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•others and as an excessive amount of chlorides is often present as an 

 impurity and these render its employment dangerous, it is very 

 necessary to insist on their absence when ordering this substance. 

 The anhydrous form is very hygroscopic and unless kept in hermetically 

 sealed vessels, it takes up water to an unknown extent, rendering it 

 impossible, without analysis, to prepare a solution of given strength ; 

 details are given of a method of titration with standardised lime-water, 

 using phenolphthaleine as an indicator. The calcium arsenate of 

 commerce generally contains a considerable proportion of impurity, 

 especially the sodium hydrate produced in the reaction, which, if not 

 removed by decantation, reduces the toxicity by 38 per cent. Calcium 

 arsenate properly prepared and free from chlorides, is said to have no 

 caustic effect on plants and, other things being equal, is superior to 

 lead and other arsenates as an insecticide ; the most useful proportions 

 for its preparation are five parts by weight of anhydrous sodium 

 arsenate and two parts of fresh unslaked quick lime, the supernatant 

 liquid being carefully decanted ; the gross product of the reaction 

 contains 23"5 per cent, of arsenic. Arsenate of lead is more difficult 

 and more expensive to prepare and is obtained by adding three parts 

 of neutral acetate of lead (sugar of lead) to one part of sodium arsenate ; 

 the acetate of soda produced in the reaction constitutes an impurity 

 and the arsenical content is 10 per cent, of the salt present. Arsenite 

 ■of soda is usually prepared by boihng together for an hour two parts 

 by weight of white arsenic of commerce and four parts of carbonate of 

 soda, or, if the commercial trisodic arsenite be used, 96 parts of it by 

 w^eight mixed with 187 parts of sulphate of copper will yield 109 parts 

 •of copper arsenite (Scheele's green) ; the gross product of the reaction 

 contains 17"4 per cent, of arsenic. Arsenate of iron is obtained by 

 mixing in solution one part of arsenate of soda by weight with two 

 parts of sulphate of iron, the gross product containing 17*2 per cent, 

 of arsenic. Arsenate of copper is prepared by mixing in solution 104 

 parts by weight of sodic arsenate with 187 parts of copper sulphate, 

 the gross product containing 16'9 per cent, of arsenic. 



The unpurified arsenate of hme therefore contains a larger proportion 

 of arsenic than any of the other compounds in the same state and it is 

 stated that over and above this, the activity of arsenate of hme is 

 G"4 per cent, greater. The necessity for adhering strictly to the pro- 

 portions given so as to have no excess of any reagents and the importance 

 of decanting the supernatant liquid to get rid of bye-products which 

 are not only useless but reduce the energy of the insecticide is insisted 

 upon. The addition of these arsenicals to Bordeaux mixture with 

 the idea of producing a combination spray against both insects and 

 mildew reduces the effectiveness of the arsenical portion by 50 per cent., 

 and it is a matter of common complaint that these mixed sprays are 

 not effective. The presence of large quantities of chlorides in the 

 water or the materials used greatly increases the risk of scorching. 

 The sodic arsenate often contains considerable quantities, and in 

 Algeria, water containing eight parts of chlorides in a 1,000 is of common 

 occurrence and even three paits in 1,000 is objectionable ; river water 

 is the best, but even with this, great care must be exercised in dry 

 weather when the streams are low and the percentage of chlorides 

 greatly increased. 



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