hillebbakd.] WATBB — HYGROSCOPIC, ZEOLITIC, CRYSTAL. 35 
gain weight, other than that of the water from the mineral, sufficient 
to introduce an appreciable error. 
The recent important research of Friedel 1 well showswhai errorsare 
possible in the determination of this easily removable water, since he 
found that certain zeolites which had been largely dehydrated but not 
heated to the point of rupture of the molecular net. could then absorb, 
instead of water, various dry gases in which they might be placed, as 
carbon dioxide, ammonia, carbon disulphide, and other-, even air in 
large quantities, and certain liquids. In the light of this observation 
the cause of the great increase of li per cent in weight of the partially 
dehydrated mineral mentioned on p. 34 may very possibly be attributed 
to air from the desiccator instead of moisture, as was at the time sup- 
posed. At any rate, as Friedel says, the danger of accepting a loss in 
weight as an index of the amount of water lost is clearly shown, and 
thus that method of determining water is for many cases fully discred- 
ited. Just what method to adopt must be largely left to the judgment 
of the operator, who will often be guided by the mineral composition 
of the rock as revealed by the unaided eye or the microscope. 
Friedel (loc. cit.) indicates a means for determining the true weight 
of water lost by minerals behaving like the zeolite-, even without col- 
lecting the water lost, namely, by driving out of the dehydrated and 
weighed mineral, under proper precautions, any air it may haveabsorbed 
in the process of drying and cooling, and collecting and measuring this 
air and thus finding its weight, which, added to the apparent loss, gives 
the true contents in water. 
Argument in fav</r of i/ncl/uding hygroscopic water in summation. — 
The question has been asked: "If the so-called hygroscopic water is 
not always such, but not infrequently includes combined water, why 
is not its determination and separate entry in the analysis entirely 
unnecessary? Why make a distinction, which, after all, may not be 
a true one?" The question involves the further consideration of the 
advisability of including in the analysis at all the loss at 100° or 110 C. 
Many petrographers desire to have all analyses referred to a moisture- 
free basis, in order that they shall be strictly comparable, and there- 
fore would omit the "hygroscopic" water from the list of constitu- 
ents. This would be eminently proper were it always possible to be 
sure that the loss at 100° truly represents mechanically held water. 
Since it very often represents more, and the determination a- to 
whether or not it does in each case is not always possible, and would 
add to the time required for the analysis, it seems accessary to include 
this water. What errors may arise from its exclusion the following 
rather extreme case well illustrates: Certain rocks of Wyoming in 
powder form lost from 1 to *1 per emit of moisture at 110°. That not 
even an appreciable fraction of this was truly hygroscopic the fact of 
i Bull. Soc Min.. Vol. XIX. pp. 14, 94, 1896; Comptes Rendus, Vol. CXXTI, p. 1001 
