24 
SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS, [bull. 176. 
LIMITS OF ALLOWABLE ERROR IN SUMMATION OF ANALYT- 
ICAL RESULTS. 
As is well known, a complete silicate rock analysis which foots up 
less than 100 per cent is generally less satisfactory than one which 
shows a summation somewhat in excess of 100. This is due to several 
causes. Nearly all rea- 
gents, however care- 
fully purified, still 
contain, or extract 
from the vessels used, 
traces of impurities, 
which are eventually 
weighed in part with 
the constituents of the 
rock. The dust enter- 
ing an analysis from 
first to last is very con- 
siderable, washings of 
precipitates may he in- 
complete, and if large 
filters are used for 
small precipitates the 
former may easily be 
insufficiently washed. 
Given the purest 
obtainable reagents, 
an ample supply of 
platinum, facilities for 
working, and a reason- 
ably clean laboratory, 
there is no excuse for 
failure on the part of 
a competent chemist 
to reach a summation 
within the limits 99.75 
and 100.50. Failure 
to attain 100 per cent 
in several of a series 
of analyses of similar 
nature should be the 
strongest evidence that something has been overlooked. Excess above 
100.5 per cent should be good ground for repeating portions of the 
analysis in order to ascertain where the error lies, for it is not proper 
to assume that the excess is distributed over all determined constit- 
Fkj. 2. — Radiator for rapid and safe evaporation. R is of sheet 
iron, also nickel (Jannasch). Various sizes. A convenient height 
is 7 cm , width at top 7 cm and at bottom 5 ctn . 
