52 SOME PRINCIPLES AND METHODS OE ROCK ANALYSIS, [bull. 176. 
will indicate absence of manganese when it is really present in quan- 
tity to give normally a strong coloration. Two fusions made side by 
side or successively, under apparently similar conditions, may in one 
case show little or no manganese, in the other considerable. This 
observation has been frequently made, and therefore the absence of a 
bluish -green color in the fusion is not to be taken as proof of the absence 
of manganese. This difference of behavior I can ascribe to no other 
cause than that of a reducing atmosphere in one of the crucibles and 
an oxidizing one in the other, even though the conditions were appar- 
ently alike. 
The contents of the crucible are placed in a rather tall covered 
beaker with some water, and hydrochloric acid of 1.1 specific gravity 
is added in excess. The depth of the pink color usually produced on 
addition of the acid allows of judging approximately as to the amount 
of manganese present. The beaker is placed on the water bath, and 
when disintegration is complete, having been assisted by gentle pres- 
sure with a blunt glass rod, the contents are transferred to a large 
platinum dish and evaporated on the bath. 
SUBSEQUENT TREATMENT. 
From this point the treatment will ordinarily be the same whether 
the boric-oxide or the sodium-carbonate method of decomposition has 
been employed. 
Drying and testing of silica. — As to the best way of rendering silica 
insoluble b} r evaporation, the writer's predilection is for a double 
evaporation instead of a single one on the water bath. By fusing with 
sodium carbonate in the forenoon, the silica is ready for the first filtra- 
tion in the afternoon. It is quite unnecessary to carry the evaporation 
beyond approximate dryness. The filtrate is again evaporated, always 
in platinum, and is ready for final filtration the following morning, 
when approximately 1 per cent of silica is recovered and added to the 
main portion. The writer's experience is that a better separation of 
silica is effected hereby, and in no more time than by a single long 
evaporation. That which is subsequently recovered from the precipi- 
tate of alumina, etc. (p. 56), rarely exceeds a half or, at the most, 1 
milligram. 
Drying in an air bath at 110° C. or higher, or on a hot plate or sand 
bath, or over a free flame, in order to render silica insoluble, offers no 
advantage unless much magnesium is present, and then the most favor- 
able temperature, according to Gilbert, 1 is 120° C. The presence of 
much calcium chloride seems to facilitate dehydration of the silica, 
while magnesium chloride above 120° C, on the other hand, by 
decomposing, forms a silicate which dissolves in hydrochloric acid and 
1 Technology Quarterly, Vol. Ill, p. Gl, 1890. Abstract in Fresenius's Zeitschr. fur anal. Chemie, Vol. 
XXIX, p. 688, 1890. 
