o() SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS, [hull. 176. 
separation by one of the approved methods, it being then in a more or 
less finely divided state, the most accurate procedure is that adopted 
in this laboratory )yy Mr. L. G. Eakins a number of years ago. The 
pycnometer used is one with a capillary stopper, provided with a milli- 
meter scale etched in the glass, the divisions being numbered both ways 
from the center and calibrated by mercury so that the value of each 
one in weight of water is known. The capacity^ of the flask filled with 
water to the zero division is then calculated for every half degree of 
temperature from 0° C. to 30° C. , by making a series of careful weigh- 
ings, in which, the capacity of the stem being known , it is quite immaterial 
at what level the water stands provided it is within the limits of the 
scale. The exact temperature is obtained by an accurate thermometer 
placed in a companion vessel of similar shape to the pycnometer and 
containing a like amount of water, both being left in the balance case 
till its temperature has been nearby or quite assumed, as shown b} T a 
second thermometer. The weighing must of course be made before 
the thread of water has sunk beneath the lowest division, which it will 
do after a time, even though at first filling the bore to the top of the 
stopper; and the corrected weight full of water to the zero mark is 
found by adding or subtracting the needed amount, as shown by the 
height of the thread on the scale. 
For each pycnometer in use, and these are of different sizes, is prepared 
a table showing its weight, the value of each scale division in grams of 
water, and the capacity of the flask at different temperatures, as indi- 
cated above. The preparation of such a series of flasks is time saved 
in the end, for the weighing of the flask full of water each time a 
density determination is made is rendered superfluous. All that is 
necessary is to look up in the table the weight corresponding to the 
temperature. 
The density of the previously weighed substance in this case is now 
determined in much the same way, after the unstoppered pycnometer 
containing it and nearly filled with water has stood with its companion 
vessel of water under the air pump the necessary length of time. The 
water needed to fill the flask is taken from its companion. 
All who have used the pycnometer method for fine substances know 
the difficulty experienced in preventing a portion from being held at 
the surface, despite all attempts at making it sink. Hence it often 
happens that a very small portion runs out around the sides of the 
stopper on inserting it. If the flask rests in a small tared dish the 
grains thus forced out may be washed down into it and weighed after 
evaporation in order to get the correct weight of that in the flask; or, 
after weighing, the contents of the flask may be emptied into a tared 
dish and the water slowly evaporated off in order to get the weight 
of the mineral. Usually this way is less to be recommended than the 
other. 
