288 Formation of Compound or Twin Crystals. 



Similar objections exist to the hypothesis of Mr. Prout. It may 

 be farther objected to it that it is hardly philosophical in accounting 

 for the formation of a solid by the existence of three axes in atoms, 

 to consider one of these axes " chemical," while the two others 

 are " cohesive." Where there is a sameness of effect we must sup- 

 pose the existence of the same cause. The perfect resemblance in 

 all the physical properties of the face of a cube, ought therefore to 

 be ascribed to the operation of the same kind of attraction. Again 

 Mr. Prout states, and correctly, (p. 41,) that particles endowed with 

 a single axis of attraction would arrange themselves in a straight line, 

 and if with two axes, in a plane. But consequently his compound 

 molecule of water instead of being spheroidal, as stated on page 148, 

 can only be either a straight line or a plane, (that is, leaving out of 

 consideration the thickness of the molecule.) The molecules of 

 Oxygen and Hydrogen are supposed by Mr. Prout to be triatomic, 

 and hence water will contain nine sub-molecules " which may be as- 

 sociated, in the first place the hydrogen with the oxygen chemically, 

 and afterwards the three sub-molecules of water with one another 

 cohesively." The latter process, as the axes of cohesion are but 

 two in number, w ill therefore according to the above principle, give 

 rise to a plane, and it is hence impossible that thus should be " con- 

 stituted a spheroidal molecule." It is also an impossibility that any 

 but rectangular molecules and rectangular crystals, should proceed 

 from rectangular axes of attraction, (see p. 41.) The rhombic and 

 rhomboidal prisms right and oblique, as also the Rhombohedrons 

 require some other hypothesis to explain their construction. It is 

 of no avail either, to suppose the existence of oblique axes in the 

 molecules of some of the elements, for combinations of those which 

 in every probability have not such, will still at times produce an ob- 

 lique form ; and for the same reason nothing is gained by the suppo- 

 sition that some molecules may have a spheroidal form. 



But I will not delay longer on this subject although it admits of 

 a much greater extension. I proceed with the explanation of the 

 formation of Compound Crystals. 



The existence of axes of attraction supposes also the existence 

 of opposite poles at the extremities of these axes. To these poles 

 may be applied the ordinary names of North and South. In the 

 Rhombohedron, the three poles about one of the dominant solid an- 



