HILLEBBAND.] SPECIFIC GRAVITY. 29 
in exceptional rases affect the second decimal by more than a single 
unit. For instance, an undried rock of 2.775 specific gravity contain 
ing in the uncrushed state the high percentage of 0.3 hygroscopic 
moisture will have a density of 2.70 when dry; a rock of 2.982 specific 
gravity, undried, will have a density of 3.00 after removal of 0.3 per 
cent of moisture. The difference becomes greater as the density of 
the rock increases. 
This method of ascertaining the specific gravity of rocks is cer- 
tainly more convenient than, and for compact rocks is believed to be 
decidedly preferable to, that of the pyenometer, in which the frag- 
ments must be reduced to small size with consequent formation of 
more or less powder, which is subject to slight loss in the various 
manipulations. To exclude this powder and employ only small frag 
ments would introduce a possible source of error, since it is likely to 
consist largely of the most easily abraded minerals and consequently 
not to have the average composition of the mass. By following the 
instructions given above, loss of material is absolutely avoided, a 
decided saving in time is effected, and considerable weights can be 
easily employed with consequent lower probable error in the results. 
To vesicular rocks, however, notably certain lavas, the above pro- 
cedure is, of course, inapplicable, unless the datum is desired for 
certain considerations in which the relative density of large rock 
masses as they occur in nature is sought, as for the comparison of 
building stones or the calculation of large known or assumed areas of 
particular rocks. 
PenfielcPs method for mineral fragments. — Peniield 1 recommends the 
following modification of the suspension method as more convenient 
than that by the pyenometer in many cases for small fragments of 
minerals. 
After boiling in water, the substance is transferred with water to a 
small glass tube about 8 mm. by 35 mm., provided with a fine plati- 
num wire for suspension. This is weighed full of water in another 
vessel of water, and again after the removal of the mineral, the weight 
of which is found after drying. 
This method is, of course, more applicable to homogeneous min- 
erals than to rock fragments, and will therefore be applied in rock 
analysis chiefly to the determination of the specific gravity of the 
mineral grains separated by heavy solutions or acids. 
PYCNOMETER METHOD. 
If the pyenometer has to be used, as is generally the case when the 
density of any one of the mineral ingredients of a rock is desired after 
Am. Jour. 8ci.,3d series, Vol. L, p. 418, 1895. 
