166 SOME SOUTH INDIAN INSECTS. ETC. [CHAP. Will. 



only kill the insects already present in the grain and will not 

 prevent insects from entering and attacking it subsequently if 

 is open to them. 



For fumigating small articles or quantities of grain, etc., use may 

 be made of a tight wooden box provided around the top of the sides 

 with a metal gutter into which fits a flange attached to the lid of 

 the box. The gutter being filled with water (allowing for displace- 

 ment of water by the flange) the substances foi fumigation are 

 plai nl m the box, the requisite quantity of Carbon Bisulphide 

 poured in and the lid applied, when the flange of the lid, dipping 

 into the gutter of water, makes a gas-tight fitting. A box of this 

 pattern may also be used for fumigation with Formalin, etc., but is 

 hardly suitable for Hydrocyanii Acid Gas, which is soluble in 

 water, and for which a tightly fitting top, pasted down with strips 

 of paper, is better. 



Hydrocyanic Aeid Gas, usually liberated by the action of 

 Sulphuric Acid on Potassium Cyanide, is a colourless, highly 

 volatile, and nearly odourless gas, which is one of the most 

 powerful poisons known. For this reason it must be used with 

 great care, although it is a most useful insecticide and absolutely 

 safe in competent hands. It is generally Used .it a strength of one 

 ounce of Potassium Cj anide (98 per cent, grade) to every one hundred 

 cubic feet of space, the gas being evolved by the help of one ounce 

 of Sulphuric Acid in three fluid ounces of water. It is especially 

 useful for the fumigation of entire buildings, such as godowns or 

 mills, which are infested with insect-pests, and may of course be 

 used for the fumigation of living-houses also if these can be 

 evacuated during treatment. For ordinary purposes on a large 

 scale, one pint of Sulphuric Acid and three pints of water are used 

 for every pound of cyanide ; it may be useful to note that commer- 

 cial Potassium Cyanide is usually only of about 45 per cent, strength 

 and the quantities to be taken must be based on the quantities given 

 of 98 per cent, grade. 



For fumigations with Hydrocyanic Acid ( ras, .is with all fumiga- 

 tions, the first requisite is to measure off accurately the cubic 

 1 apacitj of the building or vessel which is to be fumigated ; if this 

 is of irregular shape it is often of great assistance to plot its shape 

 to scale on squared paper and to count the number of squares, which 

 will give an approximately correct idea of the capacity of any 

 section, which can then be multiplied by the length to give cubic 

 capacity. All outside doors, windows, or other openings, are then 

 tightly closed and if necessary pasted over with paper. The 

 requisite amount of cyanide, at the rate of one ounce to each 100 

 cubic feet, is then taken and put up in paper packets of suitable size 



