l-'^fi JOURNAL OF THE [June, 



critical point (about 31 C), the inclusion suddenly resumes the 

 visible form it possessed before, or sometimes assumes the form 

 of two or three bubbles, or even occasionally of a cluster or of a 

 shower of bubbles. If the original gas-bubble happens to be 

 much smaller in volume than that of the inclosing liquid, and 

 the slide is warmed gently in the same way, the bubble will be 

 seen to dilate steadily, often rapidly, with a similar sudden dis- 

 appearance of the liquid layer near the critical point. In all 

 such experiments the observer must be on his guard as to the 

 temperature of the atmosphere and of the mineral section at the 

 beginning of the observation. In a warmly heated room, during 

 the winter, and on a warm day, during the summer, the critical 

 point may have been already passed and these transformations 

 have become completed. In these circumstances no indications 

 of the presence of carbon dioxide will be visible at the first ob- 

 servation unless care has been taken to keep the slide under 

 examination cool, /. e., below 30° C, which may be done by 

 previously dipping it in cool water. The temperature of the 

 air at midsummer in this city (30° to 33° C.) is often sufficient 

 alone to bring the liquid up to its critical point, under the eye 

 of the observer. 



In most mineral sections the fluid contents of the cavities 

 consist of water or some saline solution, which would usually 

 remain but little affected in form or appearance during an ex- 

 periment like that just described. Occasionally, however, the 

 bubbles in a water-cavity are excited into lively motion and 

 repelled into the farthest side of the cavity by the sudden appli- 

 cation of heat. In place of a rubber tube, the applicatio'n of a 

 warm wire, glass rod, or of the burning end of a cigar, a 

 little below the slide, may be substituted to produce the same 

 effects — or even the direct application of the warm end of one's 

 finger to the bottom of the slide for a few minutes. 



It may be here remarked that the violent explosion of granyte 

 when exposed to high temperature, as during the great fire in 

 the business districts of Boston, may be attributed largely to the 

 known abundance of liquid-cavities in the quartz-grains of that 

 rock. This is represented by the slides, on exhibition, of thin 

 sections of the coarse Quincy granyte, and of the similar horn- 

 blendic granyte from the Egyptian Obelisk now in our Central 

 Park. In both these rocks, the quartz contains many large 



