Dr. Brewster on the laws of polarisation, &c. 223 
Three 
Axes. 
By examining the preceding list, and comparing it with 
the list of crystals that have one axis, we may conclude, 
1st, that the combinations of sulphuric and tartaric acid 
with earthy, alkaline, and metallic bases, have two or more 
axes of double refraction. 
2nd, that as only about 23 crystals have one apparent axis 
of double refraction, while more than 80 have two axes, and as 
the phenomena of these 23 crystals can be referred to two 
axes, as will afterwards be shown, the general laws of polari- 
sation and double refraction remain yet to be investigated by 
experiment. 
If we compare the minerals in the preceding list with the 
most recent table of primitive forms,* it will be found that 
there is a connection by no means equivocal between the 
primitive form of a crystal and the number of its axes of ex- 
traordinary refraction. In order to establish this curious 
result, I have drawn up the following Table, in which I have 
inserted the names of the minerals and the number of their 
axes opposite to the different primitive forms under which 
they have been classed. 
"Muriate of soda 
1 
r 
Boracite 
1 
1 
Leucite 
1 
Two J 
An.alcime 
>Cube. 
Axes. } 
Aplome 
Phosphate of manganese 
I 
and iron 
J 
Sulphate of magnesia 
Prmsiate of potash 
{ Right qua- 
1 drangular 
1 prism with a 
] square base. 
* The table of primitive forms which I have used, is the one drawn up by Dr. 
Thomson in the Article Crystallography , in the Edinburgh Encyclopaedia, as it con- 
tains many of the observations made by Hauy and Bournon, since the publication 
oFHauy’s Traite de Miner alogie. 
