POTASSIUM CYANIDE (PRUSSIC ACID). 



133 



and carbon disulphide, with soft soap, only yield imperfect results. 

 It is otherwise with prussic acid ; its vapours, owing to their poisonous 

 nature, can destroy the best protected insects through the respiratory 

 organs, and as easy as any soft-skinned insect. In America the 

 benefits of the substance have been recognized, and it is in constant use 

 by farmers. To-day all fruit-growers use it with success, and owing 

 to potassium cyanide used rationally in winter, fruit trees are freed 

 from all their parasites. In America apple-trees have been particu- 

 larly the prey of the San Jose louse, but potassium cyanide, which has 

 played in this case the role of carbon disulphide in the phylloxera in- 

 vasion, has enabled this dangerous cochineal to be circumscribed and 

 efi&ciently combated. 



Use. — In spite of its toxicity and the danger incidental to its use, 

 potassium cyanide is in current use in certain countries ; it is, in fact, 

 the most radical and the cheapest means to destroy tree parasites. 

 Cyanide can only be used in a closed space. Its decomposition is 

 hastened by the addition of dilute sulphuric acid. This practice 

 requires great precautions, because the prussic acid given off is as 

 deadly to the operator as to the insects. When a greenhouse is not 

 at disposal the operation is done under cloches for small plants, and 

 under tents for trees. The latter are of packing cloth, impregnated 

 with linseed oil and ochre, or wax. These portable tents are generally 

 hexagonal in shape and must touch the ground on all sides ; they are 

 closed hermetically by beating down the soil on the edges of the cloth. 

 In America they use exclusively large cubes with a wooden frame- 

 work covered with packing cloth. In this manner trees as high as 

 20 feet may be treated. To disinfect a tree the tent is placed over it, 

 and then a solution of potassium cyanide is run into a terrine to which 

 dilute sulphuric acid is added, taking care to place this terrine quickly 

 under the tent and to retire. The following, according to Debray, are 

 the quantities to use for 5 cubic metres of air : 30 grammes of 58 per 

 cent potassium cyanide dissolved in 50 cubic centimetres of water, 

 35 grammes of sulphuric acid of 66° B. (168° Tw.) diluted' with 50 

 cubic centimetres of water, and, according to Coquelin, the doses of 

 potassium cyanide to use according to the size and force of the tree : — 



