60 ILLUSTRATIVE EXAMPLE 



from the water itself. It is colourless and perfectly 

 transparent, although it holds a mass of solid matter 

 which previously would not allow of the permeation of 

 a ray of light. Let us expose this fluid to such circum- 

 stances that the water will slowly evaporate, and we 

 shall find forming in it, after a time, microscopic par- 

 ticles of solid, light-refracting matter. These particles 

 gradually increase in size, and we may watch their 

 growth until eventually we have a symmetric figure, 

 beautifully shaped, the primary form of which is a right 

 rhomboidal prism. Thus in nature, by the action, in 

 all probability, of vegetable matter on the sulphates held 

 in solution by the water of the great rivers and the 

 ocean aided by our oxidizing atmosphere sulphuric 

 acid is produced to do its work upon the limestone for- 

 mations, and from this combination would result the 

 well-known gypsum, or plaster of Paris, which ordi- 

 narily exists as an amorphous mass, but is often found 

 in a crystalline form.* 



This is a very perfect illustration of the wonderful 

 process we have been considering,, and in which, simple 

 though it appears to be, we have set to work a large 

 proportion of the known physical elements of the 

 universe. By studying aright the result which we have 

 it in our power to obtain in a watch-glass, we may 

 advance our knowledge of gigantic "phenomena, which 

 are now progressing at the bottom of the ocean, or of 



* The transparent varieties of sulphate of liine are distinguished 

 by the name Selenite ; and the fine massive varieties are called 

 Alabaster. Gypsum forms very extensive beds in secondary 

 countries, and is found in tertiary deposits; occasionally, in 

 primitive rocks; it is also a product of volcanoes. The finest 

 foreign specimens are found in the salt mines of Bex, in Switzer- 

 land ; at Hall, in the Tyrol ; in the sulphur-mines of Sicily ; and 

 in the gypsum formation near Ocana, in Spain. In England, the 

 clay of Shotover Hill, near Oxford, yields the largest crystals. See 

 Dana's Mineralogy, second edition, p. 241. 



