CHAPTEE X. 



ORGANIZATION. 



Three 

 kinds of 

 formative 

 principles ; 



the first 

 forms 

 spherical 

 aggrega- 

 tions. 



Agate 

 nodules, 



having 

 structure 

 depending 

 on form. 



THEEE are in nature three kinds of formative princi- 

 ples, each of which produces its own characteristic 

 forms, totally unlike those of the others. 



All forces which are simply attractive produce spherical 

 forms. Gravitation and capillarity are forces of this class : 

 a planet, which owes its spherical form to gravitation, and 

 a rain-drop, which owes it to capillarity, are both instances 

 of this kind of formative principle. In such aggregations 

 there is not necessarily any structure : a rain-drop has no 

 structure : and when there is any, it consists merely in 

 stratification ; that is to say, in the deposition of layers of 

 substance round the centre of the sphere. An agate 

 nodule is a good instance of this : every one is familiar 

 with the beautiful markings, approximating in form to 

 concentric circles, which are presented when the spherical 

 nodules of agate are cut through and polished, so as to 

 show their structure, and which are due to the nodule 

 consisting of concentric layers stratified round its centre. 

 Whatever may be the kind of force to which the formation 

 of these nodules is due, whether it is only capUlarity 

 acting under special circumstances, or some quite peculiar 

 species of force, it is evidently a simply attractive force 

 Hke gra-\T.tation or capillarity, and not a polar force like 

 that of a magnet. 



It is evident also that the structure of the agate nodule 

 depends on its form, and not the converse. The relation 

 of structure to form, indeed, consists in this, that the 

 solid sphere is composed of a number of concentric hollow 



