BULLETIN 750, 1 . S. DEPARTMENT OF AGRICULTURE. 



possible probably would be preferable. On first thought, such a pro- 

 - would be the direct slaking of the stone lime with the requisite 

 strength arsenic acid solution. But such a procedure is not prac- 

 ticable. Owing to the slow surface action, the As 2 O s is locally 

 always in excess, resulting in the formation of dicalcium or even 

 monocalcium arsenate (if Ihe H 3 As0 4 is strong enough), which, 

 while it may be changed later by the excess of lime, renders the 

 product lumpy or granular. Even the use of strong arsenic acid 

 with a concentrated lime paste suffers from the same drawbacks — 

 the addition of the acid causes a further thickening of the paste. 

 and thorough and rapid mixing becomes impossible. 



SLAKING THE LIME. 



Only more or less dilute lime paste, made by the slaking of quick- 

 lime, was used in the experiments here reported. It is sell-evident 

 that the ease with which such a paste will combine with acid is 

 directly dependent on the smoothness of the paste. Consequently 

 experiments to determine the best procedure to follow in slaking 

 so that the paste would be smooth and free from lumps were made 

 first. It was soon found that the use of 3 to 3% times as much 

 water (by weight) as lime gave a paste which was entirely satisfac- 

 tory, if the lime employed was sufficiently pure. This produces a 

 paste rather thick, but not thick enough to interfere with good mix- 

 ing when working with small quantities. Best results were ob- 

 tained by using boiling water for slaking, although in bulk the 

 natural heat of reaction would be sufficient. 



PROPER PROPORTION OF ARSENIC ON1D AND LIME. 



As previously noted, it is essential thai a compound for spraying 

 purposes shall be free, or nearly so, from arsenic in water-soluble 

 forms. As seen by reference to the description of practically pure 

 crystalline tricalcium arsenate, that material is appreciably acted 

 upon by water. Other work done in the Bureau of Chemistry on 

 this subject has shown thai all of the calcium arsenates more acid 

 than this are likewise somewhat soluble (or decomposable), the 

 action increasing until the monoarsonate. which is very readily 

 soluble in water, is reached. Evidently, then, it is necessary to 

 look toward the basic side. II lias long been known Unit the addi- 

 tion of ;in excess of lime to a spraying compound renders the arsenic 

 insoluble. The object here was to find the right proportions 1o 

 produce a material combining high arsenic oxid content with suf- 

 ficiently low water-soluble As.,0, to prevent burning or reduce it to 

 the lowest possible limit. Two series of experiments were made 



