288 INSECTICIDES AND FUNGICIDES 



agitating kerosene with a solution of hard or soft soap, there 

 is obtained an emulsion which, when properly made, retains 

 its permanency for at least several days. 



An emulsion is a mechanical mixture of two liquids in- 

 soluble in each other. One is usually an oil, the particles of 

 which are very finely divided and are held in suspension 

 in the other which is of a gelatinous or viscous nature. The 

 permanency of suspension is brought about partly because 

 the particles are very small and the friction in moving 

 through the suspending liquid is sufficient to prevent their 

 accumulating rapidly; and partly because the suspending 

 liquid exerts some physical if not chemical attraction for 

 the particles, thus helping to prevent their uniting with one 

 another. Moreover, each fine particle of oil is surrounded 

 by a coating of the gelatinous or viscous suspending liquid 

 and can not easily touch another particle to coalesce with it. 

 The finer the particles of oil, the more surface exposed, and 

 hence the greater the attraction of the suspending liquid for 

 the oil, and the more friction in moving. Kerosene and soap 

 solution are liquids of the character just described. 



(c) Lime-Sulphur Boiled is made by boiling together 

 for about an hour, or until the sulphur is dissolved, 50 

 pounds of pure lime and 100 pounds of finely ground 

 sulphur in 50 to 55 gallons of water. The lime is slaked 

 before actual boiling is begun. When properly made there 

 are formed the tetra- and pentasulphides of calcium and 

 calcium thiosulphate. The exact reaction is not known, 

 but the following is given as a possibility: 



3Ca(OH) 2 + 1 IS = CaS 2 3 +CaS 4 +CaS 6 +3H,0. 



This reaction corresponds fairly well to the proportions 

 of lime to sulphur recommended, and to the amount of 

 sulphur in solution as thiosulphate and sulphides found 

 by analysis. 



Lime containing magnesia should not be used, for mag- 

 nesium forms no compounds with the sulphur and only 

 serves to increase the amount of sediment. Long boiling 

 changes the thiosulphate to insoluble sulphite (Reaction 1); 



