650 GEOLOGY OF OHIO. 
In the filtrate, precipitate magnesia with Na H PO,, and filter, 
wash in ammonia water, ignite, weigh as Mg, P, O.. 
The alkalies are determined by J. Lawrence Smith’s menod by 
lime and sal ammoniac. 
Titanic acid is determined in a separate portion of clay. Take 1 
gram; fuse with pure Na, CO, disintegrate in hot water, filter and wash. 
Residue put in casserole, dissolve in HCl, expel with H, SO, in excess, 
and evaporate till SO, is given off. Cool, dissolve in cold water, filter, 
wash and boil filtrate for TiO,, collect, fuse with KHSO,, cool, dissolve 
in water, neutralize with Na, CO,, and boil as before, weigh Ti O,,. 
Half this weight must be taken from both the alumina and silica, as the 
titanic acid does not go to either completely in ordinary analysis. 
The whole value of a knowledge of the composition of clays lies in 
the insight we gain into their properties and value, but into this most 
complex study, conjecture and hypothesis still enter in considerable 
amount. 
That the properties of clay depend on its composition is evident. 
That composition we may easily know, but in the relation of composition 
to properties is the uncertainty. Besides the effect of chemical composi- 
tion, the physical structure has weight in determining the properties of 
clay. Density, though a physical quality, influences greatly the fitness 
of clays for certain uses. The density of clay varies with its composi- 
tion and structure. The more quartz it contains, the heavier it is, for 
the specific gravity of silica is 2.5 to 2.8 and that of kaolin is much less. 
The mean of nine determinations of good New Jersey kaolins was 1.6. 
The other elements of greater specific gravity than kaolin are present in 
too small amounts to have any effect. Besides the composition of clays, 
their fineness of division and porosity are influential points. It is be- 
lieved by some that fineness of grain is detrimental to the refractory 
qualities of a clay, it being held that the finer the grain the more heat- 
ing surface is exposed in proportion to the mass, but as fineness of 
division always goes with closeness of structure, the heat has less chance 
to circulate between the particles of the mass than between the open 
grains of a sandy clay ; hence, it is doubtful whether the effect of a fine 
grain is beneficial or detrimental in this particular. The density of a 
clay is an important point in determining its use for those places where 
it will receive at once both high heat and great pressure. Such wares 
as glass house pots, whicn must be refractory, to stand and keep their 
