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PROCEEDINGS OF THE AMERICAN ACADEMY 



other, they would form crystals having a rhombic section of 60° or 

 120'^. Tlie figures 1 and 2, which we reproduce from a previous 

 paper, may help to give a more definite form to these conceptions ; but 



Fig. 1. 



Fig. 2. 



such representations are necessarily purely conventional symbols of 

 conditions of which we have as yet no accurate knowledge, and to 

 which, therefore, we can give no definite shape. The capability of 

 such molecular macling as we have described may depend solely on 

 the dimensions of the molecules ; and in our figures we have repre- 

 sented such a condition, by giving to the section,^ of the molecules the 

 form of ellipses of such dimensions that they can be inscribed in 

 the rhomb of 60° and 120°. The conjugate diameters of this figure, 

 when equal, subtend angles of 60° or 120° ; and if the poles of the 

 molecules are, as would be natural, at the ends of these lines, then, 

 when the molecules were grouped as shown in Fig. 1, the unlike poles 

 would fall directly over each other ; so that the attractive and repul- 

 sive forces, centring at the poles, would hold the parts firmly together. 

 The same molecules, if placed parallel to each other (as in Fig. 2), 

 would be also in a stable condition, and the resulting rhombic section 

 would have angles of 60*^ and 120°. On the other hand, although 

 ellipses of other dimensions might be united as in Fig. 2, so as to give 

 rhombic sections of every possible angle, yet only with ellijises of the 

 dimension we have described, or those closely approaching this condi- 

 tion, would such a grouping be stable as is represented by Fig. 1. Of 

 course, the molecules must have three dimensions ; and, as before inti- 

 mated, the ellipses are only conventional modes of expressing concep- 

 tions which are necessarily very incomplete. Tiiese symbols, however, 

 will give form to our theory, and show why, among a series of isomor- 

 phous compounds crystallizing in the rhombic system, we might expect 

 to find hexagonal crystals wherever among the various molecular 



