( 504 ) (October, 
IV. BERYLS AND EMERALDS. 
By Professor A. H. Cuurcu, M.A., &c. 
‘Xi LTHOUGH to the mineralogist the stones known as 
ah aquamarine, beryl, and emerald, constitute but one 
species, there are decided differences of optical pro- 
perties, as well as minute differences of chemical compo- 
sition, between them. Under the name “ beryl” minerals of 
considerable variety of aspect are thus included; some spe- 
cimens being nearly opaque and brownish-yellow, others 
being perfectly transparent and colourless, while others, 
again, are beautifully tinted with a pale sea-green hue. 
But other colours also occur, such as yellowish-green, pale 
blue, and even lilac. Still it is in the emerald, once thought 
to be a distinct mineral species, that the richest and most 
highly appreciated colour is found. The pure green of this 
gem has provoked several enquiries as to its nature, and, 
quite recently, Mr. Greville Williams* has taken up the 
enquiry, and published the first portion of his results. In 
presenting an account of this research to our readers, we 
will also give some fac¢ts derived from other sources con- 
cerning the transparent beryl and the emerald. 
This mineral species occurs only in crystals, while such 
divergences from constancy of composition as it exhibits 
affect scarcely more than 2 parts out of 100 in any specimen. 
It is essentially a silicate of alumina and glucina, but it 
always contains a little iron, while chromium seems a con- 
stant ingredient of the true emerald, though occurring as a 
trace merely, not amounting to a half per cent when calcu- 
lated as sesquioxide. Other metals have been recognised in 
some varieties of the beryl by different analysts, and the 
list will include calcium, magnesium, manganese, and 
tantalum. 
The specific gravity of transparent pale beryls is, as 
nearly as may be, 2°7, so that it is decidedly denser than 
rock crystal. This fact may be beautifully illustrated by 
means of a solution of mercuric iodide in potassium iodide.t 
Such a liquid may be readily obtained with a sp. gr. = 30, 
but a solution not denser than 2°67 or 2°68, nor lighter than 
2°665, is required in the particular case in point. In such 
a liquid a piece of flawless rock crystal floats, for its specific 
* Proc. Roy. Soc., No. 145, 1873. 
+ Recommended by Mr. E. Sonstadt for such experiments, in the ‘‘ Chemical 
News,” vol. xxix., p. 127. 
