ORDER II. AXOTOMOUS LEAD-BARYTE. 147 



composition often takes place parallel to the other 

 face also, and is variously repeated in parallel 

 layers, as is indicated in the stria? visible upon the 

 terminal plane. The portions of individuals have 

 their terminal planes inclined to each other at angles 

 of 179 10' and 180 50'. There is also a regular 



Pr 



composition parallel to . Massive : composition 



/ 



lamellar or granular. 



OBSERVATIONS. 



1. The two systems of coloured rings, which plates of 

 the present species exhibit in polarized light, are observable 

 with great facility. They had been discovered by Dr 

 BREWSTER, previous to the exact description of the forms, 

 as belonging to the hemi-prismatic system. Mr BROOKE 

 has called in question the accuracy of the observations in- 

 dicative of these forms, owing perhaps to his not having 

 examined a greater number of the different crystalline 

 varieties of the species. 



2. According to BERZELIUS, 100 parts of axotomous 

 Lead-baryte yield by analysis 



Carbonate of Lead 71-1. 



Sulphate of Lead 30-0. 



with traces of muriatic acid and lime, giving an excels of 

 1-1, probably owing to the existence of a subsalt of 

 lead in the mineral, and not agreeing with the definite 

 proportions. Professor IRVING had obtained a similar result. 

 Mr BROOKE had obtained three atoms of carbonate, and 

 one atom of sulphate of lead, in the proportions of 72-5 to 

 27-45. Before the blowpipe this mineral first intumesces 

 a little, and then becomes yellow, but re-assumes a white 

 colour on cooling. It effervesces briskly in nitric acid, and 

 leaves a white residue. 



3. The varieties of the present species, and among them 

 the crystallisations quoted above, occur principallv at Lead- 



