Insecticides and Related Substances. 303 



The higher amounts of lime produced more calcium tetra-sul- 

 phide, while with the smaller amounts, the mixture was more 

 nearly penta-sulphide. The largest amount of soluble sulphides 

 was formed by boiling about one hour, especially when the 

 largest amount of lime was used. The amount of sediment in- 

 creased with increased boiling, due to the formation of calcium 

 sulphite. It was found that the addition of extra lime to the 

 diluted lime-sulphur solution might seriously decrease its in- 

 secticidal value as a result of the decomposition of the higher 

 sulphides of calcium with formation of free sulphur. Where 

 pure lime was used, the sediment, found to consist of calcium sul- 

 phite, free sulphur and hydroxide and carbonate of lime, formed 

 suitable material to add in the making of a new wash. It was also 

 found that magnesium oxide when present in the lime, as in 

 dolomitic limestone, tended to decompose the sulphides of cal- 

 cium with evolution of hydrogen sulphide. The importance of 

 pure lime for this insecticide is thus emphasized. An examina- 

 tion of commercial lime-sulphur preparations revealed great 

 variations in composition. Since field experiments have demon- 

 strated that this insecticide derives its chief value from the 

 soluble lime-sulphur compounds, commercial preparations should 

 be bought on the basis of the strength and composition of their 

 supernatant liquid. 



Stewart states that the problem of making concentrated lime- 

 sulphur solutions is essentially one of preventing crystallization 

 and securing a storable product of high density. He finds that 

 the formation of crystals is largely due to an excess of lime and 

 exposure to the air when cold. Exposure to the air may be 

 avoided by covering the surface of the wash with oil. Arsenite 

 of lime, as a supplementary insecticide, has been found to pro- 

 duce least decomposition of the sulphur compounds of this wash. 



liny wood found that a one hour period of boiling dissolved 

 practically all the sulphur used for this wash. The addition of 

 rommon salt was found to have no effect so far as the sulphur 

 <-t impounds of the wash were concerned. 



