83 
No. 6. Section of Granite. The fragments of quartz in 
this specimen abound with minute more or less spherical 
cavities. Each contains a vacuole which in the large and 
medium sized cavities may be seen when highly magnified 
Q-in.) to rise to the upper part of the cavity when the 
specimen is rotated. Warming facilitates this, apparently 
rendering the fluid less viscid. The feature of interest in 
this specimen is the spontaneous movement of the vacuoles. 
At ordinary temperature the motion of the largest vacuoles 
consists in a faintly perceptible throbbing motion ; in the 
medium sized this is increased, while in the smaller cavities 
the vacuole wanders about its cell with a rapid jerking 
motion ; on applying heat the vacuoles gradually decrease 
in size, and become more active as they become smaller, 
until finally, before disappearing, their motion becomes too 
rapid to follow, but is chiefiy confined to the upper part of 
the cavity. If allowed to cool, the vacuole suddenly re- 
appears of its full volume at the lower part of the cavity, 
instantly rising to the top. The motion of the smaller 
vacuoles seem incessant, but I have never observed them at 
a freezing temperature. If the source of heat, whether a 
heated metal or glass rod, be applied either on one side or 
at the upper or lower part of the cavity, the vacuole at once 
passes to that side, and in this way may be made to pass to 
any part of its cell by moving the heat source in the desired 
direction. Whether such movement is an attraction of the 
vacuole, or a repulsion of its surrounding fluid, is not easy 
to decide. That it is not due to the nature of the heating 
body is shown by its occurrence from proximity of any 
heated substance, whether of glass or metal. The vacuoles 
in the smaller mineral cavities are interesting as being the 
smallest isolated portions of gaseous matter observable, and 
their extraordinarily active movements in a comparatively 
dense medium seems suggestive of the kinetic theory of 
matter. 
