DOUBLE REFRACTION. 



On the Double Refraction produced 

 by Heat and Pressure. 



1. Ci-ystals tenth one Axis of Double 



Refraction. 



From the examination of a great num- 

 ber of crystallised bodies, Dr. Brewster 

 found that all those bodies which crys- 

 tallised in the form of the rhomboid, the 

 regular hexahedral prism, the octohe- 

 dron with a square base, and the right 

 prism icith a square base, have one 

 axis of double refraction, and he has 

 arranged them as follows: The sign 

 indicating that the axis is negative, and 

 the sign + that it is positive. 



Table of Crystallised Bodies having one 



Axis of Double Refraction, 



i. Rhomb with an Obtuse Summit. 



Fig. 3. 



Carbonate of Lime (Iceland Spar.) 



Carbonate of Lime and Iron. 



Carbonate of Lime and Magnesia. 



Carbonate of Zinc. 



Nitrate of Soda. 



Phosphate of Lead. 



Phosphato-Arseniate of Lead. 



Levyne. 



Tourmaline. 

 _ Hubeliite. 



Ruby Silver. 



Alum-stone. 

 + Dioptase. 



+ Quurtz. 



ii. Rliomb with Acute Summit. 



Fig. 4. 



Corundum. 



Sapphire. 



Ruby. 



Cinnabar. 



Arseniate of Copper. 



iii. Regular Hexagonal Prism. 

 Fig. 5. 



-&-'- 



Emerald. 

 - Beryl. 



Phosphate of Lime. 



Nepheline. 



Arseniate of Lead. 



+ Hydrate of Magnesia. 



iv. Octohedron with a Square Base. 

 Fig. 6. 



+ Zircon. 



H- Oxide of Tin. 



+ Tungstate of Lime. 



Mellite. 



Molybdate of Lead. 



Octohedrite. 



Prussiate of Potash. 



v. Right Prism icith a Square Base. 

 Fig. 7. 



B2 



