86 



My experiences in the course of this investigation may be of 

 interest to some who wish to undertake original work of a similar 

 kind. (For the Memoir containing the investigation as a whole, 

 see Quarterly Journal of the Geological Society, vol. xxxi. p. 568.) 



The object-glass generally used was a ^-inch (by Collins) 

 with a C eye-piece, the combination magnifying 665 times. When 

 I first began my measurements, I used a neutral-tint glass reflector, 

 and traced on paper all the best defined fluid cavities, making a 

 note by the side of those in which the vacuities showed a constant 

 spontaneous movement. I soon found, however, that on a com- 

 parison of a considerable number of drawings, the fixed bubbles 

 were almost invariably relatively larger than those which showed 

 this free movement. Hence I was led altogether to reject from 

 my measurements all cases in which the bubble was fixed, and in 

 which it would seem either that gas had been primarily entrapped, 

 or that in the making of the thin slice, leakage had occurred in the 

 cavity. This last case must necessarily often occur in the making 

 of thin slices, especially as the liquid-bearing cavities are frequently 

 so irregular in shape and prolonged into horns and fine points. I 

 then gave up tracing the outlines of cavities on paper, and measured 

 the relative size of bubble and liquid-cavity directly, by means of 

 a Jackson's micrometer, with divisions equal to 10000 ths of an inch. 

 This micrometer being placed in the eyepiece, the fine divisions 

 could be brought over the bubble and liquid-cavity, and their 

 relative size at once estimated with tolerable accuracy. But a little 

 consideration will make it evident that measurement of a liquid- 

 cavity in one plane would be of little use unless the cavity be 

 exceedingly shallow, and lie along the plane, hence it became 

 necessary to take this further precaution, viz. — to rely only upon 

 the measurements of those cases in which the tiny bubble (or 

 vacuity) moved freely into all parts of the liquid-cavity without 

 going out of focus ; this would imply that the cavity was of tolerably 

 uniform depth, but little more than the depth of the diameter of 

 the bubble. And it was found that when I restricted my measure- 

 ments to these cases, there was a fairly marked uniformity in the 

 ratio between vacuities and liquid-cavities occurring in the quartz 



