(|>f the sulphur set free from the pentasulphide, tetrasulphide and thio- 

 sulphate is the effective agent, then the amount of compounds in solution cap- 

 able of furnishing the largest amount of free sulphur directly determines the 

 value of the solution. Calcium pentasulphide is capable of furnishing more 

 free sulphur by decomposition than the tetrasulphide and this more than the 

 thiosulphate. 



If the free sulphur is the material desired then the solution containing the 

 largest amount of pentasulphide is the most valuable. It is now commonly 

 believed, whether correct or not, that the solution containing the most penta- 

 sulphide is the most effective, at least in connection with the destruction of 

 scale insects. What ever may prove to be the facts in relation to the manner of ac- 

 tion of the lime-suphur solution, it is obvious that its efficiency stands close 

 and direct relation to the amount of sulphide compounds contained it it, or, in 

 other words, to the chemical composition of the solution." 



The difference in concentration and amount of Calcium thio-sulphate 

 (CaS2O3) present in home-made solutions, compared with the more concen- 

 trated form prepared by manufacturers are caused by the manufacture in pre- 

 paring a more concentrated solution, by evaporation, changes some of the cal- 

 cium thio-sulphate to calcium sulphite leaving a smaller percentage of calcium 

 thiosulphite in solution than in the home-made preparations. The calcium 

 sulphite, as formed under the conditions stated is found in the sediment as 

 such or oxidized to Calcium sulphate (CaSO4) which is insoluble and in either 

 form is a loss to the manufacturer, and the consumers. 



ARSENATE OF LEAD 



This substance was first prepared as an insecticide by Mr. F. C. Moulton 

 in 1892, while acting as chemist in Maiden, under Mr. E. H, Forbush, Field 

 Director, in charge of the work of destroying the gypsy moth. 



In the work of destroying the gypsy moth it was soon discovered that Paris 

 green would not kill many of the catterpillars, even when used in as large a 

 proportion in water as was possible without injury to the foliage of the trees. 

 It therefore seemed necessary, to discover, if possible, some insecticide that 

 would destroy the caterpillar and at the same time not injure the most delicate 

 foliage. 



The first public mention of arsenate of lead was in the report of the 

 Massachusetts Agricultural College, October, 1893, page 23. 



The name "gypsino" was given to this insecticide by Mr. Moulton, but as 

 there was an entirely different product on the market by the same name, this 

 insecticide was called arsenate of lead. 



CHEMICAL COMPOSITION 



The arsenate of lead used in spraying, operations, to be exact, is not a salt 

 whose composition may be expressed by a single formula, but instead is a 

 mixture of both diplumbic and tri-plumbic arsenates, the relative quantities of 

 each depending principally upon the source of the soluble lead salt used in 

 making it. 



