McDonnell and Smith — Lead-Chlor Arsenate. 143 



Crystals of this form are very frequent when produced by slow 

 growth. They resemble more nearly natural mimetite, which 

 rarely shows clear cut crystals but almost always characteristic 

 " barrel-shaped " forms. 



In the preceding experiments the resulting products in every 

 case (except where dilead arsenate was transposed by boiling 



Fig. 2. 



Fig. 2. Lead-chlor arsenate, artificial mimetite ( x 238). 



with sodium chloride solutions) contained chlorine in excess 

 of the theoretical amount for mimetite.* 



*This suggested that there might be other Jead-chlor arsenates containing 

 relatively more lead chloride. In fact a natural mineral (Georgeadisite) has 

 been reported (Comptes Kendrts, cxlv, 783, 1907) the analysis of which corre- 

 sponds to the formula Pb 3 (As0 4 ) 2 .3PbCla or (PbCl) 3 As0 4 . We have 

 succeeded in producing another compound of this class, having a chlorine 

 content of 3 4 per cent, which is referred to later, and evidence of a third 

 containing about 4 per cent. 



A lead-chlor arsenate is formed by double decomposition between dilute 

 solutions of di- and trisodium arsenates, or even arsenic acid, and lead 

 chloride, and also when dilute solutions of sodium and potassium dihydrogeu 



