370 FUNGICIDES INSECTICIDES 



smut in oats a solution of 1 Ib. "formalin" in 50 or 60 gallons of 

 water is recommended. The seed is to be soaked for two hours in this 

 solution, which would contain about 0'08 per cent of formaldehyde. 



Potassium sulphide. The substance used is generally " liver of 

 sulphur," a dark brown fused mass of sulphide and various poly sul- 

 phides of potassium. An American recipe for smut in oats is to soak 

 the grain for twenty-four hours in a 0'6 per cent solution (i.e., 1^ Ib. 

 of potassium sulphide in 25 gallons of water) or for two hours in 2 per 

 cent solution. 



Hot water. It is possible, in the case of many seeds, to kill the 

 spores of fungi by means of hot water, without injuring the seeds 

 themselves. This is done by dipping them into water at a temperature 

 of about 56 and taking care that every grain is wetted. Ten minutes' 

 treatment is found to suffice for destroying bunt in wheat or smut in 

 oats. If the grain be soaked for three or four hours in cold water first, 

 five minutes in the hot water is sufficient. For smut in barley, the 

 temperature should not be higher than 54 '5. 



It is said that when grain is treated with hot water or with potassium 

 sulphide, there is an increase in the yield greater than would be pro- 

 duced if every infected grain were replaced by a sound one. 1 



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

 occasionally as vapour (not sulphur dioxide), by heating sulphur to 

 the boiling-point and carefully avoiding inflammation. It can only be 

 used in this way in enclosed spaces, e.g., greenhouses. 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 de- 

 struction of insect pests, it is necessary to consider whether the parti- 

 cular insects are gnawing insects, i.e., whether they actually bite away 

 portions of the plant, or sucking insects, which derive their nourish- 

 ment 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 breath- 

 ing apparatus. 



As Food Poisons, arsenical compounds are mainly used, the favourite 

 ones being Paris green, copper aceto-arsenite [Cu(C 2 H 8 2 ) 2 .3Cu(AsO 2 ) s 

 (?)] ; London purple, chiefly .calcium arsenite and waste colouring 

 matter ; and to a less extent Scheeles green, (CuHAsO 3 ), and lead 

 arsenate, made when required 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 2 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, 



1 Swingle, Farmers' Bulletin, 75, U.S. Dept. of Agric. 

 2 Marlatt, Farmers' Bulletin, 127, U.S. Dept. of Agric. 



