150 SCIENCE AND FRUIT GROWING 



themselves under the main headings of basic sulphates, and basic 

 carbonates of copper. Both of these have been examined very 

 fully at Woburn, and the results are given in the following pages 

 (pp. 171, 183). 



Gases 



Hydrocyanic acid is used in the fumigation of hot -houses for 

 the destruction of plant lice, etc. ; it is also used in many countries 

 for disinfecting imported nursery stock, and in California it is 

 applied to peach trees in plantations for scale, each tree being 

 covered with a tent during the operation. For each 100 cubic feet 

 of space, J to J oz. of " lump " potassium cyanide is generally 

 used, and for each ounce of the cyanide, one liquid ounce of 

 acid, previously diluted with 3 to 4 ounces of water (see p. 220). 



Hydrocyanic acid is liable to cause some injury to young 

 plants, and in fumigating these they should not be exposed to 

 the direct stream of gas as it ascends from the generating vessel. 

 Its action in destroying insect eggs is, according to the Woburn 

 results, unsatisfactory, and in the case of green-houses it is often 

 necessary to repeat the fumigation after a fortnight's interval, 

 to destroy insects which have hatched out from the eggs (pp. 206, 

 219). For freeing nursery stock from woolly aphis, an advan- 

 tageous substitute for fumigation is to immerse the trees bodily 

 in water at 115 F. for 10 minutes (p. 220). 



Sulphur, as already mentioned, probably acts as an insecticide 

 owing to its volatility, and it is often used for fumigating houses 

 by heating it, taking care not to let it catch fire. Smouldering 

 tobacco and pyrethrum are used in the same way. 



