408 EXPERIMENT STATION EECORD. 



between this total estimated amount of tliiosnlpliate and the amount i ' 

 present in the solution, is the quantity that is decomposed ; and from thesv 

 the amount of sulphite can be easily calculated. 



" The initial ratio of lime to sulphur is also easily determined when one 

 knows not only the amounts of calcium and sulphur present in a given lime- 

 sulphur solution, but also the insoluble sulphite produced in the preparation 

 of the same. . . . The data given show that the proportion of lime (CaO) 

 to sulphur which reacts in the pi'eparation of the more concentrated commer- 

 cial lime-sulphur solutions, is but a mere trifle greater than 1:2. . . . More 

 dilute solutions prepared under commercial conditions [were not examined]. 

 It is evident, however, from the work of Thatcher'' and Van Slyke et al. [E. S. R., 



24, p. 663] that the ratio in this case would be somewhat greater than 1:2; 

 in some cases it would be perhaps 1 : 2.25. 



"The theoretical basis given here will not exactly apply to the preparation 

 of small amounts of solution, say 150 gal. or less, where the oxidation of the 

 polysulphids occurs to a considerable extent through contact with the air," 



In general, the insecticidal properties of lime-sulphur are due principally to 

 its power to take up large amounts of oxygen, the amount of free sulphur 

 formed in its decomposition, and its ability to soften the newly secreted wax 

 at the margin of scale insects. The question of the correct valuation resolves 

 itself into the quantitative measurement of these factors. 



The amount of oxygen consumed dei)ends upon the following reactions: (1) 

 CaS5+30=CaS.O3+3S; (2) CaS4+30=GiS2O3+2S ; (3) CaS203=CaS03+S ; 

 and (4) CaS03+0=CaS04. Calcium sulphite is formed very slowly on account 

 of the slow decomposition of the thiosulphate. The oxygen required to produce 

 sulphate is absoi'bed slowly, and it has been found that the sulphite has practically 

 no insecticidal value as far as San Jos6 scale is concerned. It may, however, 

 be important to use as a measure for insecticidal efficiency the oxygen required 

 to convert the polysulphid, using Harris's method for monosulphid (E. S. R., 



25, p. 414) into thiosulphate. It is suggested that the oxygen-consuming power 

 be expressed as the oxygen number, this term to mean the number of grams of 

 oxygen absorbed by 100 gm. of lime-sulphur solution. 



Free sulphur is liberated from lime-sulphur according to (1), (2), and (8) 

 of the foregoing equations. 



The reduction of iwlysulphid takes place rapidly and there is a correspond- 

 ingly rapid deposition of sulphur. The decomposition of thiosulphate is much 

 less rapid. No good evidence, however, exists which shows that the liberated 

 sulphur is not of equal insecticidal \-alue, and the total free sulphur which 

 would be deposited may be expressed as the available sulphur number. 



The insecticidal property of softening the so-called wax of scale insects 

 (E. S. R., 26, p. 753) is not easily estimated and the caustic properties of soluble 

 calcium polysulphid are not explained on the basis of the calcium hydroxid 

 present. " Experience in handling the spray simply verifies the correctness of 

 Shafer's statement that the ' so-called caustic action of the wash on the hands 

 seems rather due to its strong reducing power (power to absorb oxygen) than to 

 the alkalinity of the solution.' " It is possible also that the reducing power may 

 also cause the softening of the so-called wax of the scale insects. If this as- 

 sumption is found to be true the oxygen number mentioned above would be its 

 quantitative measure. " The power of the spray to soften the so-called wax is 

 evidently due to some property of the polysulphids. and in the light of present 

 knowledge no definite statement can be made regarding its exact nature or its 

 exact quantitative analytical measurement." 



"Jour. Amer. Chem. Soc, 30 (1908), No. 1, pp. 63-68. 



