238 



PHYSIOGRAPHY. 



CLASS IT. 



2. Several analyses have been published referring to the 

 varieties formerly comprehended within the Mesotype of 

 HAUY. Thus Messrs GEHLEN and FUCHS, who first dis- 

 covered the difference among several of these substances, 

 obtained from 



With these analyses, other varieties have been found to 

 agree more or less. The chemical formula of Natrolite is 

 given by BERZELIUS Na 3 Si 3 + 2 Al Si + 4 Aq, which cor- 

 responds to 15-93 of soda, 26-19 alumina, 48-64 silica, and 

 9-24 water. The Natrolite loses its transparency before the 

 blowpipe, and melts into a glassy globule ; the radiated va- 

 rieties exfoliate, and the compact ones intumesce. They 

 are with difficulty soluble in borax. Some of them assume, 

 by heat, faint degrees of opposite kinds of electricity on 

 their opposite ends, and become positively electric by 

 friction. 



3. The general repository of ail the species here under 

 consideration, are the vesicular cavities of amygdaloidal 

 rocks. They are accompanied by various species of the 

 present genus, and by rhombohedral Lime-haloide. Some 



Fig. 175. represents a crystal of the Iceland variety ; there 

 is a face of composition passing through the crystals, which 

 therefore appear to be twins. This composition has been 

 first observed by Dr BREWSTER. I obtained by measure- 

 ment the incidence of o of one individual, on the face o" of the 

 other = 1 79 0' of o on o' = 141 53', of o" on o' = 142 53'; 

 o on the adjacent face over a = 145 10'. The edges 

 marked a, and those between o and M, are often replaced, 

 by additional faces, the first of which are striated parallel 

 to the edges of combination with 0, the latter rough. H. 



