io6 American Quarterly Microscopical Journal. 



motion is spontaneous and exhibits itself in the most interest- 

 ing and mysterious form in the more minute bubbles. Many 

 of these appear to be in exceedingly rapid motion, dancing 

 from side to side with an apparent velocity which often renders 

 it difficult to follow them, and with a restless irregularity 

 against different sides of the cavity, which has suggested a 

 comparison with the movements of animalculae seeking escape 

 from a crystal prison. This rapidity is, of course, magnified 

 in proportion to the power used, and the motion of all these 

 bubbles is, in fact, very slow. Various explanations have been 

 offered to account for this motion — the vibration of the micro- 

 scope stand and table, the influence of heat rays or light from 

 the mirror (as in Crooke's radiometer), the capillary attraction 

 of the sides of the cavity, etc. ; but like the molecular motion 

 which is styled the " Brownian movement," and which in some 

 forms it greatly resembles, it is a subject which still requires 

 more careful study than it has yet received. In the examina- 

 tion of this phenomenon, skillful manipulation will be required 

 from the observer in order to bring about the best effect, as the 

 least change in the fine adjustment affects materially not only 

 the sharpness of outline and apparent motion in the bubble, 

 but the cross-section of the cavity at any moment in the focal 

 plane of the objective. 



Fluid-inclusions of all these varieties occur in many rocks; 

 e. g. in the quartz-porphyries and granites, but in remarkable 

 number in some twenty-one transparent mineral constituents 

 of rocks, and particularly quartz. And yet the opaque min- 

 erals may also abound with them. It is an interesting fact, 

 that the theory which ascribes the fluidity of melted lava in 

 part to the presence of water, has been confirqied by the dis- 

 covery, in thin sections under the microscope, of these liquid- 

 inclusions in the olivine and leucite, which are found to be 

 common, and often essential constituents of the cooled rock. 

 In some coarse granites the quartz is so richly saturated with 

 these cavities, lying at intervals of not over one-thousandth of 

 an inch, that Sorby estimates their number at a thousand 

 millions to the cubic inch (250 to a square milimeter, by Zir- 

 kel's estimate), and in some cases more than ten times as 

 many : the cavities may thus amount to 5 per cent.., and their 

 liquid contents to i or 2 per cent, of the volume of the quartz 

 in granites according to Sorby, and to 1.8 per cent, in mica 



