lime-sulphur are not very readily separable; so }he exact ir-secticidal value cf 

 each has not been carefully determined but it has been generally assumed 

 tnat the two highest sulphides are the most erncient; and of these two the one 

 higher in sulphur, the pentasulphide (CaSs ) is supposed to be of a higher in- 

 secticidal value. In freshly prepared solutions only two of these compounds 

 are present, the four sulphur compound or calcium tetrasulphide (CaS4 ) and 

 the five sulphur compound or calcium pentasulphide ( CaSs ). Calcium 

 pentasulphide contains 20 per cent of calcium and 80 per cent of sulphur, 

 which is at the rate of four parts of sulphur by weight for one part of calcium. 

 Calcium tetrasulphide contains 24 per cent of calcium and 76 per cent of sul- 

 phur, which is at the rale of 3.2 parts of sulphur by weight for one part of 

 calcium. Calcium tetrasulphide and pentasulphide produce the orange-red 

 color in the solution. In the chemical changes that take place between lime 

 and sulphur when they are heated in waler, another compound is unavoidably 

 formed, but in smaller amounts; this is a chemical compound of three elements 

 calcium, sulphur and oxygen. This calcium thiosulphate (CaSz O$ ) which 

 is easily soluble in water and is therefore contained in the solution along with 

 the sulphides of calcium. The value of this compound for spraying purposes 

 is not known but on exposure to air it is changed by oxidation to calcium sul- 

 phite (CaSO3 ) and free sulphur. Calcium sulphite is insoluble in water and 

 therefore appears in the sediment or in the undissolved portion of lime-sulphur 

 preparations, usually forming the chief part of the sediment. The free sul- 

 phur formed by the decomposition of thio-sulphate recombines with calcium 

 and goes into solution in the operation of making lime-sulphur solution, when 

 enough lime is present. The sulphite may later take up more oxygen and 

 form calcium sulphate ( CaSO4 ) an insoluble part of the sediment. The de- 

 sirable chemical changes are the formation of the soluble lime-sulphur com- 

 binations, the calcium tetrasulphide and the calcium pentasulphide and perhaps 

 the formation of the calcium thio-sulphate; the indersirable reactions are the 

 oxidizing of the thio-sulphate, after the mixture cools, with the formation of 

 sulphates and sulphites (1). 



EFFICIENCY OF LIME-SULPHUR IN RELATION TO CHEMICAL 

 COMPOSITION 



Dr. L. L. VanSlyke in Bulletin No. 329, of the New York State Experiment 

 Station at Geneva, states "that it is held, as the results of some experimental 

 work that the effect of the lime sulphur mixture is not due to the direct ac- 

 tion of calcium penta-sulphide (CaSs ) or calcium tetra-sulphide (CaS4 ) but is 

 due rather to compounds that are formed from those, either calcium thio-sul- 

 phate (CaS2O3 ) or free sulphur or both. 



If the direct effective substance is calcium thiosulphate, then the amount 

 that can be formed from the lime-sulphur solution is directly dependent upon 

 and proportional to the amount of calcium pentasulphide or calcium tetrasul- 

 phide in solution. 



(1) Bui. No.319 and No. 320 New York Experiment Station (Geneva) 



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