$52 On Crystallography. 



hardness does not admit of quite so much precision as the 

 foregoing, [t is besides iDore variable. Scmie panicles of 

 quartz, disseminated in a body of a soft nature, may, by 

 altering a little its specific gravity, make it capable cS 

 striking fire with steel. But this ch'iracter has the ad- 

 vantage of being easy and expeditious in it& application, by 

 the help of the different methods pointed out in the tabl«. 

 It is proper to disiinguiih accurately the species whose 

 varieties are but little subject to the alterations produced 

 by accidental mixtures. Thus the cymophane, or the 

 pearly chrysolite oF the lapidaries, scratches glass, whereas 

 the chrysolite uf Rome de I'lsle, since ascertamed to be a 

 calcareous phosphate, when rubbed upon glass, frequently 

 leaves a white mark of its own subsianc-e. The ferrugi- 

 nated schcelin or wolfram yields very easily to the file, 

 which has much less action on the blackish oxidized tin 

 sometimes found in the same matrix, 8cc. 



3. Brittle bodies. These must not be confounded with 

 what are called soft or tender. Talc is softer than car- 

 bonated lime, denonjinated calcareous spar., since this last 

 scratches it. But it is less brittle, as it resists percussion 

 far better. 



4. Elastic bodies are those which, after having been 

 compressed, return of their own accord to their original 

 figure. The character suggested by this' property is net 

 employed in mineralogy, except when speaking of bodies 

 which appear to us in the form, or may be easily reduced to 

 the form, of thin laminae or filaments^ and it suits but a 

 small number of species. 



5. Ductile bodies are those which are lengthened or 

 flattened by percussion or by pressure, so as to preserve 

 the figure which they have taken in virtue of the one or 

 the other of these two forces. Of this number are several 

 metallic substances, gold, silver, co|)per, &c. Whereas 

 others, such as antimony, bismuth, &;c. will break, rather 

 than submit to be flattened or elongated. 



6, Bodies endowed iviik tenacity. This is tl>e property 

 possessed by certain substances, and particularly by the 

 metals, of resisting without being broken the action of a 

 force which draws them by one extremity while they are 

 fixed at the opposite extremity. Of this kind is the re- 

 sistance presented to breaking by a harpsichord string which 

 we are tuning. This property is but little susceptible of 

 bein^ employed as a character, but it is right to mention, 

 in the description of a mineral, to what degree it possesses 

 it in comparison with others. 



7« Adiitrence to the tongue. Certain bodies adhere to 



the 



