418 THE LIFE OF MATTER. 



Organization of crystals — Vietos of TTauy, Delafosse and Bravah^ 

 and of ^\aUera,nt. — In botany, zoology, and crystallogTaphy we 

 understand by form an assemblage of material constituents coordi- 

 nated in a definite system; that is to sa}^, it is organization itself. 

 The body of man, for example, is an edifice in which 60 trillions of 

 cells ought each to find its own place fixed in advance. 



In crystallography also we understand ]jy form the organization 

 which cr3'stals present. The grouping of the elements of cr3\stals is, 

 perhaps, more simple. They are none the less organized, in the same 

 sense that living bodies are. 



Their organization, while more uniform than that of living bodies, 

 still show\s a considerable amount of variation. It should not be 

 assumed that the area of a crystal is completel}' filled with contiguous 

 parts applied one to the other by plane faces, as might be supposed 

 from the phenomena of cleavage which dissociates the parts of the 

 crystalline body into solids of this kind. In realit}^ the constituent 

 parts are separated from each other b}* spaces. They are arranged 

 in quincunx, as Haiiy says, or along the lines of a network, to use the 

 terms of Delafosse and Bravais. The intervals left between them are 

 much larger than their diameters. So it is that in the organization of 

 a crystal it is necessary to take into account two quite diflerent 

 things: An element, the crystalline particle, which is a certain aggre- 

 gate of chemical molecules having a determinate geometrical form, 

 and a more or less regular, parallelopipedic network, along whose 

 crests are arranged in a constant and definite manner the aforesaid 

 particles. The external form of the crystal indicates the existence of 

 the network. Its optical properties depend upon the action of the 

 particles, as M. Wallerant has shown. There are, therefore, to ])e 

 distinguished in a crystal two kinds of geometrical figures — that of the 

 network and that of the particle — and their characters of symmetry 

 may be either concordant or discordant. 



The crystalline particle, the element of the crystal, is then a cei'tain 

 molecular complex that repeats itself identically and is identicalh' 

 placed at the nodes of the parallelopipedic network. It has l)een given 

 difi'erent names well calculated to produce confusion — the crystallo- 

 graphic molecule of Mallard, the complex particle of other authors. 

 Some have separated this element into subordinate elements (the 

 fundamental particles of Wallerant and of Lapparent). 



These very general outlines will suflice to .show how complex and 

 adjusta])le is the organization of the crystalline individual, which, in 

 spite of its geometric regularity and its rigidity, may be compared 

 with the still more flexible organization of the living element. The 

 mineral individual is more stable, less prone to undergo change than 

 is the living individual. We ma}^ say with M. Lapparent that '•cr3\s- 

 tallizcd matter presents the most perfect and stable orderly arrange- 

 ment of which the particles of bodies are susceptible." 



