268 THE MONTHLY BULLETIN. 



DISCUSSION OF THE TABLE. 



Arsenical-fung-icide Combinations. 



Bordeaux. Again referring to the table, it is seen that both Paris 

 green and acid lead arsenate are improved by mixing with Bordeaux. 

 This is on account of the excess of lime in the Bordeaux as now com- 

 monly made. The lime forms a fairly insoluble compound with the 

 soluble arsenic in Paris green and acts in the same capacity after 

 spraying if any soluble arsenic is formed by the action of the weather 

 upon Paris green or acid lead arsenate. Calcium ars^nite can not be 

 improved in this way, as this arsenical is made with an excess of lime 

 and the additional lime of the Bordeaux could not make it more safe 

 to use. No safer arsenical is known than neutral lead arsenate ; 

 therefore, no added safety would result by admixture with Bordeaux. 

 Insufficient data is at hand to warrant the classification of the zinc 

 arsenite-Bordeaux combination. 



Lime-sulfur. Lime-sulfur is now coming to be a very formidable 

 rival to Bordeaux mixture and the other copper fungicides and is also 

 extensively used as a contact insecticide. Hence it is often desirable to 

 mix this valuable remedy with an arsenical. For this purpose, the 

 choice of arsenicals is limited to lead arsenate and very likely should 

 be entirely restricted to the neutral type. Lime-sulfur is a very 

 unstable compound and is easily decomposed by slight influences, and 

 is especially susceptible to change when mixed with other spray 

 materials. This fact is evident when it is noticed that only two of 

 its combinations are in class A. Its own decomposition products are 

 quite harmless, but its effect on its associates is very frequently of a 

 serious nature. Particularly is this true of its effect upon the arsenite 

 (Paris green, calcium arsenite, and zinc arsenite). The arsenites in 

 general are less stable compounds than the arsenates and are prone to 

 yield soluble arsenic in either alkaline or acidic solutions. The acid 

 lead arsenate is also unstable in alkaline solutions. Lime-sulfur is of 

 an alkaline nature and therefore the various arsenical-lime-sulfur mix- 

 tures must all be placed in the dangerous class, with the exception of 

 the neutral lead arsenate and possibly with the exception of the acid 

 lead arsenate under certain favorable conditions. 



Instances are known in which acid lead arsenate and even Paris green 

 have been sprayed in combination with lime-sulfur without apparent 

 injury to foliage, but it is thought that cases of this kind are very rare 

 and must have been done under unusually favorable climatic conditions. 



The acid lead arsenate-lime-sulfur combination is given a question 

 mark in the table for the reason that many of the reports of experi- 

 ments make no mention of the type of lead arsenate used. "Where the 

 distinction is made, however, the neutral type is favored. 



Iron Sulfid. The so-called iron sulfid fungicide is prepared by mix- 

 ing a solution of iron sulfate with an excess of lime-sulfur solution. 



There results a mixed precipitate of insoluble iron sulfid (black), 

 free sulfur (yellowish), and calcium sulfate (white). The excess of 

 lime-sulfvir is washed out and there is left a paste of the three precipi- 

 tates which are quite insoluble and inert toward most ordinary 

 reagents. The iron sulfid is black and is present in sufficient quantity 



