and Mechanical Condition of Amber. 33S 
and depolarising axes. I had frequently occasion to recur to 
the examination of this substance when any interesting speci- 
men presented itself ; and having now examined almost every 
variety which is likely to occur, I trust that tho results which I 
have obtained, will be considered as throwing some light on the 
origin and formation of this interesting body. 
1. In several specimens of Amber ^ the polarising structure ex- 
isted in such a manner as to display itself in irregularly parallel 
zones of colour, which suffered no change by altering the exter- 
nal shape of the mass. These different zones had their axes ly- 
ing in different directions, so that some of diem were dark when 
-others gave the tints of the first order, and vice versa. Hence 
it is obvious, that the axes of these zones must have been inclin- 
ed 45° to each other; that this structure must have been ac- 
quired during the gradual induration of the amber, and must 
have had its character modified according to the nature of the 
juice which was successively deposited round the nucleus. The 
limits, or lines of junction, of these zones appeared even in com- 
mon light, and could not have been produced either by heat or 
pressure. Some of these specimens of amber displayed various- 
ly inflected veins of different degrees of transparency like the 
agate, and in many cases the tints rose to the green and red of 
the second order. The streaks which gave the highest tints, 
were in general yellower than the rest of the mass. 
2. In some specimens of amber of a rectangular shape, I ob- 
served the same polarising structure which exists in plates of 
glass gradually heated or rapidly cooled, namely, a negative 
between two positive structures. This structure must have 
been acquired during the gradual induration of the amber at all 
its surfaces. 
3. A cylindrical piece of amber, which was cut out of a bead, 
gave the black cross and the coloured tints exactly like a cylindri- 
cal piece of glass that had been heated to redness and rapidly 
cooled, as represented in the Edin. Trans, vol. viiL Plate VIL 
Fig. 10. The thickness of this piece was about ^^^ths of an 
inch, and its highest tint was the yellow of the first order. 
4. In a piece of amber of the same thickness as the preceding, 
the polarising structure had suffered a change by drilling a hole 
through its centre ; and in numerous specimens, I found that 
