OF CENTRAL CANADA PART I. , 7 



or of the same tint as the external colour of the mineral ] but far more 

 frequently it presents a different colour. Thus, whilst Cinnabar, the 

 ore of mercury, has a red colour and red streak, Realgar, red sulphide 

 of arsenic, has a red colour and orange-yellow streak ; Copper Pyrites, 

 a brass-yellow colour, and greenish-black streak ; and so forth. In cer- 

 tain malleable and sectile minerals, the scratched surface presents an 

 increase of lustre. The streak is then said to be " shining." Finally, 

 it should be remarked, that in trying the streak of very hard minerals, 

 we must crush a small fragment to powder, in place of using the file; 

 because otherwise, a greyish-black streak, arising from the abrasion 

 of the file, might very possibly be obtained, and so conduce to error. 



Form. The forms assumed by natural bodies are of two general 

 kinds: (1) Accidental or Irregular, depending rather on external 

 conditions than on the actual nature of the body : and (2), Essential 

 or Regular. Accidental forms occur only as monstrosities in Organic 

 Nature. Amongst minerals, on the other hand, they are of frequent 

 occurrence ; but the Mineral Kingdom possesses also its definite or 

 essential forms. These, whether transparent or opaque, are termed 

 crystals. This term was first applied to transparent vitreous speci- 

 mens of quartz or rock-crystal, from the resemblance of these to ice ; 

 but as it was subsequently found that many opaque specimens of 

 quartz present exactly similar forms, and that opaque as well as 

 transparent forms of other minerals occur, the term, in scientific 

 language, gradually lost its original signification, and came to be 

 applied to all the geometrical or regular forms of minerals and other 

 inorganic bodies, whether transparent, translucent, or opaque. As 

 already remarked, minerals of a metallic lustre are always opaque ; 

 and many of these, galena, iron-pyrites, arsenical-pyrites, &c., occur 

 frequently in very regular and symmetrical crystals. 



As regards the regular or essential forms of Nature, two distinct 

 and in a measure antagonistic form-producing powers Vitality and 

 Crystallization thus appear to exist. Forms which arise from a 

 development of the vital force, exhibit rounded and confluent out- 

 lines ; whilst those produced by crystallization are made up of plain 

 surfaces, meeting, in sharp edges, under definite and constant angles.* 



* This law is affected within slight limits by isomorphous replacements, and also by changes 

 of temperature. The law itself appears to have been discovered by Nicolaus Steno (a 

 naturalized Florentine) as early as 1669, but its true importance was not appreciated until the 



