Carbonic Acid in Mineral Cavities. By Walter N. Hartley. 171 



returned after a sliort interval. 2nd. The liquid hud totally dis- 

 appeared at 31° "5, and returned on cooling. 3rd. The liquid was 

 invisible at 31^, but returned almost immediately after contact with 

 the microscope stage. 4th. At 31° there was no liquid to be seen, 

 but it was observed to be filling in immediately afterwards. 5th. 

 Again at 31° did the liquid vanish. 6th. At 30^ "To the margin 

 of the liquid was visible, but was not so sharply defined or so high 

 up in the cavity as it afterwards became. 7th and 8th. On 

 being warmed almost to 31°, the liquid was still visible, but the 

 margin became more distinct immediately afterwards. 9th. At 

 31^^-5, hquid invisible. lOth. At 31^, upper portion of the 

 liquid invisible ; lower one not. 11th. The hquid invisible at 81°, 

 in the upper cavity, but not in the lower. 12th. At 30° "75 the 

 hquid was seen in the larger cavity ; the quantity, however, in- 

 creased to treble immediately afterwards. 13th. At 31° the upper 

 cavity appeared empty ; the lower one full. It is evident, then, 

 that the critical point hes between 3U°'75 and 31° C. The critical 

 point of pure carbonic acid, as determined very precisely by 

 Andrews, lies at 30° "92 C, or very nearly 87°- 7 F. Hence I 

 conclude that the identity of this liquid with carbonic acid is 

 established in a most convincing manner. It was noticeable that 

 in whatever position the slide was placed, the liquid generally con- 

 densed on the same spot. Varying the method of heating the 

 liquid by applying a hot wire to the surface of the quartz, I dis- 

 covered what was at first by no means apparent, namely, that the 

 upper and lower cavities were connected by a small fissure, and that 

 water occupied the intervening space ; the upper cavity was then 

 seen to have the shape drawn in Fig. 3, and marked a. This 

 presence of water, no doubt, determined the place of condensation, 

 so that no matter what the position of the specimen, the carbon 

 dioxide always condensed on the surface of the water, because of its 

 adhesion to this fluid being greater than to the quartz. The 

 concavely curved surface of the carbon dioxide is due to adhesion to 

 the moist sides of the cell ; the convex curvature indicating where 

 the two liquids are in contact is caused by the greater adhesion of 

 the water to the same surface. Before the specimen had been 

 heated in such a way as to drive the liquid from the smaller into 

 the larger cavity, it contained more of the carbonic acid than has 

 collected in it since, and it was noticed on two or three occasions, 

 that the action of heat was to diminish the gas -bubble very rapidly, 

 by expansion of the liquid, until it had the appearance shown in 

 Fig. 4 ; the bubble then as quickly increased in size, by contraction 

 of the liquid to its original dimensions, when the source of heat was 

 removed ; likewise, when the heat was continued, the gas-bubble 

 increased by vaporization of the liquid, as in Fig. 2. The appear- 

 ance caused by the expansion and contraction resembled the dilata- 



