208 MINERALOGY AND LITHOLOGY. 



they lie in some other fluid than water; for the circles of their outer 

 edges are not broad and black, as are those of bubbles in water, but 

 their outlines are simply sharp lines; and this indicates that the bubbles 

 are in a fluid, the index of refraction of which is less than that of water. 

 On warming the section, the bubbles all simultaneously disappear at the 

 temperature of 35° Centigrade, which is a trifle above that at which the 

 disappearance has been usually noticed. There are two ways by which 

 such bubbles in liquid carbonic acid disappear: the first is, when the 

 relative size of the bubble to that of the cavity is small, and then the 

 fluid expands and fills the space, below the critical point of the fluid ; 

 and in this case the bubbles begin to grow small as soon as heat is 

 applied. But when the size of the bubble is relatively large, when heat 

 is applied the fluid evaporates into it ; the bubble grows larger, and at 

 the critical point suddenly expands and fills the space. In the cavities 

 under consideration, the bubble disappears in this second manner. It 

 is very interesting to watch this.* The increase in the size of the bub- 

 ble at first can be detected ; but the disappearance and reappearance of 

 the bubble are instantaneous, and nothing further can be seen than that 

 at one instant the bubble is plainly there, and then it is gone, and, on 

 removing the source of the heat, it shortly is there again. 



In the figure the mineral above is orthoclase, which includes some 

 apatite crystals ; the one on the left is hornblende including a mica scale, 

 and on the right there is a grain of calcite. The minutest of the little 

 cavities in the quartz, when very highly magnified, are found to contain 

 the same fluid. Where the quartz is in contact with the calcite, it ap- 

 pears dirty and troubled ; but this apparent impurity, with the highest 

 magnifying power, is also resolved into a dense aggregate of excessively 

 minute cavities. This, too, is significant. 



Besides the carbonic acid, the presence of grains of calcite in an erup- 

 tive acidic rock is interesting. It confirms what has been said in regard 

 to the mode by which such rocks became plastic. After a pure igneous 

 fusion no such combination of minerals as is here found could exist, save 

 as a result of some subsequent decomposition. It would seem that if the 



* A liitle apparatus has been contrived by which a section and a thermometer are warmed together on the 

 stage of the microscope. This is accomplished by an electric current in Vogelsang's contrivance ; but in a 

 simple and inexpensive apparatus that comes with Mr. Rosenbusche's microscope that I have described, a cur- 

 rent of warm air is carried across the stage in a chimney, and is brought in its passage in contact with the 

 section and thermometer. 



