STATE GEOLOGIST. 285 
It is thus seen that in these raw mixtures there is practically no differ- 
ence in the fineness attained in grinding between limestone-clay and marl 
mixes. ‘The figures of the analysis themselves are of interest inasmuch as 
they show how far the fineness is really carried in practical work. The last 
column of figures showing the amount of matter finer than 200 mesh 1s the 
best gauge of the fineness. 
The question of the grinding of quartz being an int resting one from 
the practical standpoint, it seemed worth while to the writer to determine 
on a small scale, if possible, how far the grinding of quartz has progressed 
during a certain period of time during which the same weight of a 
limestone clay mixture has been ground to the desired fineness, passing 
through a 100-mesh sieve and leaving no residue after ignition and treat- 
ment with hydrochloric acid and sodium carbonate solutions. This work 
was carried out by taking 25 pounds of a raw cement mixture consisting 
of Wellston limestone and silicious clay obtained from a drift deposit on 
the State University campus passing a 10-mesh sieve and grinding the 
mixture in an iron ball-mill with 136 pounds of flint pebbles till it 
answered the above tests, which required four hours. ‘This time could 
thus be taken as representing the time necessary for the thorough grind- 
ing of a commercial cement mixture. 
A charge of 25 pounds of glass sand whose mechanical analysis was 
as given below was placed in the mill with 136 pounds of pebbles and 
ground somewhat over four hours. 
Per cents. 
Residue: onus Omateshysieve as-is ean eee PS ee 49.16 
IVESTAUC ONE OOMMeShesieGvie mins aisy ee pe eons ie 33.26 
Residuevony so Uemeshecieviencr myn: ceiie ects aes sO 282, 
Residue omy LOOmmesh' sieves are =e sre ee er 6. 64 
IRCsiickos @in, WAD iaesll SMOWSs 2 oo 5 con uo aes ooo ok eae ea Le We 
estdueromeliaOnmeshysievenctase snc star wet 1er03 
esrdmerone2 OOsmeshysieverrr arian 0 eee nt 0.29 
Finer than 200 mesh sieve............ 0.54 
There was only 0.83 per cent. of chemically available material in this ~ 
sand. At intervals of 15 and toward the end of 30 minutes the mill was 
stopped and a small sample removed for mechanical analysis. This was 
carried out as described in Chapter III. Everything passing the 150 
mesh sieve was called chemically available, as previous experiments had 
‘shown that quartz of this fineness was rendered soluble in acid and alkali 
by ignition with lime. - 
The results of these experiments are shown by the accompanying 
‘curve, figure, 46, in which we find that in 250 minutes the sand contained 
821% per cent. of chemica!ly available material. This means, therefore, that 
in the time required to grind 25 pounds of a limestone clay mixture suf- 
‘ficiently fine, the same amount of quartz sand ground under the same con- 
‘ditions (27 revolutions per minute of the mill already described else- 
‘where) had but 0.8 of the quartz chemically available. 
