Thoughts on Causality. 



249 



important points. The first is, that we discern more than 

 a single mode of activity; in other words, the forces of 

 nature are not all mutually convertible. Some of the 

 molecular forces seem to be so. Heat may perhaps be 

 transformed into electricity ; electricity into heat, and so 

 on. And yet, even amongst these, we note a want of simi- 

 larity. Magnetism and electricity are polar forces ; but 

 it is not pretended that heat, light and affinity are such. 

 Though light and heat are both molecular vibrations, and 

 hence congeneric, they can hardly be regarded as conspecific, 

 equivalent and intertransmutable, since they are vibrations 

 of different intensities. Electricity, magnetism, chemical 

 and cohesive attractions, though sustaining undoubted 

 correlations with heat and light, are not known or believed 

 to be vibrations or modes of motion ; and it seems like a 

 stretch of evidence to pronounce them conspecific with 

 phenomena which are such. Repulsion, moreover, is a 

 molecular force looming distinctly above the horizon 

 of discovery; and there are indications that its intensity 

 is inversely as the fifth 'power of the distance, while chemi- 

 cal affinity varies as the cube of the distance. Gravity 

 is a force varying inversely as the square of the distance; 

 and it is, moreover, a force which has never, to our know- 

 ledge, resulted from the transformation of any other force ; 

 nor does it sustain quantitative or any other correlations 

 with any other force. Here, then, in the field of inorganic 

 nature, we find forces producing three classes of pheno- 

 mena — attractions, repulsions and vibrations. Of the 

 attractions, certain ones affect aggregates, and others, 

 molecules ; the former are again differentiated into uon polar 

 (gravitation) and polar (magnetism and electricity) while 

 the later embrace cohesiou and affinity. The vibrations, 

 moreover, are different intensities as before stated. We have, 

 therefore, three different genera of inorganic force, and at 



Trans. viii.'\ 



32 



