22 Wells—Compounds containing Lead and extra Lodine. 
I have made a large number of crops of the compound, all 
of which agreed with Johnson’s description in forming rect- 
angular crystals of a black color having a marked brassy luster 
upon four of the six faces, and occurring usually in intergrown 
groups of nearly square, flat plates. in preparing these pro- 
ducts the conditions were varied considerably. As a starting- 
point 80¢. of potassium iodide and 50g. of iodine were 
invariably used. These amounts give a slight excess of iodine 
over the proportion required for potassium triiodide. From 40 
to 100 g. of crystallized lead acetate were used, and it was 
found that beyond these limits the preparation was unsuccessful. 
The solvent varied from absolute alcohol, diluted only with 
the water of crystallization of the lead acetate, to alcohol 
diluted with one-half its volume of water. Several crops 
were prepared in the presence of glacial acetic acid, and a 
volume of this amounting to 74 of the total liquid (20°) was 
used with success. The total volume of solvent varied from 
200 to 500%, the larger amounts being used when it was not 
expected to obtain the product by simple cooling. It was cus- 
tomary to dissolve the potassium iodide and iodine in about 
one-half of the solvent to be used and the lead acetate in the 
remainder. The solutions were sometimes mixed boiling hot, 
while at other times a lower temperature was employed. A 
precipitate, evidently consisting chiefly of lead iodide, was 
always produced by mixing the two liquids, but its quantity 
was usually small. The effect of the presence of iodine in 
preventing the precipitation of lead iodide to a great extent is 
very remarkable. ‘The solutions were filtered, sometimes while 
hot, sometimes after a longer or shorter period. The products 
- obtained by cooling formed coherent crusts composed of very 
small, intergrown crystals, while by evaporation over sulphuric 
acid much larger isolated crystals, or groups of crystals were 
deposited. All the analyses given below were made upon 
crops obtained by evaporation, except in one instance. Two 
partial analyses of products made by cooling are not ineluded 
in the list, because the results varied rather widely from each 
other and from the results obtained with the products of evapo- 
rations. The omitted results differed still more from Johnson’s 
analysis than the others. Two or three successive crops were © 
often obtained by evaporating a single solution, and the twelve 
products, analyses of which are given, represent six different 
original solutions. The products were well crystallized and 
most of them seemed entirely satisfactory in regard to purity. 
They were all examined microscopically, and as far as could be 
judged from the appearance of an opaque substance, no 
impurities were present. The samples for analysis were very 
carefully pressed upon filter-paper in order to remove the 
mother-liquor. The salt is practically stable in the air, so that 
decomposition was not to be feared during the drying operation. 
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