ARSENATE OF LEAD. 61 



rarely met with, and have but a limited use in medicine ; 

 while sodium arsenate, from its abundance in the market and 

 low cost, is the only one that may be profitably considered 

 as a source of soluble arsenic acid. Several grades of this 

 salt, containing from 50 to 75 per cent, of arsenate of soda, 

 are found in the market. 



The sodium arsenate of commerce was formerly prepared 

 by dissolving common white arsenic in a hot solution of 

 caustic soda, with the addition of a sufficient quantity of 

 sodium nitrate, the whole being evaporated to dry ness and 

 fused, when the sodium arsenite became oxidized to sodium 

 arsenate. At the present time, commercial arsenate of soda 

 is chiefly produced as a secondary product of the German 

 aniline industries. I ani informed by Mr. John S. Rigby, 

 F.R.S., of Liverpool, that English manufacturers of sul- 

 phuric acid are now using white arsenic in the place of 

 sulphuric acid for decomposing nitrate of soda in the nitre 

 pots, and are thus producing arsenate of soda as a by- 

 product. 



The pure crystallized salt sold by druggists is prepared by 

 dissolving the crude arsenate of soda in water and concen- 

 trating the solution, when di-sodium arsenate having the 

 formula NagHAsO^HgO crystallizes out. 



Pure di-sodium arsenate is isomorphous with di-sodium 

 phosphate, and possesses almost identical physical and chem- 

 ical properties. Under ordinary conditions di-sodium phos- 

 phate crystallizes with 12 molecules of water, but di-sodium 

 arsenate crystallizes with 12 molecules of water only when 

 the solution is evaporated and crystallized below the tem- 

 perature of 18 C. (Fresenius, J. pr. Chemie 56, 30.) In 

 discussing the preparation of arsenate of lead Mr. K. P. 

 McElroy unfortunately overlooked the fact that the arsenate 

 of soda containing 12 molecules of water is quite unsuitable 

 for the purpose, and hence the directions given by him 

 ("Proceedings of the Association of Economic Entomolo- 

 gists," 1895, p. 24) are misleading. The amount of water 

 in the commercial salt is dependent upon its manner of 

 preparation and care in storage. All the commercial grades 

 contain less water of crystallization than the crystallized 

 salt, Na. 2 HAsO 4 7H 2 O, commonly sold by druggists, and when 



