114 FRAGMENTS OF SCIENCE. 



steps resembling those up which the Egyptian traveller is 

 dragged by his guides. The human mind is as little dis 

 posed to look unquestioning at these pyramidal salt-crys 

 tals as to look at the pyramids of Egypt without inquiring 

 whence they came. How, then, are those salt-pyramids 

 built up ? 



Guided by analogy, you may, if you like, suppose that 

 swarming among the constituent molecules of the salt, 

 there is an invisible population, controlled and coerced by 

 some invisible master, and placing the atomic blocks in 

 their positions. This, however, is not the scientific idea, 

 nor do I think your good sense will accept it as a likely 

 one. The scientific idea is that the molecules act upon each 

 other without the intervention of slave labor ; that they 

 attract each other and repel each other at certain definite 

 1 mints, or poles, and in certain definite directions ; and that 

 the pyramidal form is the result of this play of attraction 

 and repulsion. While, then, the blocks of Egypt were laid 

 down by a power external to themselves, these molecular 

 blocks of salt are self-posited, being fixed in their places by 

 the forces with w T hich they act upon each other. 



I take common salt as an illustration because it is so 

 familiar to us all ; but any other crystalline substance would 

 answer my purpose equally well. Everywhere, in fact, 

 throughout inorganic Nature, we have this formative power, 

 as Fichte would call it this structural energy ready to 

 come into play, and build the ultimate particles of matter 

 into definite shapes. The ice of our winters and of our 

 polar regions is its handywork, and so equally are the 

 quartz, felspar, and mica of our rocks. \ Our chalk-beds are 

 for the most part composed of minute shells, which are also 

 the product of structural energy ; but, behind the shell, as 

 a whole, lies a more remote and subtle formative act. These 

 shells are built up of little crystals of calc-spar, and to form 

 these crystals the structural force had to deal with the 



