176 ROYAL SOCIETY OF CANADA 



quite evident that some action had taken place, for the powder became 

 changed into more or less flocculent particles, while the solution gave 

 an alkaline reaction. This was filtered oit", and it was found that 

 ammonia gave quite a perceptible precipitate of aluminium hydrate after 

 warming the solution and allowing to stand for some time. The ex- 

 planation of the apparent insolubility of the residue left after ignition 

 appears to be that, although the sodium aluminate dissolves, most of the 

 aluminium is reprecipitated as hydrate as fast as it goes into solution, 

 the latter salt being soluble only when an excess of soda is present. 

 The precipitate obtained on adding anmionia to the filtered liquid is 

 due to the small amount of alumina which the soda is able to hold in 

 solution. 



The experiment has only a negative value, since even if the sodium 

 and aluminium are not in direct combination with each other in the 

 mineral, we would expect sodium aluminate to be formed after the 

 removal of the carbon dioxide and water by intense ignition. 



Dawsonite and Nesquehonite : a compakison. 



Although as regards their chemical composition, these two minerals 

 can scarcely be described as nearly related, yet, when we compare their 

 crystallograpliical properties, a very curious and strikins: similarity is 

 Been to exist between them, as v/ill be apparent from the following table : 



Dawsonite. Nksquehonite." 



AljOj . Na,0 . 2C0, . 2H,0. MgO . CO, 3H,0. 



Orthorhombic. Orthorhoml)ic. 



o ; 6 •• c = 0.6475 : 1 : 0.5339 a : b : c = 0.6445 : 1 : 0.4568. 



M : M = 65° 51'. M : M = 65° 36'. 



Cleavage ||(110), perfect. Cleavage H (110), perfect. 



Birefringence,— ^e, strong. Birefringence,— Y, strong. 



Ax. PI. i (001); Ac. Bis. i (100). Ax. PI. H 001); Ac. bis. 1 (100). 



Dispersion, small, p < v Dispersion, small, p < v. 



a = 1.466, P =- 1.542, y = 1.596 a = 1.495, B = 1 501, y = 1.526. 



^ry=7G°46'. ^Fj, = 53°5'. 



We would have no hesitation in saying that two minerals such as 

 these, whose properties are so closely allied as to be identical in many 

 respects, were isomorphous, provided there was some similarity also in 

 their chemical composition; and if we are to regard the geometrical and 

 optical properties of crystals as being intimately connected with the 

 ftruetures of their molecules, whether physical or chemical, then we are 

 forced to the conclusion that the molecule of dawsonite must be in 



•Genth and Penfield, Am. J. Sc, 39,121, 1890. 



