FACTS 



165 



Except in cases of isomerism, the measure of the diedral 

 angles of the crystal enables us to know its composition. 

 But even this supposes that the crystal has not been moulded 

 hollow in a plastic substance and subsequently filled by a 

 flow of glass or any other fusible substance. Geologists are 

 acquainted with such mouldings in what are called pseudo- 

 mor phases sometimes found in nature. 



But a crystal is essentially solid ; the recent discovery 

 of liquid crystals is really a play upon words like that which 

 speaks of the life of dead substances. The crystal is solid, that 

 is, under the conditions in which we observe it, 1 it carries its 

 form with it independently of external circumstances. For 

 the environment in which we live we may say that the 

 crystalline form of quartz is a property of quartz, even con- 

 sidered independently of all variations in the environment. 



On the contrary, the form of living bodies, under the 

 conditions in which they are observed by us who are also living, 

 is the result of two factors the living being itself and the 

 medium in which it lives. 



This is also true for crystals while they are forming ; for, 

 although certain characters remain constant in their geo- 

 metrical form, there are others which vary according to the 

 conditions of crystallization. On this account one quartz 

 crystal has its prismatic part longer, another its pyramidal 

 part surbased ; there are individual characters in crystals 

 of the same species on account of the conditions of their 

 formation. But as soon as they have been formed and are 

 solid, they remain what they are without being subject to the 

 least transformation in any medium where they are observed 

 by us. 



1 This restriction has to be made because, in certain conditions 

 which have been verified experimentally, the most resistant crystals 

 can be liquefied. 



