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Proceedings of the Royal Society 
g. When a crystal of ammonia iron alum is placed in a solution of 
ammonia alumina alum, replacement goes on very slowly, rust at 
the same time being thrown down. 
h. Potash chrome alum grows on crystals of potash alumina alum. 
i. If a crystal of ammonia iron alum he placed in a solution of 
potash chrome alum, the iron alum is turned out, whilst a skeleton 
in chrome alum of the original crystal is left ; if this skeleton be 
placed in a solution of ammonia alumina alum, the latter alum 
grows on it, completing the form of the original crystal of ammonia 
iron alum. 
6. On a New Method for the Separation of Yttrium and Erbium 
from Cerium, Lanthanum, and Didymium. Part I. By 
J. Gibson, Ph.D., and R. M. Morrison, D.Sc. 
The method for the separation of these two groups hitherto in use 
was first proposed by Berzelius, and has been followed by almost 
all chemists who have investigated these earths. For the details of 
this method we must refer to his “Handbuch;” but we may briefly 
state that it depends on the relative solubilities of the double sul- 
phates of these metals with potassium in a saturated solution of 
potassic sulphate. The yttrium and erbium double sulphates are 
said to be easily and completely soluble, while the double sulphates 
of cerium, lanthanum, and didymium are said to be perfectly 
insoluble. Wishing to prepare pure salts of yttrium and erbium, 
in order if possible to obtain the metals and to determine their 
specific heats, we tried this method, and found that, although it is 
a good rough method, the separation is by no means complete. 
We repeated the separation six times, but never obtained the 
earths pure, the spectroscope always showing the characteristic 
absorption-spectrum of didymium, provided we examined a suffi- 
ciently thick layer of a saturated nitric acid solution. The incom- 
pleteness of this method is indeed acknowledged by Bahr and 
Bunsen in their well-known paper on these metals.* The test 
given for the presence or absence of these two groups by these 
chemists was the presence or absence of the absorption-spectra of 
didymium and erbium respectively. We found, however, that 
* Ann. d. Chem. u. Pliar. cxxxvii. 
