PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 819 



cal equilibrium is an impossibility, but, iu all aggregations of 

 atoms and molecules, dynamical equilibriums exist, where the 

 accelerations of atomic motion, caused by the augmentations of 

 gravity due to nearer proximity, produce what is known as pressure, 

 namely, a contraction of the orbits, accompanied at the instant of 

 its production by an elevation of temperature. On again separa- 

 ting the atoms, the pressure undergoes a corresponding diminution, 

 accompanied by a depression of temperature. 



In both the gaseous afld liquid conditions we find a perfect or 

 nearly perfect freedom of motion of the molecules among each 

 other, so that they yield, so far as change of form is concerned, to 

 the slightest external force, and we see nothing in the forms of 

 the orbits or in their connections, one with the other, to prevent 

 this, but we come now to the third ov solid condition of matter, iu 

 which rigidity of form is the important characteristic. 



If we investigate the effect of a still further diminution in the 

 heat or velocity of atoms, we shall probably find that the long 

 elliptical, or nearly parabolic orbits, become gradually somewhat 

 reduced in size and changed in form, so that at a certain period a 

 second or lateral interlinking, at the extremities of the conjugate 

 axes or diameters takes place, and a form of molecule is originated 

 which gives this rigidity of form to solid bodies. 



Doubtless the most direct path to positive knowledge iu regard 

 to the forms of solid molecules, is through the study of crystalline 

 forms and firces. It is now generally understood that cohesive 

 attraction, in homogeneous substances, is identical with the polar 

 force of crystallization. A glance at the general features of crys- 

 tallogeny, as now taught, will be likely, therefore, to render essen- 

 tial aid in the present investigation. 



Prof. J. D. Dana, probably the highest authority on the subject, 

 reduces all the various forms of crystals to primary ellipsoids, 

 which he supposes to be held together by a powerful attractive 

 force which suddenly comes into existence at the instant when 

 crystalization takes place, at the extremities or poles of conjugate 

 axes of the ellipsoids. 



While this hypothesis is amply sufficient to account for all the 

 YQXions forms of crystals, which it does in a most simple and beau- 

 tiful manner, the source of the polar force, by which the previously 

 liquid molecules are compacted into the hard unyielding cr3^stal, 

 is left as much in the dark as before. What is the nature and 

 constitution of this ellipsoid ? Is it solid, hard, clastic, etc. ? 



