1885.] NEW-YORK MICROSCOPICAL SOCIETY. 135 



The chemical nature of the liquids and gases which occupy 

 the cavities can be readily detected by chemical means. For 

 example, the carbon dioxide, to the presence of which in the 

 cavities of some specimens allusion has already been made, can 

 be identified by simply crushing a fragment of the quartz in a 

 mortar, under baryta-water, or by examining the gas with a 

 spectroscope after expelling it by heat from a flask into a Geiss- 

 ler tube. A few simple microscopical accessories may be also 

 employed for the same purpose The expansion of this gas by 

 a slight increase of temperature above 20° C. is so great that 

 advantage can be taken of its peculiar sensitiveness in this re- 

 spect for its identification, on this minute scale, by very simple 

 means. The simplest of all is a piece of rubber tubing, about 

 one foot in length and one-eighth of an inch 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 carbon 

 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 sufficient to convert the liquid car- 

 bon 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 substance (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 dis- 

 appears 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 con- 

 dition affecting all liquids, that is, all condensed gases ; — at a 

 certain fixed temperature — 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 



