36 SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS, [bull.176. 
the uncrushed rocks Losing the same amount fully demonstrates; yet 
the rule followed by many chemists and petrographers would have 
involved the removal of all this water as a preliminary to beginning 
the analysis, and not only would a most important characteristic have 
passed unnoticed, but the analyst would have reported an incorrect 
analysis, inviting to false conclusions and possibly serious confusion. 
Separatt entry of hygroscopic and combined water. — To revert now 
to the primary question, it may be said that the estimation of the loss 
at L00° or 110° C. and its separate entry in the analysis is advisable as 
not infrequently affording at once to the lithologist an indication of 
the mineral character of one or more of the rock constituents, thus 
perhaps confirming the microscopical evidence or suggesting further 
examination in that line. An unusually high loss at 100- would be 
regarded as probable evidence of the presence of zeolites or other min- 
erals carrying loosely combined water. It has been objected that the 
true hygroscopic moisture varies with the degree of comminution of 
the sample and with the condition of the air at the time of weighing, 
and that ii is therefore improper to incorporate it in the analysis: but 
this variation is ordinarily not at all great. Perhaps the time nuvy 
come when it will be the rule to ascertain by additional heating at a 
higher temperature whether the water lost at 100° is to be regarded as 
purely hygroscopic. In such case it would be proper to omit it. and 
a distinct advance would undoubtedly be scored. 
Is all trm hygroscopic water eospelled at 100°? — it has been tacitly 
assumed in the foregoing that true hygroscopic water can all be 
expelled at 100°, which perhaps is not to be accepted as universally 
true Eminent authority holds that it is impossible, in the cases of 
certain foliaceous minerals, notably the micas, to thus entirely remove 
it, but that a part is only driven oh 1 ' at higher temperatures. If this 
is true a further uncertainty is introduced in its determination, which 
not only strengthens the argument in favor of entering all water in 
the tabulation, but also serves to emphasize the difficulties of the 
situation. 
APPARATUS FOR THE DIRECT DETE KM 1 NATION OF WATER AT DIFFERENT 
TEMPERATURES. 
A form ^\' drying oven devised by Dr. T. M. Chatard 1 is in use in 
this laboratory for determining water at different temperatures up to 
35< > ( \ . and gives entire satisfaction. It is an asbestos-covered copper 
box B, shown in different aspects and parts in the accompanying tig. 
3. The box is so constructed that the tube with its contents can be 
removed without detaching from either the drying or collecting tubes, 
which is a great advantage if it is desired to afterwards apply the direct 
1A111. Chcm. Jour., Vol. XIII, p. 110. 1891; Bull. U.S. Geol. Survey X,.. 78, p. 84. 
