Crosby.] 
122 
[May 21, 
residue, which appears at first to be pure clay, is found by testing 
to contain a large proportion of almost impalpable gritty matter 
which it would be a still more hopeless task to separate by ma- 
nipulation with water. Even the tenth product, the residue left 
after the evaporation of the water which has been siphoned off 
from number nine, is not pure clay, although the rock flour must in 
this case be truly superfine, a product of mechanical disintegration 
rivalling in fineness the pure claj^, which is a product of chemical 
disintegration. But here the resemblance ends, for the rock-flour 
is still hard and gritty when tested with the teeth, and lacks the 
soft and truly impalpable character of the clay. Microscopic ex- 
amination shows, further, that even the superfine rock-flour is sim- 
ply comminuted quartz ; and hence we must admit a real and 
fundamental distinction between rock-flour and clay. 
Despairing of determining their proportions in the eighth, ninth 
and tenth products, by mechanical means, resource was had to a 
chemical test. It was deemed sufficient to determine a single con- 
stituent of either the rock-flour or clay, which could be regarded as 
fairly constant and not common to both ; and the combined water 
of the clay appeared to be on the whole the most convenient and 
satisfactory. The proportion of water in kaolin or pure clay va- 
ries from about 12 to 15 per cent. But to avoid underestimating 
the clay, the minimum amount, or 12 per cent., was assumed to 
constitute a true clay. Each sample was carefully dried at a tem- 
perature not exceeding 100° Cent, and then strongly ignited. In 
nearly every instance a more or less marked reddening resulted 
from the ignition. This may be explained partly as due to the 
dehydration of ferric oxide, especially in the products from the ox- 
idized or buff-colored till ; but it must be in part, also, attributed 
to the peroxidation of ferrous oxide. These two causes of error 
thus tend to neutralize each other, but the latter probably predom- 
inates in most cases, and to this extent the apparent loss of water 
suffered by the clay must be less than the real loss. It is scarcely 
possible, however, that this excess of oxidation over dehydration 
of the iron oxide should lead to too low an estimate of the pure 
clay, on account of the very low proportion of water (12 per cent.) 
assumed for a normal clay. 
GENERAL RESULTS OF THE ANALYSES. 
The analyses may be summarized as follows, the numbers giving 
