62 SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS, [bull. 176. 
oxide, its weight being always very small, and is then tested for cobalt 
in the borax bead. 
It is somewhat unsafe to consider traces of copper found at this stage 
to belong to the rock if the evaporations have been conducted, as is 
usually the case, on a copper water bath, or if water has been used 
which has been boiled in a copper kettle, even if tinned inside. There- 
fore, and because of its contamination by a little platinum, it is better 
to determine copper in a separate portion if its presence is indicated 
with certainty. (See p. 54.) 
X. CALCIUM AND STRONTIUM (BARIUM). 
SEPARATION FROM MAGNESIUM. 
Precipitation and ignition of the oxalates together. — The platinum 
derived from the dish in the silica evaporation, except for the small 
portion precipitated with the manganese sulphide, is now wholly in 
the nitrate from the latter. Its separation at this or any other stage 
•is quite unnecessary; nor is the removal of ammonium chloride usually 
demanded, since there is no undue amount present in most cases, the 
first precipitation of alumina, etc., having been by sodium acetate. 1 
Therefore, without destroying ammonium sulphide the calcium and 
strontium are thrown out by ammonium oxalate at boiling heat, the 
precipitate, often darkened by deposited platinum sulphide, is ignited 
and redissolved in hydrochloric acid, boiled with ammonia to throw 
out traces of alumina sometimes present and reprecipitated as before, 
but in a small bulk of solution. It is weighed as oxide, transferred 
to a small flask of 20 cm. 3 capacity, dissolved in nitric acid, evaporated 
to diyness at 150 to 160° C, and the separation of strontium from 
calcium effected by ether-alcohol 2 as described below. 
The weight of strontia found deducted from that of the two oxides 
gives that of the lime. 
Necessity for two precipitations by ammonium oxalate. — It may be 
said with regard to the separation of calcium from magnesium that 
two precipitations by ammonium oxalate are essential to the attain- 
ment of correct results, not only for the complete removal of mag- 
nesium but of sodium as well, the retention of compounds of the 
latter element by calcium oxalate being now generally known. For 
the treatment of the filtrates, see Magnesium, p. 64. 
1 If two or three precipitations by ammonia alone are depended on, the second and third nitrates 
are evaporated rapidly to dryness and the ammonium salts removed by ignition. 
2 See Fresenius: Zeitschr. fur anal. Chemie, Vol. XXXII, pp. 189, 312, 1893, for the latest improve- 
ments in this method. 
