332 Mr, Brooke's Reply to Dr. Brewster, [Nov. 



Dr. Brewster also supposes these planes c to remain upon the 

 compound crystal, whereas it is evident from fig. 2, that they 

 entirely disappear from that form. The measurement of the 

 planes e over M, in the bypyramidal crystal, is 112^ 44' very 

 nearly, instead of 114° as quoted by Dr. Brewster. 



Dr. Brewster also alludes in his paper on this salt to crystals 

 with one axis and crystals with two axes of double refraction, — a 

 fact which seems to place another stumbling block in the way 

 of Dr. Brewster's optical system, and which affords additional 

 evidence that the optical characters of minerals are liable to 

 modifications from causes not yet understood. 



I would have here closed my observations upon the article quoted 

 above from Dr. Brewster's review, had not the Doctor chosen 

 to tack to its tail what I suppose he intended for its sting — an 

 observation about Apophi/llite, or rather, as I suppose he means, 

 about his favourite TesseHte, On this mineral, as I have shown 

 elsewhere. Dr. Brewster has allowed his imagination to revel to 

 the top of its bent; and whatever may be the fate of any other 

 of Dr. Brewster's novel speculations, his extraordinary disco- 

 very in crystal building * relative to this imaginary species, is 

 not likely to have any other claimant, or ever to find its way 

 among " Decisions on dupuied Inventions." It will remain a 

 memorial of the great extent of the Doctor's knowledge of nature, 

 and may at last be fortunate enough to occupy a niche in the 

 temple of Fame as a companion to the celebrated optical system 

 of Miss Margaret Macavoy. 



But seriously, for really these sprightly effusions of the Doc- 

 tor's pen, in which, as regards myself, he has indulged himself 

 wherever an opportunity has been aflbrded him, scarcely merit a 

 serious attention, if the claim of Tesseliteio be ranked as a sepa- 

 rate species had any real foundation, it might have been expected 

 to be so distinguished in the latest work on mineralogy which 

 has been published in this country, I mean that by Dr. Brews- 

 ter's coadjutor, Mr. Haidinger. 



But the observations of this gentleman on Tesselite, although 

 they are, perhaps, calculated to soothe the froward philosopher, 

 do not compromise his own judgment as a mineralogist. 



♦ The following extract from a paper by Dr. Brewster in the Edinburgh Philosophi- 

 cal Transactions for 1 823, announces the discovery here alluded to : 



'' The Tcsselile could not,''' Dr. Brewster says, *' have bexn formed hy the ordinary 

 process of crystallization^ but that a foundation appears to be first laid by means of 

 uniform homogeneous plates^ the primitive form of which is pyramidal ; a central pillar 

 whose section is a rectangular lozenge, thr,n rises perpendicularly from the base, and 

 consists of similar particles. Round this pillar are placed new materials, in the form 

 of four trapezoidal solids, the primitive form of whose particles is prismatic, and in 

 those solids the lines of similar properties are at right angles to each other. The crys- 

 tal is then made quadrangular by the application of four triangular prisms of unusual 

 aculrness. 'These nine solids arranged in this symmetrical manner, and joined by tran- 

 aparent lines performing the functions of a cement, are then surrounded by a mall com" 

 posed of numerous films, deposited in succession, and the whole of this singular assem~ 

 blago ii finally roofed in by a plate exactly similar to that which formed its foundation.'^ 



