182 
Mineral. — The Different Colours of Labradorite. A 
microscopical examination of a number of specimens of this 
mineral in the collection of the Ecole Polytechnique des 
Pays-Bas, all -from the Labrador coast, has enabled M. 
Vogelsang to give an explanation of the splendid play of 
colours often exhibited by it. In the coloration of labra- 
dorite its more or less crystalline structure plays an essential 
part, for the coloured specimens show usually a better cleav- 
age than the colourless ones. The bright blue reflected by 
some specimens depends upon a certain crystalline state of 
the mineral, and is a phenomenon of polarisation produced 
by the passage of rays refracted by one lamina into another 
lamina, the planes of vibration of which do not coincide with 
those of the first, the result being a difference of phase and 
an interference of the luminous rays on reflection, just as 
with the ordinary colours of polarisation. The golden- 
yellow colours proceed from a total reflection from interposed 
microlites, which consist of magnetic oxide of iron, or else of 
diallage; the red colour results from the reddish colouring 
of small lamellae of diallage ; the association of these colours 
with the bluish reflection accounts for the green and violet 
play of colours ; lastly, the coloured metallic reflection from 
laminae of diallage gives rise to the effects of coloured aven- 
turiue. — Archives neerlandaises des Sciences exactes et 
naturelles. 
The Application of the Microscope to Mineralogy. — Mr. 
H. C. Sorby exhibited specimens illustrating this subject at 
the soiree of the lloyal Society. 
The following substances can be recognised in transparent 
minerals or blow-pipe beads by means of the characteristic 
absorption bands seen in the spectra, even when they are 
much coloured by the oxides of iron, manganese, or nickel, 
viz., Didymium, Erbium , 1 U ranium, Cobal, Chromium, Copper, 
Manganese (when it occurs as permanganic acid), a new 
earth, for which the name Jargonia is proposed, and another 
substance, perhaps also new, but not yet sufficiently studied. 
Jargonia is an earth closely allied to zirconia, existing in 
small quantity in zircons from various localities, but consti- 
tuting the chief ingredient of some of the jargons from 
Ceylon. It is, however, distinguished from zirconia and all 
other known elementary substances by the following very 
remarkable properties. The natural silicate is almost, if not 
quite colourless, and yet it gives a spectrum which shows 
above a dozen narrow black lines, much more distinct than 
even those characteristic of salts of didymium. When melted 
1 Erbium ofBunseu— Delafoutaine's Terbium. 
