Accidental Characters.
Colour. Bardiglione is most frequently colourless; but it is sometimes of a pale violet-red ; such is the crystalline variety of Hall, as well as that of Sweden, and that of the lead mine of Pessi near Mont Blanc. It is of a deep grey at the salt-works of Bex, and of a bluish tint at Vulpino, which has occasioned some mineralogists to confound it with the bluish variety of sulphate of strontian.
Electricity. The transparent violet-coloured bardiglione of Hall is pretty strongly electrical by friction. That of the salt-works of Bex is likewise electrical, but in a less degree. I could not find this property in any of the other varieties of this substance: even in that of Hall the intensity of the electricity varies considerably; for in some pieces it is very weak, and is excited with difficulty.
Phosphorescence. This character, like the preceding, varies very considerably. Some of its varieties, as that of the salt-works of Hall, give a slight and bluish phosphorescent light. In others, as that of Sweden, which I have already noticed in the 77th number of the Journal des Mines, and which I have said is mixed with actinote, yellow copper, and magnetic iron ore (fer oxidulé), the phosphorescent light is of a yellow colour inclining to orange. In some of the varieties of this substance, which come from Vulpino, this light is of a very deep orange yellow, and with the brightness of a burning coal. Several other varieties, among which is that belonging to the salt-rock of Arbonne near Mont Blanc, as well as that of a deep grey with a somewhat reddish tint of the salt-works of Bex, are totally destitute of this character.
