84 PROF. A. dattbree on incrustations by thermal 



minutes with boiling nitric acid diluted with 5 times its volume of 

 water, the results being as follows :— ? 



Brick A. Brick B. 



Soluble portion 1-397 1-488 



Insoluble portion 8-603 8-512 



Together 10-000 10-000 



Analyses of the soluble portion — 



Silica 19-39 8-85 



Alumina 17-33 19-73 



Ferric oxide 5-37 5*95 



Lime 51-40 60-34 



Magnesia 0-75 0*47 



Potash 5-94 5-07 



Soda 0-33 0-13 



Total 100-51 100-54 



The large percentage of potash in the bricks containing zeolites is 

 remarkable. For although it is well known that this alkali is an 

 invariable constituent of clays, the bricks of Plombieres contain it not 

 only in larger amount than is found in natural clays, but in such a 

 condition as to be rendered readily soluble by weak acids, which is 

 not the case with the alkalies contained in clays. It appears there- 

 fore to be due, in part at any rate, to the alkaline constituent of the 

 thermal water. Possibly the potash found in green earth, where that 

 substance replaces crystals of pyroxene, as in the Tyrol and elsewhere, 

 may be derived from a similar source. 



Note 4. On the crystalline forms of chahasiU and christianite of 

 contemporary formation. — The chabasite is in rhombohedra, which 

 are partly simple and partly macled in the same manner as natural 

 crystals. 



The christianite of Plombieres appears to be identical in form 

 with the crystals found at Marburg and Somma ; i. e. they are cruci- 

 form penetration-twins of two individuals not showing any re- 

 entering angles. According to M. Descloizeaux, the plane of the 

 optic axes is parallel to the macropinakoid, and their first median 

 line to the macrodiagonal. The primitive form is a rhombic 

 prism of 111° 13'. The crystals from Oran appear, however, to 

 be macled on the type of harmotome, and the optic axes to be 

 similarly placed to those of that species, the primitive form being 

 probably a rhomboidal prism of 128° 11'. The crystals are too 

 small^ however, to afford any very accurate measurement. 



