On Hyalite. 
243 
(b) The effects of heat. 
It was observed that, when a fragment of clear hyalite was heated on 
platinum foil in a bunsen flame, thin flakes split off from its surface, and 
that these flakes eventually curled and became portions of spherical surfaces ; 
in addition they lost their glassy appearance and turned opalescent. 
A somewhat thick diametral section, placed on a glass slip and heated 
as above, became opalescent and curled into a concavo-convex shape, 
whilst at the same time a small spherical kernel split out from its centre. 
When the concavo-convex portion was examined under the microscope, 
it was found to have a pale-brown colour by transmitted light ; further, 
between crossed nicols in plane polarised light it yielded an extinction figure 
having a positive sign — that is, having the same sign as the naturally 
occurring opalescent hyalite previously described. 
Portions of some of the flatter opalescent shells obtained by heating 
behaved in a very similar manner to tangential sections of the naturally 
occurring form. 
■(c) Chemical examination. 
Material was submitted to Mr P. K. v. d. Eiet Copeman for analysis, 
with a view to finding out what chemical change had accompanied the change 
in sign produced by heating. From a series of silica determinations, and 
assuming the general formula of hyalite to be Si0 2 .nH 2 0, he arrived at 
the following conclusions : — 
1. The loss— presumably water — on heating the clear hyalite at bunsen 
heat was 2-98 per cent. 
2. The clear contains 10-28 per cent, water. 
3. The naturally occurring opalescent hyalite contains 7-55 per cent, 
water. 
From the above it appears that the artificially produced opalescent 
hyalite has almost the same water content as the naturally occurring 
opalescent form. 
Remarks in Conclusion. 
Our chief object in writing this note has been to record observations 
pending the arrival of more material for further investigation. At present 
we feel we can only write tentatively regarding the reasons why hyalite 
is doubly refracting. We consider that the explanation that it is due to 
tension is, at the best, an insufficient one ; we are inclined to regard the 
occurrence of spheroids composed of concentric layers of positive and 
negative material as making this view — -considering it as being the sole 
explanation — untenable. 
We would suggest that hyalite is a substance of which the component 
