FLUIDS IN MINERALS. 381 



with a very feeble reflected light, the dense fluid N N 

 with a light perceptibly brighter, and the vacuity V V 

 with a light of considerable brilliancy. The boundaries 

 a b c d, e f g h are marked by a well-defined outline, and 

 also by the concentric coloured rings of thin plates produced 

 by the extreme thinness of each of the fluids at their edges. 



If the temperature of the room is raised slowly to 

 58, a brown spot will appear at x in the centre of the 

 vacuity V V. This spot indicates the commencement of 

 evaporation from the expansible fluid below, and arises 

 from the partial precipitation of the vapour in the roof of 

 the cavity. As the heat increases, the brown spot en- 

 larges and becomes very dark. It is then succeeded by a 

 white spot, and one or more coloured rings rise in the 

 centre of the vacuity. The vapour then seems to form a 

 drop, and all the rings disappear by retiring to the centre, 

 but only to reappear with new lustre. During the 

 application of heat, the circle efgh contracts and dilates 

 like the pupil of the eye. When the vaporization is so 

 feeble as to produce only a single ring of one or two tints of 

 the second order, they vanish instantly by breathing upon 

 the crystal, but when the slight heat of the breath reaches the 

 fluid, it throws off fresh vapour, and the rings again appear. 



If a drop of ether is put upon the crystal when the rings 

 are in a state of rapid play, the cold produced by its 

 evaporation causes them to disappear, till the temperature 

 again rises. When the temperature is perfectly uniform, 

 the rings are stationary, as shown between V and V in 

 Fig. 79 ; and it is interesting to observe the first ring 

 produced by the vapour swelling out to meet the first 

 ring at the margin of the fluid, and sometimes coming so 

 near it that the darkest parts of both form a broad black 

 band. As the heat increases, the vacuity V V diminishes, 

 and disappears at 79, exhibiting many curious phenomena 

 which we have not room to describe. 



