526 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Examining^ Fluid-cavities in Quartz.* — Dr. A. A. Julien, after de- 

 scribing the selection of the material and its preparation (by grinding 

 tliin sections or chipping oflf tliin flakes), mounting and examination, 

 points out that the chemical nature of the liquids and gases which occupy 

 the fluid-cavities in quartz can be detected not only by chemical means, 

 but by the use of a few simple microscopical accessories. 



The expansion of the carbon dioxide by a slight increase of temperature 

 above 20° C. is so great that advantage can be taken of its peculiar 

 sensitiveness in this respect for its identification, on this minute scale, 

 by very simple means. The simplest of all is a piece of rubber tubing, 

 about 1 ft. long and 1/8 in. in bore. If the peculiar limpidness and delicate 

 outline of the liquid in a fluid-cavity should lead the observer to suspect 

 it to be liquid cai'bon dioxide, he has but to put this tube to his mouth, and 

 blow a gentle stream of warm air for a minute or two upon the slide, from 

 either above or below the stage. The simple warmth of his breath (about 

 32° C.) will be sufiicient to convert the liquid carbon dioxide into a gas 

 and thus to render its identification at once complete ; for that temperature 

 allows at least one degree to spare in reaching the point in the pure sub- 

 stance (31° C) at which this change of state takes place. If there 

 happens to be a gas-bubble of large size in relation to the layer of liquid 

 in the cavity, the increase of temperature tends at the same time to 

 expand the gas, and to cause the liquid to evaporate into the inner space. 

 These two actions usually so counteract each other that hardly any change 

 is visible. At other times, an appearance of boiling is produced. But 

 when the temperature of 29° to 31° C. is reached, in an instant the liquid 

 layer disappears, and nothing is visible within the cavity except the 

 blurred outlines of its walls. The precise temperature at which liquid 

 carbon dioxide thus passes entirely into the gaseous form within the cavity 

 is termed its " critical point." This is a condition afiecting all liquids, 

 that is, all condensed gases ; at a certain fixed temj)erature — which varies 

 with the gas — the liquid flies into the gaseous state when heated in an 

 inclosed cavity the walls of which are strong enough to resist the enormous 

 pressure so resulting. When the slide has cooled back to the 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 

 disappearance of the liquid layer near the critical point. 



In all such experiments, however, the observer must be on his guard 

 as to the temperature of the atmosjjhere, 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 observation, unless care has been taken to keep 

 the slide under examination cool, i. e. below 30° C, which may be done by 

 previously dipping it in cold water. The temperature of the air at mid- 

 sumnier (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 



* Juuin. X. York Mi*, fcjoc., i. (1885) pp. 129-44. 



