hillebband.] WATER HYGROSCOPIC, ZEOLITIC, CRYSTAL. 33 
further, with every opening of the tube moisture-laden air enters and 
is inclosed with the remainder of the dry powder. It therefore may 
very well happen that a powder at first dry will, after several open- 
ings of the tube, especially at considerable intervals, be nearly as 
moist as when first inclosed. 
It is preferable to weigh the air-dry powder and to make a special 
determination of moisture. If all the portions necessary for an analy- 
sis are weighed out one after another, or even at different times on the 
same day, the error due to difference of hygroscopicity in dry and 
moist weather, which for most of the separate portions is an entirely 
negligible quantity, is eliminated. Only in the main portion, in which 
silica and the majority of the bases are to be estimated, can it ever be 
an appreciable factor. 
Temperature of drying. — As to the temperature to be adopted for 
drying in order to determine so-called hygroscopic moisture, the prac- 
tice has varied at different times and with different workers, ranging 
from 100° to 110° C. For the great majority of rock specimens it is 
quite immaterial which of these temperatures is adopted, since no 
greater loss is experienced at the higher than at the lower tempera- 
ture, given a sufficient time for the latter. It is the present practice 
in this laboratory to employ a toluene bath giving a temperature of 
about 105° C. Should the results show a very unusually high loss, 
the powder is reheated at, say, 125°, in order to learn if the loss is 
progressive with increased temperature. In the affirmative case it 
may be well to repeat the drying at 100°, for a portion of the loss at 
105° was probably due to combined water from a mineral or minerals 
in the rock; but in that case even the loss at 100° may sometimes very 
well include combined water, in which case drying over sulphuric acid 
alone may be desirable, or over dry sand. 
Cautionary hints. — In this latter connection it is proper to point out 
certain pitfalls in the path of the unwary, which, however, are far more 
likely to be encountered in the analysis of minerals, where their 
influence may be of far-reaching consequence. 
A mineral which loses a great deal of water over sulphuric acid — 2 or 
3 per cent, for instance — may need an exposure of several days or even 
weeks for its complete extraction. If the weighings are made from 
day to day, the apparent limit may be reached long before all water 
really removable has been taken up by the acid. Whenever the cruci- 
ble, after weighing, is replaced in the desiccator it is no longer in a 
dry but a more or less moist atmosphere, and its contents, even when 
covered, sometimes absorb a part of this moisture and retain it so per- 
sistently that the acid is unable to bring the powder beyond its previous 
state of diyness in the next twenty-four hours. In fact, it may be 
unable even to reach it unless greater time is allowed. An experiment 
Bull. 176 3 
