FLUIDS IN MTNEEALS. 337 



which chemical operations are constantly going 

 on, and beautiful optical phenomena continually 

 displaying themselves. Let ABDC, for example, 

 be the summit of a crystallized cavity in topaz, 

 S S representing the dense, N N the expansible 

 fluid, bounded by a circular line abed, and V V 

 the vacuity in the new fluid, bounded by the 

 circle e f g h. If the face ABDC is placed 

 under a compound microscope, so that light may 

 be reflected at an angle less than that of total 

 reflexion, and if the observer now looks through 

 the microscope, the temperature of the room 

 being 50, he will see the second fluid S S shin- 

 ing 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 pro- 

 duced 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 ex- 

 pansible fluid below, and arises from the partial 

 precipitation of the vapour in the roof of the 

 cavity. As the heat increases, the brown spot 

 enlarges 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 efy h contracts 



