SEGREGATION. 477 



set of atoms are driven off rather than the others; and 

 even then not a few of the others accompany them. The 

 most interesting and instructive example, however, is fur- 

 nished by certain phenomena of crystallization. When 

 several salts that have little analogy of constitution, are 

 dissolved in the same body of water, they are separated 

 without much trouble, by crystallization: their respective 

 units moved towards each other, as physicists suppose, by 

 polar forces, segregate into crystals of their respective 

 kinds. The crystals of each salt do, indeed, usually con- 

 tain certain small amounts of the other salts present in the 

 solution — especially when the crystallization has been rap- 

 id: but from these other salts they are severally freed by 

 repeated re-solutions and crystallizations. Mark now, how- 

 ever, that the reverse is the case when the salts contained in 

 the same body of water are chemically homologous. The 

 nitrates of baryta and lead, or the sulphates of zinc, soda, 

 and magnesia, unite in the same crystals ; nor will they crys- 

 tallize separately if these crystals be dissolved afresh, and 

 afresh crystallized, even with great care. On seeking the 

 cause of this anomaly, chemists found that such salts were 

 isomorphous — that their atoms, though not chemically 

 identical, were identical in the proportions of acid, base, 

 and water, composing them, and in their crystalline forms : 

 whence it was inferred that their atoms are nearly alike 

 in structure. Thus is clearly illustrated the truth, that units 

 of unlike kinds are selected out and separated with a readi- 

 ness proportionate to the degree of their unlikeness. In the 

 first case we see that being dissimilar in their forms, but sim- 

 ilar in so far as they are soluble in water of a certain tem- 

 perature, the atoms segregate, though imperfectly. In the 

 second case we see that the atoms, having not only the like- 

 ness implied by solubility in the same menstruum, but also 

 a great likeness of structure, do not segregate — are sorted 

 and parted from each other only under quite special con- 

 ditions, and then very incompletely. That is, the incident 



