ATTRACTION. 151 



they become pyramids, shewing half a tetrahedron, half an octohe- 

 dron, &tc. which would be completed, by continuing the group down- 

 ward, in the same form. The marbles used for play, by children, 

 may be made use of for similar illustrations. But it is obvious that 

 the truth of this view, beautiful and probable as it is, cannot be de- 

 monstrated, nor is it perhaps inconsistent with that of Haiiy ; for if 

 the ultimate integrant particles of bodies are spheres or spheroids ; as 

 they may, by the supposition, be grouped so as to produce Hauy's in- 

 tegrant molecules, and these may be the last term of mechanical 

 analysis, although the ultimate particles of which they are composed, 

 may be spheres ; and when they are inconceivably small, there will 

 be no appreciable difference between the plane and curved faces. 

 Indeed, in Hauy's theory, the passage by increment and decrement, 

 is supposed to be by particles so minute, that the steps cannot be or- 

 dinarily perceived, although the imperfection of the process some- 

 times renders them more or less obvious.* 



5. CHEMICAL ATTRACTION OR AFFINITY. 



(a.) It is exclusively, a corpuscular power. 



(6.) Its three principal characteristics, are : it is exerted at insen- 

 sible distances ; between particles only ; and those particles are al- 

 ways heterogeneous. 



(c.) Its effects are, a change of properties more or less complete : 

 it is unlike cohesion, which induces no change of properties, but 

 merely of bulk or form. 



(d.) The change of properties, in the cases where weak affinities 

 are exerted, is often slight ; giving in many instances only the mod- 

 ified properties of the parent substances ; as examples, we can men- 

 tion watery solutions generally, as of salts, gum and sugar, and often 

 alcoholic solutions, as of resins ; and among fluids, alcohol and water, 

 and water and acids ; the union in such cases, is quiet, and attend- 

 ed with no remarkable appearances. 



(e.) But the union is permanent and cannot be destroyed by mechan- 

 ical means. Solutions of salts, sugar, gum, and alcohol, in water, are 

 instances in point ; they are not decomposed by repose, by agitation 

 or by filtration, thus proving that the union is not merely mechanical. 



(f.) This class of compounds should be considered as midway be- 

 tween mere aggregation and energetic chemical combination ; The 

 union is chemical, inasmuch as it is not subverted by mechanical 

 means ; but these compounds partake of the nature of aggregates, in- 

 asmuch as they present the mitigated properties of the parent sub- 

 stance and no new properties. 



* Mr. Daniel, in a paper in the Eng. Jour, of Science, Vol. I, p. 24, has with great 

 ability, illustrated Dr. Wollaston's theory ; but the limits of this work do not allovv 

 ns to go farther into these metaphysics of crystallization ; a subject which is per* 

 Jiap<? more closely allied to mechanical than to chemical philosophy. 



