372 
Proceedings of the Royal Society of Edinburgh. [Sess. 
of oceanic Red Clay * was treated side by side with the glauconite. The 
powders were then assayed for moisture by simple ignition. The results 
take the following form : — 
Concentration of acid. 
Tension of 
aqueous 
vapour. 
Glauconite ; 
percentage 
of moisture. 
Red clay ; 
percentage 
of moisture. 
1 per cent. .... 
9*2 mm. 
30-35 
24-19 
30 „ .... 
6-8 „ 
18-29 
13-29 
45 „ .... 
4*3 ,, 
13-22 
11-43 
i 55 .... 
23 „ 
11-31 
1053 
65 „ .... 
1-2 „ 
9-47 
9-28 
75 „ .... 
0’5 „ 
8-45 
8-36 
Fully dried .... 
7T2 
6’94 
The figures in the last line refer to the combined water remaining after 
the minerals have been dried to constancy at 110°, which may be regarded 
as water of constitution. 
From the above table it will be perceived, firstly, that glauconite becomes 
a highly hydrated mineral in presence of moist air (whilst still remaining 
an apparently dry powder). No doubt this is the condition in which it 
exists in its native element, a third or more of its weight consisting of 
water. Secondly, it is evident, without drawing up a curve, that the 
hydration decreases continuously with the tension of aqueous vapour in 
equilibrium, whence it follows that there are no definite hydrates re- 
presenting a series of distinct molecular species. Thirdly, there is a 
marked parallelism as regards hydration between glauconite and Red Clay. 
It is well known that this kind of water absorption, which is especially 
characteristic of colloids, is also shared by the unquestionably crystalline 
zeolites. The inference, then, which we may now draw as to the nature 
of glauconite is that it is certainly not an ordinary crystalline silicate like 
felspar or mica, but that it must be either a zeolite or a colloid. As 
between these two alternatives, it is less easy to decide. The property 
possessed by glauconite of absorbing dyes is again common to both colloids 
and zeolites. In favour of the view that glauconite is a colloid, we have 
the absence of crystal-contours and the ease with which it forms colloidal 
suspensions and solutions. Evidence of crystalline habit, on the other 
hand, is afforded by the optical anisotropy of glauconite. To this, however, 
it may be rejoined that isotropic matter in a state of strain is equally 
capable of showing birefringence. That glauconite may exist under some 
such strain seems not unlikely when we consider the structure of glauconite 
* Analysis under No. 1 in Proc. Roy. Soc. Edin ., xxx. p. 190, 1910. 
