240 CRYSTALLIZATION. 



AMMONIO-PHOSPHATE OF MAGNESIA 



is a salt frequently met with in animal secretions which 

 have undergone decomposition ; they belong to the rhombic 

 system, but their varieties are endless. Stellate and pen- 

 niform crystals are frequently found in urine. 



URIC ACID, OR LITHIC ACID. 



This acid abounds in animal secretions, in the excrement 

 of birds, serpents, &c., and the urine of mollusca and 

 carnivorous mammalia. The crystals belong to the right 

 rhombic prismatic system, but are various in form and size. 

 They polarize light splendidly. 



NITRATE OF POTASH, NITRE, OR SALTPETRE. 



This salt is dimorphous ; it crystallizes in various forms, 

 but they all belong to the right rhombic prismatic system ; 

 sometimes six-sided prisms with dihedral summits are on 

 the slide, and sometimes obtuse rhombohedral crystals, resem- 

 bling those of nitrate of soda ; but they all polarize, and 

 exhibit the phenomena of analytic crystals. 



Analytic crystals are those which possess the power of 

 analysing light, like the tourmaline used in the ordinary 

 polariscope. 



SALICINE, 



an alkaloid extracted from the bark of the willow tree, and 

 crystallizing in beautiful forms, either in discs exhibiting 

 the cross and concentric circles of colour, or in prismatic 

 crystals in stellate and irregular groups, polarizing ad- 

 mirably. 



NITRATE OF SILVER, 



crystallized on a slide, shoots over the glass in most bril- 

 liant feathery crystals, and is also a splendid object viewed 

 with a spotted lens or parabolic reflector. 



