25 



VOGELSANG and GEISSLER (1) applied the spectroscope to the determination 

 of the fluid in the inclusions. Their experiments were principally made on 

 the well-crystallised minerals of veins and drusy cavities, and therefore have 

 not such a direct bearing on geological problems as those of Dr. SORBY. 

 They clearly proved that the expansible substance first noticed by Sir DAVID 

 BREWSTER, and observed by the authors in quartz, topaz and amethyst was 

 liquid carbon dioxide. In one experiment quartz from the granite of Johann 

 Geordenstadt, Saxony, (1) was used, and the inclusions were found to contain 

 water and a small amount of carbon dioxide. The latter substance, however, 

 could not be detected in the liquid form when thin sections of the granite 

 were examined under the microscope. The same authors also constructed an 

 apparatus for examining the effects of heat in expanding the liquid of 

 inclusions while the object was being examined under the microscope, and 

 thus proved that in cavities containing liquid carbon dioxide, the bubble 

 disappeared at 32 C., i.e. at about the critical point for this substance. The 

 expansibility of carbon dioxide, and the disappearance of the bubble at about 

 32 C. furnish a ready method of determining the existence of this substance 

 in microscopic inclusions. The critical point as determined by the dis- 

 appearance of the bubble in fluid inclusions is, however, not perfectly constant. 

 Thus HARTLEY (2) found, in the specimens which he examined, that it varied 

 from 27-27 C. to 33'7 C. According to the experiments of Dr. ANDREWS 

 it is 30-92 C. 



PFAFF (3) constructed an ingenious apparatus for estimating the amount 

 of water mechanically enclosed in the minerals of certain rocks. It consisted 

 of a pestle and mortar, so arranged that a rock or mineral could be pulverised 

 in a closed space through which a current of perfectly dry air was passing. 

 He examined, by means of this apparatus, granite from several localities, 

 syenite, porphyry, gneiss, mica- schist, lavas from Vesuvius and Etna, and 

 obsidian from Iceland. The granitic rocks and the schists invariably yielded 

 water ; the actual amount varying from O'll p.c. in the case of syenite from 

 Meissen to 1*8 p.c. in that of a mica schist from Davos. The lavas and 

 obsidian gave only negative results. It thus appears that the experiments of 

 PFAFF agree with the results of microscopic examination. The minerals of 

 plutonic rocks and the crystalline schists frequently abound in fluid cavities, 

 while those of lava are generally free from them. 



Attempts have been made to determine the conditions of temperature and 

 pressure under which rocks have been formed by considering the phenomena 

 presented by the fluid inclusions. In Dr. SORBY'S experiments on the 

 formation of inclusions in salt crystals, it was shown that bubbles arise in 

 consequence of the difference in the contraction of the enclosed liquid and the 

 enclosing crystal, as both cool down from the temperature at which the crystal 

 was formed. In these cases the ratio of the volume of the bubble to that of 

 the liquid is constant for all inclusions formed under the same conditions. If 



(1) P. A. Vol. CXXXVIL, p. 56. 1869. 



(2) Jour. Chem. Soc. 1876, vol. II. p. 237. 



(3 liber den Gehalt der Gesteine au mechaiiisch eingeschlut-senem Wasser und Kochsaltz. 

 P.A. Vol. CXLIIL, p. 610. 



