hillebrand.] PREPARATION OF SAMPLE. 31 
HEAVY SOLUTIONS NOT SUITABLE FOR ROCKS. 
Because of their roughness, porosity, and complex mineral composi 
tion the density of rock fragments can not be accurately determined 
b}' that of heavy solutions in which they may remain suspended. 
III. PREPARATION OF SAMPLE FOR ANALYSIS. 
QUANTITY OF ROCK TO BE CRUSHED. 
In the great majority of cases a few chips from a hand specimen will 
well represent the average of the mass, but with rocks in which a por- 
phyritic structure is strongly developed the case is different. Here a 
large sample should be provided, gauged, according to the size of the 
crystals, and the whole of this should be crushed and quartered down 
for the final sample. Unless this is done, it is manifest that the analy- 
sis may represent anything but the true average composition of the rock. 
CRUSHING. 
Mechanical appliances for reducing samples to line powder are much 
in use in technical laboratories, where they answer their purpose more 
or less satisfactorily, and something similar is needed in those scientific 
laboratories where rock anatysis is of daily occurrence and many sam- 
ples must be reduced to fine powder in a short space of time. For 
accurate analyses the use of steel crushers and mortars is out of the 
question, because of the danger of contamination by particles of metal 
and the impossibility of cleansing the roughened surfaces after they 
have been in use a short time. Extraction by the aid of a magnet of 
steel particles thus introduced into the powder is quite inadmissible, 
since the rocks themselves, almost without exception, contain magnetic 
minerals. The method of rough crushing on a small scale found to be 
most satisfactory in practice is to place each fragment as received on a 
hard steel plate about 4| em. thick and 10 cm. square, on which is like- 
wise placed a steel ring 2 cm. high and of about cm. inner diameter, 
to prevent undue flying of fragments when broken by a hardened 
hammer. In this way a considerable sample can soon be sufficiently 
reduced for transfer to the agate grinding mortar with a minimum of 
metallic contamination. 
For breaking large pieces of rock to small sizes a thick iron plate 
with specially hardened surface and a similarly hardened pounder, such 
as street pavers use, will probably render the best service, but the 
hardening must be done with extreme care. 
<;kinding. 
Of the various grinding arrangements on the market purporting to 
fulfill their purpose few, if any, observed have met the conditions 
