1820.] Dr. Thomson's Analysis of NeecUestone. 407 



not furnish us with any historical details respecting the speci- 

 mens which they examined, excepting merely the localities, viz. 

 Ferro, Iceland, and the Tyrol. Now if these specimens hap- 

 pened to be taken from rocks possessing the very same charac- 

 ters and situation, it might very well happen that they contained 

 nearly the same proportion of constituents, though the natrolite^ 

 and skolezite of which they were composed were only mechani- 

 cally mixed. 



The specific gravity of a mechanical mixture of two parts 

 skolezite and one part natrolite would be 2'228. 



Dr. Freyssmuth lately examined a fibrous zeolite from Bohe- 

 mia, which he showed to belong to the mesolite of Fuchs and 

 Gehlen. Its specific gravity, as I have already mentioned, was 

 2'333, and its constituents were as follows : 



Silica 44-562 



Alumina 27-562 



Lime 7-087 



Soda 7-688 



Water 14-125 



Oxide of iron Trace 



101-024 



Here the proportion of soda is increased, and that of lime 

 diminished, and both are nearly what they would be on the 

 supposition that the mesolite analyzed was a mixture or combi- 

 nation of equal parts of natrolite and skolezite ; for the consti- 

 tuents of such a mixture or combination would be 



Silica 47-25 



Alumina 26-10 



Lime 7-10 



Soda 8-10 



Water 11-45 



100-00 



Now this approaches very near the above analysis as far as 

 the lime and the soda are concerned. The quantity of water 

 present in the mineral analyzed by Freyssmuth is greater than it 

 ought to have been while there is a corresponding diminution in 

 the silica and alumina ; yet this analysis appears to me sufficient 

 to show that mesolite is not always exactly the same in its con- 

 stitution. We must either admit that there are two species of 

 '^^r mesolite, one composed of two parts of skolezite and one 

 part of natrolite, and another composed of equal parts of these 

 two minerals, or we must acknowledge that it is merely a 

 mechanical mixture of natrolite and mesolite in various and inde- 

 finite proportions. The symbols for the mesolite of Fuchs and 

 Gehlen and of Freyssmuth are obviously the following : 



