uit from fumigation. We 
have no complaints of any effect on the keeping quahties of the fruit.' 
To generate the gas, ' the required quantities of cyanide and water 
are first placed in the generating vessel, the cyanide being broken into 
small pieces about the size of lump sugar. The operator then adds the 
acid, pouring it slowly into the vessel to avoid splashing, and immediately 
withdraws.' 
The above treatment is suitable for fruit and hardy plants. Tender 
garden plants are usually imported in Wardian cases, and may be treated 
separately. We have in the Wardian case an air-tight chamber ready 
to hand, in which the plants can be fumigated before their removal. 
After a large series of experiments with various fumigating media, I find 
that hydrocyanic acid gas remains by far the most efficient insecticide 
and the least injurious to the plants. But with delicate succulent plants 
I find it has to be applied rather differently. A more concentrated dose 
of the gas applied for a shorter period is most satisfactory in its results. 
In a Wardian case containing about 16 cubic feet I find a dose of ^ oz. 
cyanide, ^ oz. acid, and 1 oz. water, with an exposure of half an hour, 
will kill every individual of a colony of Orthesia (the most resistant of all 
coccids) without in the least affecting the plants. The treatment should 
be carried out only after sunset. According to Mr. Lounsbury's tables, 
these proportions of chemicals should be sufficient for a space of 140 cubic 
feet with a longer exposure." 
Mr. Green further describes the treatment of orchard trees on a large 
scale, cjuoting again from Mr. Lounsbury's method : — 
" Generation of the Gas. — Hydi'ocyanic acid gas is generated by the 
action of sulphuric acid on potassium cyanide in the presence of water. 
The required quantities of the cyanide and water are first placed in 
the generating vessel, the cyanide being broken into small pieces, not 
above the size of lump sugar. The tree is then covered with the tent or 
sheet, and the vessel slipped under almost to the base of the tree ; 
reaching in, the operator then adds the acid, pouring it slowly into the 
vessel so as to avoid its splashing and thus burning his hand or the 
