calcines, sinters, residues, and other miscellaneous smelter materials. Treatment of such samples with HN03 before evaporation with HC104 is recommended to avoid explosion hazards.Large amounts of iron may be extracted by 25 ml. of isopropyl ether from the perchlorate residue dissolved in 7M HC1. Following the iron separation, evaporate residual ether from the aqueous layer and add 0.5 ml. of HC104. Evaporate to remove HC1 and excess HCIO4 and proceed with the HBr extractions. One gram of ascorbic acid added to the HBr phase 10 minutes prior to adding the isopropyl ether reduces the interference of iron. For high-lead or high-copper materials, use two separate 5-ml. HBr washes rather than a single 10-ml. portion.Any of these modifications is likely to change the over-all efficiency of extraction. Consequently, for accurate results, it is necessary to prepare the standard curve by carrying standard samples through the entire procedure with which the unknown samples are to be treated.