74 Proceedings of the Royal Society of Edinburgh. [Sess. 
may be taken in two ways : either the specific gravity of a sample of the 
clay can be taken, or the specific gravity of the material of which the rock 
is formed ; the latter method takes no account of the voids or pore spaces in 
the rocks. For our purpose it is evident that the voids or pore spaces must 
be considered, as the absorptive capacity of a rock must depend chiefly upon 
its porosity. It is therefore possible to make a comparison with spent shale, 
which, though it has been somewhat affected in mineral composition during 
retorting by loss of alkalis, etc., must contain a considerable proportion of 
voids formerly occupied by the kerogen. It is found that the spent shale, 
as is natural, has also a low specific gravity — lower, that is, than other 
clays such as brick clays or fireclay (2*51) or kaolin (2*60). The average 
is approximately 2*20. 
Turning to the question of absorptive capacity, we find that of all clays 
that known as fuller’s earth is the most absorptive. 
Fuller’s earth is unfortunately a somewhat loose term, and is applied to 
any clay that can be used in fulling cloth. The “ fulling ” is the removal of 
grease and oils by absorption in the clay. 
The composition of fuller’s earth differs considerably in different forma- 
tions and in different countries, but there are some characteristics that appear 
to be common to all deposits of this nature. Their specific gravities are low, 
they do not become plastic with water but crumble away, and they all contain, 
or can absorb, a large percentage of water, up to as much as 24 per cent. 
The following table gives the chemical composition of some fuller’s 
earths compared with brick clays and shales and spent oil-shales : — 
Analysis of Clays, Fuller’s Earths, and Spent Shales. 
Fireclay 
(Geikie). 
Brick Clay 
(Geikie). 
Average 106 
American Clays. 
English Fuller’s 
Earth (Geikie). 
English Fuller’s 
Earth, 
Californian 
Fuller’s Earth. 
Spent Broxburn 
Shale, 
Spent Broxburn 
Shale. 
Spent Broxburn 
Shale (Mills). 
Spent 
Torbanite. 
Si0 2 . 
73*82 
49*44 
52*6 
53*0 
54*20 
54*32 
49*72 
55*97 
55*60 
567 
A1 2 0 3 . 
15*88 
34*26 
21*0 
10*0 
14*30 
18*18 
18*8 
31*21 
22*14 
362 
Fe 2 0 3 . 
2*95 
7*74 
14*8 
9*75 
6*30 
6*50 
16*8 
2-84 
12*23 
3*2 
CaO . 
j- Traces 
/ 1*48 
3*58 
*50 
1*25 
1*0 
2*4 
•59 
| 1*55 
f 13 
MgO . 
\ 5*14 
2*98 
1*25 
2*72 
3*22 
2*2 
1*87 
1 -4 
K20 . 
Na 2 0 . 
} 9 
/ 
•” l 
3*1 
1*1 
} 3*79 
4*21 
*62 
* 
2*2 
Water . 
6*45 
1*94 
2*8 
24* 
17 44 
11*86 
1*07 
... 
Sp.gr. 2*51 ... 2-4 ... ... 1*85-2 2*1-2 3 1*2 
* 8*27 soluble in water, probably chiefly salts of K and Na. 
