April, "15] PARROTT AND SCHOEXE: SULPHIDES AND POLYSULPHIDES 205 



tant consideration than the nature of the base of the sulphides and 

 polysulphides. On the assumption that the compounds of the different 

 bases are equal or nearh^ equal against the scale, the chief factors that 

 enter into a choice of a proprietary insecticide are safeness to foliage 

 when combined with arsenate of lead and economj^, in which respects 

 the advantage appears to lie with the calcium and barium sprays, 

 the former being less expensive of the two. 



In tests to determine their values as stomach poisons, none of the 

 compounds alone, apparently, including those of barium, were harmful 

 to various species of leaf-eating caterpillers or beetles. In most in- 

 stances arsenate of lead with, the sulphides and potysulphides of sodium 

 and potassium was somewhat quicker in manifesting its toxic proper- 

 ties than when combined with the other compounds, which may 

 be explained by the formation of soluble arsenic in the reactions be- 

 tween the poison and the sulphides. Because of the chemical reac- 

 tions, the sodium and potassium sulphides, while more rapid in their 

 poisonous effects upon insects, were generally much more liable to 

 cause injuries to the foliage of fruit trees than the calcium and barium 

 compounds in combination with arsenate of lead. 



According to their nature and mode of action, various substances 

 are incorporated in spraying mixtures to give penetrating and adhesive 

 properties. The agents that are commonly used are silicate of soda, 

 saccharates, soap, glue, gelatine, resin, and home-made and commer- 

 cial oil emulsions. Tests show that glue, soap, glycerine, sodium sili- 

 cate and oil emulsions of various formulas may be combined with the 

 sodium and potassium sulphides, and that similar combinations are 

 possible with, the calcium and barium sulphides except in the case of 

 soap and oil emulsions. As to the influence of such combinations on 

 effectiveness, no marked results have been observed, except when soap 

 and oil emulsions were employed with the sodium or potassium sul- 

 phides against aphides or oil emulsions with the same sulphides for 

 the control of the San Jose scale. The principal gain in the latter 

 case appears to be in an increased rate of toxicity. The destructive 

 action of the sulphides alone on the scale is apparently slower, and 

 while prolonged over a more extended period may, however, prove no 

 less efficient. 



President H. T. Fernald: The next paper is by W. M. Scott. 



