150 Abbe Haliy^s Observations on Double Refraction. 
as I have not had leisure to multiply my researches sufficiently, 
in order to ascertain if a crystal, which gives only a single image 
of objects, would produce two after being cut in a particular 
manner. 
44 Another observation which will be of some use in generalis- 
ing the theory of these phenomena, consists in this, that all 
the substances whose integrant molecule is remarkable by its 
symmetry, have single refraction. Such are those which have for 
their primitive form the cube, the regular octohedron, and the 
rhomboidal dodecahedron. 
44 Experiments on this object have hitherto been made only on 
those bodies which are called Stones. I have extended these 
experiments to several called Salts, and also to inflammable sub- 
stances, and to metallic substances oxidated and united to other 
substances, and I have found that there is no class of minerals 
which does not present bodies endowed with double refraction.” 
% After mentioning the methods which he employed for ob- 
serving the double refraction, but which are entirely superseded 
by methods equally simple and much more certain, our author 
remarks, 44 That it would be difficult to find a character more 
prominent than that which is drawn from double refraction, 
since it belongs to the very essence of the minerals in which it 
exists and that 44 it is fortunate that we are able, in this way, 
to supply the disappearance of crystalline forms by a physical 
observation, and thus to read in a certain degree in the interior 
of the stone, when its exterior no longer speaks to the eye.” 
The following is the list of substances which have double re- 
fraction, as given by the Abbe Haiiy. 
Hatty's Table of Doubly Refracting Crystals . 
Carbonate of Lime. Strong . Corundum. 
Sulphate of Lime. 
Barytes. 
Strontian. 
.'S Borate of Soda. 
Quartz. 
Zircon. Very strong . 
Cymophane. 
Topaz. 
ilO Emerald. 
Euclase. Strong . 
Feldspar. 
Peridot, Strong . 
15 Mesotype. 
Sulphur. Strong. 
Mellite. 
Carbonate of Lead. 
Sulphate of Iron. 
20 Arragonite. Strong . « 
