Miscellaneotis, 335 



these deductions, and at the same time taking away the amount 

 of water necessary to render the MgOSOg rrrMgOSOg -f- "7 aq. : 

 (the hydrated sulphate of soda would of course become anhydrous 

 on exposure to dry air) ; the results then become ; 



Calculation. 



99-98 537-1 100-00 



corresponding to the formula, 



NaO, 3CaO, B9O.3 + I2HO." 



The late Prof. Robb remarks as follows on its crystalline 

 form : — 



"In spite of your odd formula, the mineral, just as I got it, un- 

 touched and unwashed, is perfectly crystalline in every particle. 

 A good power is required, but with a magnifying power of about 

 350 diameters there is no difficulty, the form comes out as sharp 

 as possible. The crystals are excessively thin translucent tables 

 or plates. They have a rhombic outline and the angles probably 

 = 80* or more, owing to their excessive thinness I could not say 

 whether they could be called right or oblique rhombic prisms. I 

 suspect the latter from analogy. By care the ' Tiza ' (Natro-boro- 

 calcite) can be shown to consist of very fine prisms, sharp, angu- 

 lar and long, but too fine for me to state their form. The diam- 

 eter was less than '00118 of an English inch. The long prismatic 

 needles of the Tiza are in great contrast to the broad tables of 

 the recent mineral in your last letter ; of that the plates are about 

 •0048 of an inch from side to side, but some are a little larger, 

 others a little smaller. In some you see regular cleavage, that is, a 

 small rhomb chipped out of one side. As I'ar as form goes therefore 

 it would seem to be a distinct and definite species. I presume it 

 was formed in a dry place for the angles were quite sharp. The 

 connection between these borates and sulphate of lime and sul- 

 phate of soda is very curious." 



Prof. How thus notices its bearing on the question of the mode 

 of formation of gypsum : — 



"The truth of the last sentence in Prof. Robb's letter is very 

 apparent. In my former paper on the subject I adverted to the 

 existence of Natro-boro-calcite in the Gypsum here as confirming 



