STATE GEOLOGIST. 225 
In this manner the mineral character of the clay becomes at once apparent. 
This, of course, is done by dividing the percentages of the constituents, 
silica, alumina, ferric oxide, lime and magnesia by the respective molecular 
weights, and then dividing each quotient thus obtained by the alumina * 
quotient. This will make the alumina equivalent equal to unity. 
A clay of the composition : 
Per cents. 
SUN Caesarean eaten eeeira es drat rad one mrss runes Me maker Sire 63.73 
PNIORTONNOEE 5 6 oo Ga OBR Sie S Ber Mee RAR eee ae a em Lee 
TEVSFOTTIG OSAUG Is Sas os ais oe ee Pee ON et or me a 5 5 
iT eRe ree ero Oren en sweaty tile ath ee Sat). str 0.58 
IN [yO OVERST 2) cere als aohist cae Reams amcor cee a ea 0297 
RO Gas Mee paW re rice een rie oe ee yA oe Gonos aii DY) Bx 
SOclarpepacaecen were Uneer nt a Man tah calmed MES oy fA, 0.67 
Combinedswateraewi ses eee cee ee ees 4.90 
94.00 
would thus have a formula corresponding to: 
o.21KNaO | it ONIO, | 62:80 
o0.2z0CaMeO 0.22Fe,O, | 
This tells us at once that this clay contains at least 4.32 molecules 
of free silica. Since the limits of silica to alumina given are 3:1 and 
A:1 expressed in parts by weight, or percentages, they can also be ex- 
pressed in molecular proportions in which the limits are: 5 molecules 
of silica to 1 of alumina and 6.66 silica to 1 alumina. 
The limit set for the sum of the silica and alumina provides for 
fluxes which are necessary to promote the formation of an easily fusible 
magma and especially for a liberal amount of ferric oxide. The lower 
the fusing point of this magma, the more readily will the cement mix- 
ture vitrify and the lower will be the cost of burning. 
As to the magnesia limit, much has been claimed pro and con, and the 
last word in this matter has not been said. Dyckerhoff probably has 
done more work than any one else in this respect, and it is his opinion 
that magnesia is a dangerous ingredient, if present in large quantities, 
owing to its slower hydration, which takes place when the calcium 
compounds have already become hardened, thus causing cement to ex- 
pand unduly and destroying its structure more or less. 
Ferric oxide, if present in not too great a quantity, is a desirable 
fluxing ingredient. Sulphur compounds are undesirable, since they form 
either compounds of lower oxidation, which on the hydration of the 
cement oxidize to sulphates with increase in volume, or, if already oxi- 
dized or oxidized in the kiln, take away as much lime from the silica 
as is necessary to form calcium sulphate, thus lowering the content of 
available lime. Calcium sulphate in, itself is an undesirable ingredient 
15—S. G. Bull. 3. — 
