of the same rock. Thus, take as an example two different rock- 

 sHces of the same granite : — 



No. 



•154 

 •154 

 ■154 

 •166 

 •180 

 •180 

 •154 

 •154 



f 



[►•162 



No. 2 



•166 



•125 

 •166 

 •180 

 •200 

 •166 

 •166 

 •166 



■154 

 •166 

 ■142 

 •142 



^ -164 



J 



In No. I there are five cases in which the whole liquid-cavity is 

 6>< times the size of the vacuity, one case in which it is 6 times 

 the size, and two cases in which it is only 51^ times. The mean 

 •162 is probably not far from the truth. 



It may be difficult to realize the size of these liquid-cavities 

 and their contained bubbles. Those reliable for measurement are 

 extremely small, sometimes less than T^r^th of an inch in diameter. 

 Much larger cavities generally occur in plenty; but these seldom 

 exceed W^th of an inch in length, and in them, the bubbles eithe: 

 have no movement, or but a very slight or sluggish one. In 

 fact, such is the minuteness of these cavities and their number, in 

 many cases, that more than a thousand million might be contained 

 easily within a cubic inch of quartz, and sometimes the contained 

 water must make up at least five per cent, of the volume of the 

 containing quartz. In some cases the liquid-cavities are much 

 arranged along lines, as in the quartz crystals occurring in the 

 Armboth Dyke. Occasionally, however, liquid-cavities are met 

 with in quartz crystals of very considerable size, such that the 

 movement of the bubble can even be recognized by the naked 

 eye. 



Having accumulated a sufficient number of reliable measure- 

 ments, and struck the mean for any one rock, such as the Skiddaw 

 Granite, the calculation of the pressure under which the granite 

 was found is proceeded upon by mathematical formula furnished 



