H. L. Wells — Halides of Potassium and Lead. 191 



K 3 Pb 2 I,4H 2 

 K 3 Pb 2 Br 8 .4H 2 



These formulae may be also written, 



3KI.2PbI 2 .I.4H 2 and 3KBr.2PbBr,.Br.4H 9 0. 



The composition of these salts is very remarkable on account 

 of the small amount of the extra halogen that they contain. 

 They apparently do not correspond to any other chemical com- 

 pound that is known. 



The Iodide, K % PIJ % AHJ) : — This salt forms brilliant, 

 black, prismatic crystals, sometimes a centimeter or two 

 in length and three or four millimeters in diameter. Although 

 the crystals have fine prismatic faces, they never appear to 

 have definite terminations. The ends usually appear fibrous, 

 as though made up of numerous small crystals in parallel posi- 

 tion. When the crystals are crushed on paper it is evident 

 that they enclose much mother-liquor. The salt is deposited 

 from nearly or quite saturated solutions of potassium iodide 

 containing lead iodide and iodine. It is deposited at ordinary 

 temperature, usually slowly, after the lapse of several hours or 

 even after several days. In preparing the compound, the lead 

 iodide and the iodine can be varied considerably, but it is 

 formed only in very concentrated potassium iodide solutions 

 and it is difficult to obtain crops of it which are not evidently 

 contaminated with this salt in the form of crystals. The salt 

 is stable in the air but it is instantly decomposed by water or 

 alcohol, so that it cannot be washed. 



Six separate crops have been analyzed, and great care has 

 been used in selecting them and in drying them on paper for 

 analysis. In two cases the product was rapidly and finely pul- 

 verized during the drying operation, but without any effect 

 upon its composition. The results of the six analyses agree 

 with remarkable closeness, but in spite of this fact it must be 

 assumed, from considerations which will be given subsequently, 

 that all these products were seriously contaminated with potas- 

 sium iodide. The fibrous nature of the crystals and the con- 

 centration of the mother-liquor make the possibility of such a 

 contamination very evident, but the constancy of this contam- 

 ination, as indicated by the uniformity of the analyses, is very 

 remarkable in view of the fact that some of the products were 

 made at wide intervals of time, covering a period of about six 

 months, so that there were considerable variations in the labo- 

 ratory temperature. 



The products were made under the following conditions : 



