McDonnell and Smith — Lead-Chlor Arsenate. 141 



The precipitates resulting from the preceding experiments, 

 when examined under the microscope, were seen to consist in 

 the main of amorphous material, but many crystals were 

 observed, usually small prismatic forms, apparently hexagonal, 

 though too small for definite determination. After many 

 attempts, we were successful in obtaining crystals sufficiently 

 large to permit of the determination of their optical properties. 

 Uniformly crystalline material was first obtained by dissolving 

 pure dilead arsenate in hydrochloric acid (30 cc of concentrated 

 acid to S00 tc of water) adding ammonia until a precipitate 

 was just about to form and then pouring the solution into 10 

 liters of cold water. A precipitate formed immediately and 

 when examined with the microscope was found to consist 

 mostly of very small crystals with numerous long prismatic 

 ones, apparently hexagonal prisms doubly terminated by pyra- 

 mids of the same order, or a combination of the pyramid and 

 basal pinacoid. Two such preparations were made and gave 

 when analyzed the following results : 



& i b 



Lead oxide, PbO 7 5/ 0.6 £ 74-83$ 



Arsenic pentoxide, As 2 B ' 22-84 22-76 



Chlorine, CI 2-GC 2-96 



100-56 100-55 

 O, equivalent to CI 60 -67 



99-96 99-88 



Considerably larger crystals (see fig. 1*) were obtained by 

 treating a solution of FbHAs0 4 in dilute hydrochloric acid 

 with lead acetate to incipient precipitation and allowing it to 

 cool to room temperature. In one such preparation many 

 crystals were obtained *07 by -01 mm , while a few measured -13 

 by -03 mm . The chlorine content of products prepared in this 

 way varied from 2-52 to 2-83 per cent. 



When lead-chlor arsenate crystallizes from a solution con- 

 taining hydrochloric acid it is sometimes accompanied by 

 dilead arsenate or lead chloride. In one case crystals of all 

 three compounds were obtained from the same solution. 



Dilead arsenate is slightly soluble in concentrated sodium 

 chloride solutions (probably because of the acid liberated) but 

 only a few crystals were obtained from a solution of 150 grams 

 of salt in 500 cc of water, which had been boiled in contact with 

 dilead arsenate for one hour and the clear filtrate allowed to 



* Photomicrographs made by G. L. Keenan, micro, analyst, Bureau of 

 Chemistry. 



