106 COHESION CRYSTALLIZATION. SECT. XIV. 



merable. The spherical form of rain-drops ; the difficulty of 

 detaching a plate of glass from the surface of water ; the force 

 with which two plane surfaces adhere when pressed together ; 

 the drops that cling to the window-glass in a shower of rain 

 are all effects of cohesion entirely independent of atmospheric 

 pressure, and are included in the same analytical formula 

 (N. 162) which expresses all the circumstances accurately, 

 although the laws according to which the forces of cohesion and 

 repulsion vary are unknown. It is more than probable that the 

 spherical form of the sun and planets is due to the force of 

 cohesion, as they have every appearance of having been at one 

 period in a state of fusion. 



A very remarkable instance has occasionally been observed 

 in plate-glass manufactories. After the large plates of glass 

 of which mirrors are to be made have received their last polish, 

 they are carefully wiped and laid on their edges with their 

 surfaces resting on one another. In the course of time the 

 cohesion has sometimes been so powerful, that they could not be 

 separated without breaking. Instances have occurred where two 

 or three have been so perfectly united, that they have been cut 

 and their edges polished as if they had been fused together ; and 

 so great was the force required to make the surfaces slide that 

 one tore off a portion of the surface of the other. 



In liquids and gases the forms of the particles have no in- 

 fluence, they are so far apart ; but the structure of solids varies 

 according to the sides which the particles present to one another 

 during their aggregation. Nothing is known of their form further 

 than the dissimilarity of their different sides in certain cases, 

 which appears from their reciprocal attractions during crys- 

 tallisation being more or less powerful according to the sides 

 they present to one another. Crystallisation is an effect of 

 molecular attraction regulated by certain laws, according to 

 which atoms of the same kind of matter unite in regular forms 

 a fact easily proved by dissolving a piece of alum in pure water. 

 The mutual attraction of the particles is destroyed by the water ; 

 but, if it be evaporated, they unite, and form in uniting eight- 

 sided figures called octahedrons (N. 163). These however are 

 not all the same. Some have their angles cut off, others their 

 edges, and some both, while the remainder take the regular form. 

 It is quite clear that the same circumstances which cause the 



