Dr Brewster on a New Species (^Double Refractmn, 255 
experiments with a considerable degree of accuracy, on the evo- 
lution and absorption of almost any kind of gas. Subsequently, 
finding that it was often desirable to ascertain the quality of gas 
evolved at different stages of a process, I thought of the varia-^ 
tion which is the subject of the present paper. 
Paisley, ) 
December 2. 1823. j 
Aet. X . — On a New Species of Double R^ractioiiy accompany^ 
ing a remarltable Structure in Jnalcime. By D. Beewstee, 
LL.D. F.R.S. Lond. and Sec. R. S. Ed. ^ Communicated 
by the Author. 
The mineral to which the Abbe Haiiy has given the name 
of AnalcimCj has been ranked among those crystals which have 
the Cube for their primitive form. No distinct cleavage-planes, 
however, so far as I can learn, have been observed in it. Crys- 
tallographers presumed that such planes must exist, and, allow- 
ing conjecture to supply the place of observation, they considered 
Analcime as differing in no respect from other crystals of the 
same series. This opinion was first rendered doubtful by the 
observation which I made many years ago, that at thicknesses 
of ^ jth of an inch, it displayed a considerable action upon polar- 
ised light ; but though this fact indicated something singular in 
its organization, yet, owing to the great difficulty of obtaining 
proper specimens, I have been baffled in repeated attempts to 
investigate its structure. 
Having lately received from the Rev. Dr Fleming some very 
transparent crystals of Analcime from the Macdonalds’ Cave in 
the Island of Eigg, and having also been favoured with a very 
fine specimen from Montecchio Maggiore in the -Vicentine, 
through the liberality of Mr Heuland, I have been enabled to 
resume the inquiry, and have obtained the results which it is the 
object of this paper to describe, 
* This paper is an abstract of the original one, which was read before the Royal 
Society of Edinburgh oh the 7th January 1822, and will appear in Vol. X. Part I. 
of their Transactions, which is on the eve of publication. 
s 2- 
