STATE GEOLOGIST. 149 
clay. The sand and silt are separated from each other by repeating the 
subsidences and decantations in the manner just described. 
“In this way there is removed from the sediment, on the one hand, a 
portion of silt free from sand and dust, and, on the other hand, a portion 
of sand free from silt; then is obtained a second intermediate portion 
consisting of sand and silt, but less in amount than the first and containing 
particles of diameters much more nearly approaching 0.05 mm. By re- 
peating this process a few times this intermediate portion will be reduced 
to particles whose diameters are very near 0.05 mm. and which may 
be divided between sand and silt, according to judgment. The amount 
of this is usually very small. As soon as portions are separated which 
the microscope shows to be pure sand or pure silt, they are added to the 
chief portions of these grades already obtained. | 
“The same process is applied to the separation of silt from dust. 
When all the silt has been removed from the dust and clay, the turbid 
water containing the dust and clay is set aside and allowed to settle in 
a cylindrical vessel for 24 hours. ‘The vessel is filled to a height of 20 cm. 
According to Hilgard, the separation of the dust from the clay during a 
subsidence of 24 hours will give results of sufficient accuracy, although 
the clay then suspended will not be entirely free from measurable fine 
particles up to .oOI or .002 mm. diameter. 
“Small beakers and small quantities of distilled water are used at 
first for the decantations, as thus the duration of subsidence is less and 
more decantations can be made in a given time. Beakers of about 100 cc. 
capacity are convenient for the coarser grades, but it is necessary to 
use larger vesels for the fine sediments from which turbid water accumu- 
lates that cannot be thrown away, as may done with the clear water, 
from which the coarse sediments settle out in a short time. The water 
should be kept as small in amount as possible. | 
“Pestling with a soft rubber pestle and boiling will help to break up 
the fine lumps of clay. 
“A mechanical analysis is made in about 5 or 6 hours, exclusive of 
the time necessary for collecting the dust and separating the clay, for 
which a subsidence of 24 hours is allowed. 
“The sediments are prepared for weighing by allowing them to sub- 
side completely, decanting the clear water as far as possible, rinsing them 
into a weighed platinum dish, evaporating and igniting. The dish is 
cooled in a dessicator.” 
Although soil analysis for agricultural purposes requires great ex- 
actness as to the percentages of very fine material, for the purpose of the 
cement industry this very elaborate differentiation is not necessary. The 
cement chemist requires to know only how much of a clay is mechanically 
fine enough for the chemical reaction involved. Generally speaking, we 
might say that everything passing the 150 mesh sieve (150 mesh to 
