98 SOME PRINCIPLES AND METHODS OE ROCK ANALYSIS, [bull. 176. 
a hole in a piece of stout asbestos board clamped in a vertical position, 
and heated for about ten minutes by a low, flat flame placed at con- 
siderable distance beneath. As soon as the odor of ammonia is no 
longer perceptible the nearly full flame of two Bunsen burners is 
applied, and continued for forty to fifty minutes. The sintered cake 1 
detaches readily from the crucible as a rule; if not, it is softened up in 
a few minutes by hot water and digested in a dish until thoroughly 
disintegrated. It is first washed by decantation, and any lumps are 
broken up by a pestle, then thrown on the filter and well washed with 
hot water. The residue should dissolve completely in hydrochloric 
acid without showing the least trace of unattacked mineral, not even 
of quartz. 
Separation of calcium and sulphuric acid. — All but a trifling amount 
of the calcium is separated at boiling heat in a large platinum dish by 
double precipitation by ammonia and ammonium carbonate. The com- 
bined filtrates are evaporated to dryness and the ammonium salts arc 
carefully driven off. From the aqueous solution of the residue — but 
a few cubic centimeters in bulk — the rest of the calcium is thrown out 
by ammonia and ammonium oxalate, this last being more effective than 
the carbonate. The filtrate, caught in an untared platinum crucible 
or small dish, is evaporated to dryness and gently ignited; the residue 
is moistened with hydrochloric acid to decompose any alkali carbonate 
that may have been formed, again evaporated, ignited, and weighed. 
On solution in water a few tenths of a milligram of fixed residue is 
invariably left, which should be collected, ignited, and weighed in the 
same crucible or dish in order to arrive at the weight of the chlorides. 
If the rock contains sulphur this will be in part found with the 
chlorides as sulphate. Therefore, if the sulphur is at all considerable 
in amount it must be removed by a drop of barium chloride before 
the final precipitation of the calcium. The excess of barium is removed 
by ammonium carbonate and the last of the calcium by ammonium 
oxalate, as above. If the sulphur is not thus removed there is danger, 
if not certainty, of the potassium -platinic chloride carrying sodium 
sulphate. A faint reaction for sulphate can usually be obtained, 
anyway, if the evaporations have been made on a water bath heated 
by gas. 
Precipitation of potassium. — To the solution of the chlorides in a 
small porcelain 2 dish an excess of platinic chloride solution is added. 
The dilution should be such that when heated on the water bath any 
precipitate that may form wholly redissolves. Evaporation is then 
carried on till the residue solidifies on cooling. It is then drenched 
1 To avoid the formation of a melted cake with silicates very high in iron it is advisable to increase 
the proportion of calcium carbonate. 
- Preferred to platinum because of the possibility, under certain rare and ill-understood conditions, 
of the formation of an insoluble subchloride of platinum, probably by reaction between the platinum 
of the dish and that of the salt. (See also Bohn, Zeitschr. fur anal. Chemie, Vol. XXXViil, p. 349. 1899.) 
