1921.1 



PUBLIC DOCUMENT — No. 31. 



41a 



materials, but in practice this proved to be only a few 

 minutes. 



Where very finely divided arsenic can be obtained, therefore, 

 its use in homemade Bordeaux mixture seems to be profitable, 

 at least as a spray for potatoes. 



Dry Sulfides as Substitutes for the Lime-sulfur Concentrate. — 

 Dry sulfides have appeared on the market in recent years as 

 substitutes for the liquid lime-sulfur. Reports as to their value 

 have been conflicting, some claiming that they give good re- 

 sults, while others consider them worthless. The difficulties 

 with the concentrate are several. Being a liquid it is more 

 inconvenient and more costly to ship, and the material itself 

 will not keep long if exposed to the air. It is also spoiled by 

 freezing. These difficulties are all avoided by the use of dry 

 materials, and if their eft'ectiveness as insecticides is equal to 

 that of the concentrate, the latter will surely be entirely dis- 

 carded after a time. 



This department has no opportunity to carry out field tests of 

 these materials, but with the aid of the Chemical Department 

 of the Station has made some studies on their composition. 



The lime-sulfur concentrate contains calcium thiosulfate and 

 calcium polysulfide. It is the general belief, sustained by ex- 

 perimental tests, that the polysulfide is the active agent of the 

 mixture. The relative efficiency of these compounds is best 

 measured in terms of their sulfur content. An investigation 

 was made of the amount of polysulfide sulfur present in the 

 lime-sulfur concentrate, in dry lime-sulfur, in barium tetra- 

 sulfide (B. T. S.), and in soluble sulfur (sodium polysulfide). 

 In the comparisons wdiich follow, spray made from the lime- 

 sulfur concentrate, strength 1 to 8, was taken as the standard: — 



