SECT, xiv.] CKYSTALLJSATION. 123 



of the plate. A sufficiently pure metallic surface can only be 

 obtained by immersing the platina in very strong hot sulphuric 

 acid and then washing it in distilled water, or by making it 

 the positive pole of a pile in dilute sulphuric acid. It appears 

 that the force of cohesion as well as the force of affinity, exerted 

 by particles of matter, extends to all the particles within a 

 very minute distance. Hence the platiua, while drawing the 

 particles of the two gases towards its surface by its great 

 cohesive attraction, brings them so near to one another that 

 they come within the sphere of their mutual affinity, and a 

 chemical combination takes place. Dr. Faraday attributes 

 the effect in part also to a diminution in the elasticity of 

 the gaseous particles on their sides adjacent to the platina, 

 and to their perfect mixture or association as well as to the 

 positive action of the metal in condensing them against its 

 surface by its attractive force. The particles when chemi- 

 cally united run off the surface of the metal in the form of 

 water by their gravitation, or pass away as aqueous vapour 

 and make way for others. 



The particles of matter are so small that nothing is known 

 of their form, further than the dissimilarity of their different 

 sides in certain cases, which appears from their reciprocal 

 attractions during crystallisation being more or less powerful, 

 according to the sides they present to one another. Crystal- 

 lisation is an effect of molecular attraction regulated by cer- 

 tain laws, according to which atoms of the same kind of 

 matter unite in regular forms a fact easily proved by dissolv- 

 ing 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. 160). 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 aggregation 

 of a few particles would, if continued, cause the addition of 

 more ; and the process would go on as long as any particles 



