The first knowledge which the public has had about it is due to Mr.
Erasmus Bartholinus, who has given a description of Iceland Crystal and of its chief phenomena.

But here I shall not desist from giving my own, both for the instruction of those who may not have seen his book, and because as respects some of these phenomena there is a slight difference between his observations and those which I have made: for I have applied myself with great exactitude to examine these properties of refraction, in order to be quite sure before undertaking to explain the causes of them.
3.

As regards the hardness of this stone, and the property which it has of being easily split, it must be considered rather as a species of Talc than of Crystal.

For an iron spike effects an entrance into it as easily as into any other Talc or Alabaster, to which it is equal in gravity.
[Illustration] 4.

The pieces of it which are found have the figure of an oblique parallelepiped; each of the six faces being a parallelogram; and it admits of being split in three directions parallel to two of these opposed faces.