314 M. Huygens on the cause of the 



tides of solid bodies resist separation in different planes, it be- 

 comes of great use in ascertaining the form of these ultimate 

 particles ; and in the case of crystals like calcareous spar, which 

 possess other remarkable and well determined physical proper- 

 ties, it may enable us to discover some relation between these 

 properties and the form of its ultimate particles. 



One example of this species of inquiry has been given by 

 the celebrated Huygens in the fifth chapter of his Traitt de la 

 Lurniere ; and as we believe it has never appeared in our lan- 

 guage, we shall lay a translation of it before our readers in 

 the subsequent article. 



Allerly, August 27, 1828. 



Art. XXI. — On the Cause of the extraordinary figure of Cal- 

 careous Spar, and on its Cleavage in three different direc- 

 tions. By Christian Huygens. 



There are several vegetable and mineral bodies and crystal- 

 lized salts which are formed with certain angles and regular 

 figures. Among flowers, for example, there are many which 

 have their leaves arranged in regular polygons, to the number 

 of three, four, five, or six sides, but not more ; and it deserves 

 to be particularly remarked, not so much that the figure is a 

 polygon, as that it never exceeds the number six. 



Rock crystal commonly grows in hexagonal prisms, and dia- 

 monds occur which are formed with a square point and polished 

 surface. There is a kind of small flat stones, heaped closely 

 the one above the other, which are all of a pentagonal figure, 

 with their angles rounded and their sides a little bent inwards. 

 The grains of salt which are produced from sea-water affect 

 the form, or at least the angle of the cube ; and in the crystal- 

 lization of other salts, and of sugar, we find other solid angles 

 with surfaces perfectly smooth. Small snow falls almost al- 

 ways formed in minute stars with six points, and sometimes 



* Translated from his Traite de la Lurniere, chap. v. See the preced- 

 ing article. 



