VII.] CRYSTALLIZATION. 71 



cannot be permeated by water so long as it retains the 

 crystalline structure. In this respect, also, crystals are 

 contrasted with organisms ; for the substances that compose 

 organisms are always in some degree permeable by water. 

 On this depends the fundamental difference between the 

 mode of growth of crystals and of organisms. Crystals 

 grow by addition of substance at the surface, and at the grow at 



„ , . , I J. 1 j-i • 1 the surface 



surface only : organisms, on the contrary, both animal o^iy . 

 and vegetable, absorb into their interior the nutritive 

 materials that are to be added to their substance. In 

 other words, crystals grow by superficial accretion, organ- 

 isms by interstitial accretion. 



Closely connected with the last-mentioned is another 

 difference, which also is a fundamental one. A crystal is 

 in a state of perfect molecular immobility : its molecules are mole- 

 cannot acquire mobility without the crystalline structure immobile 

 being destroyed. In an organism, on the contrary, a cer- Contrast 



tain degree of molecular mobility is the first condition of ?^ o^gan- 

 ° -^ . isms, 



life : it is constantly excreting matter, while other matter is 

 brought in from without to supply the loss. This double 

 process goes on with most rapidity in those organisms 

 where the life is most energetic ; that is to say, in warm- 

 blooded animals. 



I have now stated the chief among what may be called 

 the physiological resemblances and differences between 

 crystals and organisms. Before I go on to their morpho- 

 logical resemblances and differences, it will be necessary 

 to give a brief account of the morphological, or formative, 

 principles of crystallization ; of the optical properties of 

 crystals, which stand in close connexion with their forms ; 

 and of the classes, or systems, of crystals. 



For every crystalline species three lines may be found 

 which stand in a simpler geometrical relation than any 

 other to all the forms which are possible in the species. 

 These lines intersect in a point within the crystal. They 

 are called the crystallographic axes, or simply the axes. Crystalio- 

 They are never all in the same plane, and they all may be at f^es."'' 

 right angles to each other. The angles of their intersection 

 are constant " crystallographic elements " for the species. 



