d8 SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS, [bull. 176. 
V. WATER— TOTAL- OR COMBINED. 
ARGUMENTS AGAINST " LOSS ON IGNITION" METHOD. 
In a few cases the simple loss on ignition of a rock will give the 
total water with accuracy, but in the great majority there are so many 
possible sources of error that this old-time method can rarely be used 
with safety. Only when the rock is free from fluorine, chlorine, sul- 
phur, carbon, carbon dioxide, and fixed oxidizable constituents can 
the loss be accepted as the true index of the amount of water present, 
and it is rarely that a rock is met with fulfilling these conditions, 
especially as to the absence of ferrous iron. Blast ignition in presence 
of carbon dioxide alone of the above list may give a correct result, 
after separate estimation of the carbon dioxide, provided this ema- 
nates from carbonates of the earths and not from those of iron or 
manganese. The long-maintained and still upheld idea that in pres- 
ence of ferrous iron a sufficiently correct result is obtainable by adding 
to the observed loss an amount needed for oxidizing all ferrous iron is 
not justifiable. There can be no certainty that the oxidation has been 
complete, especially in the case of readily fusible rocks, and at the 
high temperature of the blast a partial reduction of higher oxides is 
not only possible but sometimes certain. The inability to insure com- 
plete oxidation by simple ignition is illustrated in the case of precipi- 
tated ferric hydroxide which has been ignited in contact with its filter 
paper. If the quantity was in any degree large it is sometimes decid- 
edly magnetic, presumably from presence of magnetic oxide, which 
no amount of heating wholly oxidizes, especially in the larger grains. 
Neither is evaporation with nitric acid and reignition sufficient to 
destroy the magnetic property of the oxide, as has been claimed. 
Direct weighing of the water evolved is then imperative in most 
cases, and of the numerous methods advocated, or in general use, sev- 
eral will now be considered. 
DIRECT WEIGHING OF THE WATER WITHOUT THE USE OF ABSORPTION 
TUBES PENFIELD'S METHODS. 
For minerals easily deprived of their water. — If no other volatile 
constituents than water are present, the beautifully simple method first 
used by Prof. G. J. Brush and extended by Prof. S. L. Penfield 1 
leaves nothing to be desired for accuracy. It consists simply in heat- 
ing the powder in a narrow tube of hard glass, enlarged at the closed 
end and provided with one or two further enlargements in the middle 
to hold the water and prevent its running back and cracking the 
hot glass. A capillary glass stopper fitted in with rubber tubing pre- 
lAm.Jour. Sci., ::<! scries, Vol. XLVIII, p. 31, 1894; Zeitsch. fur anorg. ChQmio, Vol. VII, p. 22, 
1894. 
