376 MISCELLANEOUS PRODUCTS USED IN AGRICULTURE. XVI. 



than would be produced if every infected grain were replaced 

 by a sound one." 



Sulphur. Used in fine powder for certain forms of mildew ; 

 occasionally as vapour (not sulphur dioxide), by heating sul- 

 phur to the boiling point and carefully avoiding inflammation. 

 It can only be used in this way in enclosed spaces, e.g., green- 

 houses. A mixture of finely-divided sulphur and lime is also 

 employed as a remedy for mildew. 



Insecticides. In the choice of a substance to be used for 

 the destruction of insect pests, it is necessary to consider 

 whether the particular insects are gnawing insects, i.e., 

 whether they actually bite away portions of the plant, or 

 sucking insects, w r hich derive their nourishment by imbibing 

 the sap or juice of the plant. 



If the former, any violent poison which does not harm the 

 plant may be distributed over the leaves or stems and may be 

 effective ; in the latter the substance must act upon the insect 

 in some other way than as a poison, either corrosively upon its 

 body or through its breathing apparatus. 



As Food Poisons arsenical compounds are mainly used, 

 the favourite ones being Paris green, copper aceto-arsenite 

 [Cu(C 2 H302) 2 .3Cu(AsO 2 )2 (?)] ; London purple., chiefly calcium 

 arsenite and waste colouring matter ; and to a less extent 

 Scheele's green (CuH'As0 8 ), and lead ar senate, made when re- 

 quired by mixing solutions of sodium arsenate and lead acetate. 

 Free arsenious oxide is not suitable for the purpose, on 

 account of its corrosive effect upon foliage. According to 

 American experiments! the arsenical preparations are less 

 likely to injure the plant the less soluble the arsenic is, the 

 order of solubility being given thus, beginning with the least 

 soluble lead arsenate, Scheele's green, Paris green, London 

 purple. They are best applied in suspension in water as a 

 spray; but sometimes they are used in the dry state, either 

 alone or mixed with flour. In some cases they are employed 

 as poisoned bait, mixed with sugar and bran. For spraying, 



* Swingle, Farmers' Bulletin 75. U.S. Dept. of Agriculture, 

 t Marlatt, Farmers' Bulletin No. 127, U.S. Dept. of Agriculture. 



