34 
SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS, [bull.176. 
on 1 gram of tyrolite, made and published some years ago, seems to 
illustrate this point in part: 
Bours 
exposed. 
Loss. 
Hours 
exposed. 
Loss. 
18 
26 
23 
24 
23 
24 
25 
Gram. 
0.0231 
. 0083 
.0029 
.0012 
. 0008 
. 0001 
. 0003 
24 
24 
48 
24 
283 
Oram, 
0. 0002 
. 0001] 
.0000 
.0002 
.0380 
The experiment might reasonably have been considered ended after 
the one hundred and fifty-eighth hour, when a loss of but 0.1 milligram 
was shown during twenty-four hours; but nevertheless a nearly steady 
loss of 0.3 milligram per day took place for six days more, and might 
have been longer observed but for the interruption of the experiment. 
Again, it is a common practice to determine the water given off by 
hydrous minerals in an air bath at temperatures far above 100 c C. To 
insure accuracy this experiment should not be made in crucibles or 
dishes which must be cooled in a desiccator. One instance will suffice: 
A gram of a mineral mixture containing about 17 per cent of water, of 
which about 3 per cent was driven off at 100° and 8 or 9 per cent at 280°, 
was, after several hours' heating at the latter temperature, placed in 
a desiccator over sulphuric acid and weighed as soon as cold, then 
replaced and again weighed the next day. It had regained 1J per 
cent of its original weight, although the desiccator was tightly closed 
and the crucible covered, showing apparently a drying power superior 
to that of the acid. 
A specimen of tyrolite was found on one occasion to lose 10.31 per 
cent at 280° C, and on another occasion 11.33 per cent. In the latter 
case the drying and heating at progressive temperatures had continued 
during a period of 528 hours, the weighings being made usually from 
day to day; whereas in the former the duration of the experiment was 
much shorter and the intervals between weighings were but a few 
hours each. 
J '*ror, dure irn special cases. — For experiments of the kind just indi- 
cated the powder should be heated in a weighed tube, through which 
a current of dry air can be passed, and allowed to cool therein, or 
else the water given off should be collected and directly weighed in 
suitable absorption tubes, even though the long time often required 
is an objection to this lattter method, since the absorption tube may 
