182 JOURNAL OF THE [June, 



the smoky quartz from all these localities are on exhibition here 

 to-night. 



But in addition to these natural sources of material, there are 

 artificial substances, represented also by slides on exhibition, 

 which may well illustrate many of the conditions that have 

 affected the origin and development of fluid-cavities in natural 

 crystals. Examples of these are exhibited in crystals of 

 common salt, colored reddish or yellowish by potassium 

 dichromate. The readiest mode of their preparation is the 

 following.' A solution of potassium dichromate is taken, just 

 strong enough to appear red to the eye. This is saturated with 

 common salt in fine powder, and the mixture is freed from the 

 excess of salt and from impurities by filtration. The solution is 

 allowed to crystallize very slowly in a flask, loosely covered by 

 paper, in a warm place. The hopper-shaped crystals of salt are 

 not produced under these conditions on the surface, but little 

 cubes are deposited over the bottom. These do not fall down 

 in the ordinary transparent form of crystals of common salt, but 

 are clouded, and colored yellowish to reddish. On examination 

 of the slides under the microscope, it will be seen that the cause 

 of the cloudiness is the saturation of each salt-crystal with 

 thousands of fluid-cavities that are partly filled with liquid, 

 partly with a gas, the liquid being a solution of potassium 

 dichromate caught up during the crystallization of the salt, and 

 being red or yellow according to its strength, and often contain- 

 ing still more minute crystals of the same red salt. These 

 clouded crystals are not mounted in the original colored solu- 

 tion, but simply in a colorless saturated solution of common salt, 

 or in castor oil, or in Canada balsam, inclosed in a wax cell. 

 We have in the simultaneous formation of these artificial 

 crystals, their cavities, and their inclusions, conditions and results 

 closely corresponding in a general way to those which have 

 occurred in nature in the formation of crystals with fluid-cavities. 



In preparing for microscopical study the common material 

 from this island, two easy methods may be employed. One is 

 the grinding of thin sections. This method has been fully 

 described, in the English language, in the well-known works of 

 Rutley and Beale.* A single suggestion may be added to 



^H. C. Sorby : Quar. Joiir. Geol. Soc, 1858, XIV., pp. 4-6. 



"F. Rutley : "On the Study of Rocks," p. 59.— L. S. Beale : " How to Work wi|th 

 the Microscope," Fifth Edition, p. 213. 



