INSECTICIDES, FUNGICIDES, ETC. 149 



and it has formed the subject of some considerable inquiry at 

 Wo burn, an account of which will be found below (p. 152). 



(6) Arsenical compounds. These supply some of the most 

 powerful insecticides for leaf-eating insects. An aceto-arsenite 

 of copper (Paris green), made by the action of arsenious oxide on 

 verdigris, was one of the first of these substances to come into 

 use, but it is liable to injure the foliage, the leaves becoming 

 scorched, and often falling : this action is intensified if the 

 Paris green contains excess of arsenious oxide, as it often does, 

 and it is well to guard against this by the addition to it of an equal 

 weight of lime. Calcium arsenite, made by boiling arsenious 

 oxide with lime, is also used, although more frequently in the 

 form of London purple, which is a waste product from dye- 

 works, containing 75 per cent, of the arsenite. The compound 

 which has come most into favour of recent years, chiefly on 

 account of its causing very little leaf -scorching, is lead arsenate, 

 also called gypsine, from its being first used for the gipsy-moth 

 caterpillar. The results of an inquiry into the nature and action 

 of this compound will be found on pp. 169, 227. 



(7) Lime has been used of late years together with salt, as an 

 insecticide in special cases (apple psylla). The lime-salt wash 

 most recently recommended is made by slaking 15-20 Ibs. of 

 lime, adding 2-3 Ibs. of salt, J-i Ib. of water-glass, and making 

 up to 10 gallons. It is applied to the trees, as hot as possible, 

 a few weeks before the buds open. The wash appears to act, 

 partially by destroying the eggs, and partially by glueing them 

 up and preventing the egress of the insects. What function the 

 salt performs it is difficult to see. Strong brine will destroy 

 insect eggs by depleting them of their liquid contents, but the 

 salt is hardly present in this wash in sufficient proportion for such 

 an action (p. 210). As a detergent for removing moss, lichen 

 and dead bark from trees, which is a most important operation in 

 combating any insect attack, lime is inferior to a stronger alkali 

 such as caustic soda. This is one of the subjects which has been 

 examined at Woburn (p. 168). 



B. Fungicides. Sulphur, and the sulphides previously de- 

 scribed, are important fungicides, the latter being of special use 

 in the case of surface moulds or mildews. Phenol and the salts 

 of iron appear to have but very feeble fungicidal properties, and 

 mercuric chloride, although of great value in the laboratory, has 

 not been successfully applied to plants. By far the most im- 

 portant fungicide appears at present to be copper, and it has 

 been applied in a great variety of forms, but these all group 



