( 239 ) 
ON HYALITE. 
By J. S. v. d. Lingen and A. R. E. Walker. 
The investigation of hyalite, some of the results of which are recorded 
in the following note, first suggested itself to the authors whilst they were 
engaged in studying the optical behaviour of liquid spherulites. 
An attempt was being made to find a solid spherulitic body analogous 
— both as regards internal structure and optical behaviour — with the type 
of liquid spherulite described by Lehmann, in which the molecules of the 
substance are regarded by him as being arranged either along lines radiating 
outwards from the centre of the spherulite, or on the surfaces of spheres 
possessing a common centre — the centre of the spherulite. 
As a result of a preliminary examination of a number of substances 
selected for this purpose, hyalite was chosen as being the most promising. 
In this way one was led to consider the nature of what is commonly 
termed the " anomalous double refraction " of hyalite. In this connection 
Dana * writes as follows : 
" (Opal) often shows double refraction similar to that observed 
in colloidal substances due to tension. The mammillary form Hyalite 
often yields the uniaxial interference cross of a negative substance in 
parallel polarised light ; f this is referred to tension by Schultze, Ber. 
nied. Ges., 69, 1861." 
Similarly Rosenbusch J : 
" Hyalite is doubly refracting, and, under certain conditions, 
exhibits the interference cross of a uniaxial substance with isochromatic 
curves. This appearance is referred to conditions of strain occasioned 
by the concentric shell-like structure of the mineral. The character 
of the double refraction is negative. The hyalite cross often separates 
into hyperbolas during a rotation of the section between crossed nicols, 
which would necessarily be the case if the layers were not regular 
spherical shells." 
* Dana, A System of Mineralogy, 6th ed., p. 194. 
f The italics are ours. 
J Rosenbusch (Iddings), Microscopical Physiography of Rock-making Minerals, 
4th ed., p. 194. 
