1 21) SOME SOUTH INDIAN INSECTS. ETC. [CHAP. XI. 



any large scale, it is more convenient to have proper tents which 

 are clearly marked on the outside so that the cubic capacity may 

 he seen exactly when they are erected, in order that the proper 

 dosage of gas may be given. It tin capacity ol the tent is not 

 known the following rough rule may be followed : Multiply the 

 distance around by the distance over the top of the tented tree and 

 divide by 133, the result being the number of ounces of Potassium 

 Cyanide required for ordinary work. Thus, the distance around 

 the tented tree being 35 feet and the distance over (from ground to 

 ground) being 20 feet, the cyanide required will be 35 x 20 -s- 133= 

 5'26, or five and one-quarter ounces. For every ounce of Potassium 

 Cyanide there are required one fluid ounce of Sulphuric Acid and 

 three fluid ounces of water to liberate the gas. The acid is added 

 to the water in a deep wide-mouthed earthenware vessel; the water 

 must never be poured into the acid, as the acid and water combinJ 

 \ iolentlj and in the latter case the acid will probably be spattered 

 about and do damage. The tent or box being ready overthe plants 

 to hi- fumigated, the water and acid, ready mixed in the earthen- 

 ware vessel, are slipped inside and the weighed quantity of cyanide, 

 loosely wrapped in clean paper, is dropped into the acid and the 

 box or tent immediately closed down and earthed up around the 

 bottom and left overnight. Next morning the tent is opened up. or 

 the box lifted, and, after an interval to allow the escape of any gas 

 remaining, may be removed. It must be clearly remembered that 

 Hydrocyanic Acid gas is one of the most poisonous substances 

 known and for this reason alone it should only be used under strict 

 supervision. Potassium Cyanide is also a violent poison if taken 

 into the system, although it is fairly safe to handle if there are no 

 cuts or sores on the hands ; but it is always better to use gloves 

 if any quantity is being dealt with. Sulphuric Acid will burn 

 violently into the skin and flesh and the vessels containing it 

 must be handled with great care. For these reasons fumigation 

 ul trees and plants in the field should never be tarried out except 

 under competent personal supervision. 



Besides its use for the control of insect-pests already attacking 

 plants, fumigation with Hydrocyanic Acid gas has also an impor- 

 tant use m checking the spread into hitherto unaffected localities 

 i,i pests, especiallj Scale-insects, which would otherwise he 

 brought in with growing plants. That there is actually great dan- 

 ger of this may be realized from even a casual inspection oi most ol 

 the plants, such as young mangoes, so commonly seen being carried 



• ,V,>,'. In calculations ol this sort, it is always assumed that the cyanide is of 

 1 1 rni. purity. Ordinarj commercial cyanide is often ol onl) aboul 45 per cent. 



j>uiity and tin i. i 



