98 R. J. ANDERSON [august 



a few, and at last even to these. Yet even a superficial study of such 

 figures and forms must lead one on to the consideration of the forces 

 at work. There is exhibited on approaching the living form a remark- 

 able feature which living things possess beyond inorganic forms, viz. the 

 greater power and facilities which a living organism has to express what 

 it cannot conceive or understand, and the capacity of adjusting most 

 complex forces to meet others which it can neither measure nor weigh. 



The forces that are at work in moulding bodies are external or 

 internal ; amongst the latter may be placed surface tension in fluids. 

 The external compression that causes a soft substance to assume a 

 spherical form is more familiar to us than the mode of action of the 

 cohesion forces that cause the particles to swing into position to form 

 the crystalline body. Yet one may in inorganic bodies see that the 

 forces that press, or the pressure that acts all around a sphere, may be 

 so distributed as to form a cube, if divided into three equal pressure 

 sets, each two forces acting opposite to one another on equal areas and 

 at right angles to the directions of the other two pairs. The cube, 

 octohedron, or dodecahedron (with rhombic base), may be easily pro- 

 duced by similar compressions, and these symmetrical irregular bodies 

 may be divided into two equal symmetrical parts by three planes or 

 more passing through certain axes. It is evident that a quadrilateral 

 symmetry may be noted in a cube lying on one side, by making 

 sections with suitable planes, and a triangular symmetry in sections 

 made perpendicular to a through diagonal. A suitable adjustment of 

 the compressing forces leads to the production of the square prism. 

 The side pairs of pressure sets will in this case be equal, whilst the end 

 pair is greater or less, but each pressure pair acts at right angles to each 

 of the other pressure pairs. The lateral compressing forces, if one 

 opposing pair do not act at right angles to the other opposing pair, will 

 give rise to a rhombic prism. The three main axes must stand at 

 right angles. If the compression be so applied that an oblique prism 

 is produced, one plane only can be found which will divide the crystal 

 into symmetrical halves. "Where a crystal is doubly oblique, the form 

 may be imitated by proper pressure planes, no plane of symmetry can 

 be found ; symmetry here is only discoverable in individual planes. 

 The hexagonal prism form seen in beryl and other minerals is con- 

 nected with the rhombohedron, and the rhombohedron is a cube crushed 

 out of shape. The tetrahedron and pentagonal dodecahedron are 

 asymmetric crystalline forms, although regular solids. 



Angular bodies are not limited, as is well known, to inorganic 

 nature. The elements of which organic bodies are composed are often 

 constrained to assume forms with an angular outline. Polyhedra, 

 hexagonal prisms, tessellated pavements, brick shaped and stellate cells, 

 are a few of the varieties well known to the student. These forms, 

 although correctly attributed to external pressure, are largely under the 

 influences of forces inorganic and organic within the elements themselves. 



