December, '14] TARTAR: VALUATION OF LIME-SULFUR 465 



California perhaps a quarter of a century against almond red spider. 

 The experimental work carried on by the experts of the California 

 Agricultural Experiment Station^ and the Bureau of Entomology, 

 United States Department of Agriculture^ shows that towards certain 

 insects free sulfur has marked insecticidal properties. 



There is also the possibility that hydrogen sulfide, a gas poisonous to 

 insects, may be liberated from lime-sulfur when it combines with the 

 carbon dioxide of the atmosphere or that given off in the breath of 

 insects. So far as the writer knows, no means has been found to ascer- 

 tain the extent to which this occurs. Experimental work carried out 

 at this laboratory, however, indicates that if hydrogen sulfide is Hber- 

 ated under normal conditions it is in very small quantity, and, 

 evidently, is not an important matter to consider here. 



From the discussion preceding, it appears that, in general, the in- 

 secticidal properties of lime-sulphur are due principally to the follow- 

 ing-named properties: 



(1) Its power to take up large amounts of oxygen, (2) its ability 

 to soften the newly secreted wax at the margin of scale insects, and (3) 

 the amount of free sulfur formed in its decomposition. If this be true, 

 then the question of the correct valuation resolves itself into the 

 quantitative measurement of these factors. 



The amount of oxygen consumed depends upon reactions as repre- 

 sented in the following equations: 



CaSs -f3 = CaS203+3S 



CaS4 +3 = CaS203+2S 



CaSaOa =CaS03+S 



CaS03+ = CaS04 

 The combination of oxygen with the moist polysulfides is very rapid 

 and quantities of the tetrasulfide or pentasulfide containing the same 

 amount of calcium would absorb the same amount of oxygen and con- 

 sequently produce the same amount of thiosulfate. This last named 

 substance decomposes very slowly under ordinary conditions. For 

 this reason, calcium sulfite is formed very gradually and the oxygen 

 required to form the sulfate is absorbed slowly; too slowly, in the 

 writer's opinion, to make it of insecticidal importance. Investigations 

 made by the entomologist of this station indicate that calcium sulfite 

 has practically no insecticidal effect upon San Jos6 scale. 



The oxygen required to convert the polysulfides present in a given 

 solution into thiosulfate can be easily estimated by the use of the 

 methods of Harris.^ The titration used in the determination of " mono- 



1 BuU. 154 and 234 Calif. Agr. Exp. Sta. 



» Private correspondence. 



3 Tech. Bull. No. 6, Mich. Agr. Exp. Sta. 



