484 wherry: classes of crystals 



sarily result. As all available data indicate that the atomic 

 volume of potassium is decidedly greater than that of chlorine, 

 this symmetry would be expected with potassium chloride. It 

 may be pointed out here that in the case of sodium chloride the 

 atomic volumes of the two elements are essentially identical, and 

 no gyrohedrism should be exhibited; and as a matter of fact, the 

 etch-figures on this substance, as well as all of its other proper- 

 ties, are completely holohedral. On the other hand, in practi- 

 cally all the other alkali halides, including those of ammonium, 

 a difference of , volume must be present, and gyrohedral sym- 

 metry is to be expected; and this has been thoroughly confirmed 

 by etching experiments. The crystallization of these halides 

 may therefore be described as: 



System, cubic; structure, holohedral; structure-units, gyrohe- 

 dral. 



The relation in the case of cuprite corresponds to that of the 

 haUdes, except that here for some obscure reason the gyrohe- 

 drism is shown by fonns occasionally found upon the natural 

 crystals rather than by the etch-figures. The explanation of the 

 symmetry is no doubt the same as with the halides, copper having 

 a less volume than oxygen. The crystalHzation of this sub- 

 stance may accordingly be described as: 



System, cubic; structure, holohedral; structure-unit, gyro- 

 hedral. 



The pyrite group is a particularly good illustration of the 

 relation here under consideration. Many crystallographers have 

 considered all the members of this group to be tetartohedral, 

 because of occasional tetrahedral development of forms capable 

 of showing it, accompanied by typical dyakis-dodecahedral or 

 ''pyritohedral" symmetry of other forms. The thermo-electric 

 behavior is also regarded as tetartohedral in character. The 

 etch-figures appear, however, not to depart from simple pyi'ito- 

 hedral relations ; and no information can be obtained from optical 

 properties, as no method of measuring optical rotatory power on 

 opaque, metalUc, minerals has as yet been developed. The X-ray 

 studies made by the Braggs of three members of this group, 

 pyrite (FeSa), hauerite (MnS2), and cobaltite (Co S As), show the 



