386 
CHEMISTRY: RICHARDS AND SCHUMB 
to say that so far as our present knowledge extends the metamorphoses of 
the Phyllopods into the two great branches of the barnacles were essentially 
con temporaneous . 
REFRACTIVE INDEX AND SOLUBILITIES OF THE NITRATES 
OF LEAD ISOTOPES 
By Theodore W. Richards and Walter C. Schumb 1 
WOLCOTT GlBBS MEMORIAL LABORATORY, HARVARD UNIVERSITY 
Communicated October 31, 1918 
The refractive index of the nitrate of pure common lead was compared very 
carefully with that of the nitrate of uraniolead prepared from the Australian 
material already mentioned in previous reports. The two samples of ni- 
trates were separately recrystallized with care, and finely developed crystals 
were measured in the Abbe-Zeiss total-reflecting crystal refractometer. A 
solution of sulphur in methylene iodide having a refractive index of 1.79 was 
used as the contact liquid, and very careful measurements were made of many 
crystals in varying positions. The refractometer was standardized by means 
of a polished glass prism possessing an exactly known refractive index which 
could be verified by the method of minimum deviation. The refractive in- 
dex of common lead nitrate at 20° was found to be 1.7815, and that of the 
uraniolead nitrate was found to be 1.7814 — each value being a mean of many 
concordant determinations. Thus the difference in atomic, weight shown by 
the two specimens of lead (the atomic weights are respectively 207.20 and 
206.41) has no appreciable effect on the refractive index of the salt — a highly 
interesting and newly discovered fact. 
The two specimens of lead nitrate which had been purified (crystals of which 
had been used in the preceding investigation) were further investigated with 
great care as to solubility, using a method recently described, 2 with small but 
convenient improvements. Greater difficulty was found in obtaining an 
exactly saturated solution than was the case with sodium sulphate, at least 
twenty-four hours at perfectly constant temperature being needed in order 
to obtain constant results. The weighed portions of solution saturated at 
25.02° were evaporated with excess of sulphuric acid and the sulphate was 
gently heated at 350° until constant in weight. Four such determinations 
with ordinary lead nitrate gave values from 37.33 to 37.36 (average 37.342) 
grams of nitrate in 100 grams of solution, and nine determinations with 
uraniolead nitrate gave values from 37.26 to 37.30 (average 37.280) grams of 
nitrate in 100 grams of solution.. These results are precisely proportional 
(within the limit of error of experiment) to the different molecular weights 
of the two samples of nitrate. Expressed in other terms, the molal solubilities 
of the two samples per thousand grams of water are respectively 1.7993 for 
