18 



inclusions of Dr. SORBY are formed. Like the fluid inclusions, already referred 

 to, they frequently contain bubbles. 



Crystals formed by sublimation contain only gas- and vapour-inclusions. 

 Such inclusions, however, are by no means limited to crystals formed in this 

 way. Liquids possess the power of absorbing gases to a certain extent. 

 Water will take up a certain amount of oxygen, nitrogen, and carbon dioxide 

 from the air ; molten silver will absorb twenty-five times its volume of oxygen ; 

 slags contain a considerable amount of gas. When solidification occurs the 

 dissolved gases are either expelled, or included mechanically in the solid. It 

 therefore happens that gas- and vapour-inclusions are frequently associated 

 with solid- and fluid-inclusions. 



In recognising the various kinds of inclusions under the microscope the 

 breadth of the black border, which marks the outline of the inclusion or of 

 the bubble, is of the greatest utility. This border is due to the refraction and 

 total reflection of light which occurs at the surface of contact of two media 

 having different refractive indices ; and, other things being the same, its breadth 

 is dependent on the difference between these indices. If the refractive index 

 of empty or gas- filled space be taken as 1, that of quartz is 1*547 ; of glass 

 (obsidian), 1*488 ; and of water, 1*336 (1) 



The external boundary of a glass inclusion in quartz (difference in 

 refractive indices *059), will therefore be less marked than that of an ordinary 

 liquid inclusion (diff. = *211), whereas the boundary of the bubble will be 

 more marked in the former (diff. = *488) than in the latter case (diff. = *336). 



Dr. SORBY draws the following conclusions from his experiments and 

 observations on artificial products. 



(1) Crystals possessing only inclusions containing water, more or less 



saturated with various salts, were formed by being deposited 

 from solution in water. 



(2) The relative size of the bubbles in normal fluid inclusions depends 



on the temperature and pressure at which the crystals were 

 formed, and may in some cases be employed to determine the 

 actual or relative temperature and pressure. 



(3) Crystals containing only glass- or stone-inclusions were formed 



by being deposited from a substance in the state of igneous 

 fusion. 



(4) Crystals containing only gas- or vapour-inclusions were formed 



by sublimation, or by the solidification of a fused homogeneous 

 substance ; unless they are liquid inclusions that have lost all 

 their liquid. 



(5) Other circumstances being the same, crystals containing few 



inclusions were formed more slowly than those containing more. 



(6) Crystals possessing liquid - inclusions which contain a variable 



amount of gas or vapour, and pass gradually into gas or vapour 

 inclusions, were formed under the alternate presence of a liquid 

 and a gas. 



(1) Zirkel. Mikroskopische Beschaffenheit, &c. 1873, p. 71. 



