378 Analyses of Books. [May, 



On this account the mirrors should be at least nine or ten inches 

 long, to a breadth of about one inch, in order to produce a 

 sufficient effect. The illumination is further extended by giving 

 the instalment the form of a truncated pyramid, instead of a 

 prism, with the aperture for the eye at the smaller end. 

 I am, Gentlemen, your most obedient servant, 



P. M. ROGET. 



Article XVI. 



Analyses of Books. 



Traite dcs Characters Physiques des Pierres Precietttes, pour 

 servir a leur Determination lorsqu'elles ont tte taillees. Par 

 M. L'Abbe Haiiy, 8cc. 



To those who are acquainted with the " Traite de Mine- 

 ralogie " and the "Tableau Comparatif" of M. Haiiy, the 

 present volume offers but little information. In the attempt to 

 make a popular book, or rather to lower the subject to the 

 understanding of mere amateurs, the author has, in many in- 

 stances, abandoned the precise terms of science, and has sub- 

 stituted in their place the vagueness of ordinary language. The 

 narrow bounds within which he has restricted himself, added to 

 the veiy elementary form that he has chosen to give to his work, 

 have rendered it at once superfluous and defective. The deserv- 

 edly high esteem, however, in which the writings of M. Haiiy 

 are held, demands that a summary of the contents of the volume 

 before us should be laid before our readers. 



• After some pages, for the most part explanatory of the terms 

 made use of in crystallography, the author proceeds to give a 

 brief description of the species and principal varieties of crystal- 

 lized gems, as far as relates to their figure and structure, in the 

 following order : topaz, quartz, zircon, corundum, cymophane, 

 spinelle, emerald, dichroite, garnet, essonite, felspar, tourmaline, 

 peridot, and diamond. The only novelty in this chapter relates 

 to the structure of einnamon-stone, called byM. Haiiy essonite. 

 This mineral has hitherto been met with only in amorphous 

 fragments ; some of these have afforded indications of structure, 

 from which it may be inferred that the primitive form is a right 

 prism with rhombic bases, the alternate angles of which are 



L02 c 40', and 77° 20'. 



He next proceeds to treat of the physical characters of gems, 

 beginning with their habitudes with regard to light, under which 

 term he includes the colour, properly speaking, of the substances, 

 as well as their power of refracting and reflecting the rays of 

 light. The colours of the gems, with the exception of the 

 spinelle and the emerald, he attributes to oxide of iron ; but 



