152 



PRACTICAL MICROSCOPY. 



forms an illustration of the production of crystalline forms 



when molten vitreous matter is allowed to cool slowly. 



This mineral is of a beautiful crimson colour, and under a 



binocular microscope few objects 

 can vie with it for beauty. 



The two figures, 139 and 140, 

 show the artificial production of 

 similar crystals during the manu- 

 facture of glass. When the 

 " metal " is cooled slowly, crys- 

 tals of certain forms are obtained, 

 which vary in form according to 

 the differences existing in the 

 quantities of the materials from 

 which the glass is made. 

 Books which may be consulted: 'Sur les Crystallites/ 



Vogelsang ; Allport on the " Microscopical Examination 



FIG. 140. 



of Rocks and Minerals," p. 98, 'Monthly Microscopical 

 Journal' ; 'Mineralogy and Study of the Rocks/ Rutley. 

 MOLLUSCS. The entire series of the Mollusca form very 



