PREFACE. 7 



ance, and there appears to be no reason to divest it of the functions com- 

 mon to all matter, or superficially to appropriate it to certain affections. 

 Again, the phenomena of transparency and opacity are, to my mind, more 

 easily explicable by the former than by the latter theory ; as resulting from 

 a difference in the molecular arrangement of the matter affected. In re- 

 gard to the effects of double-refraction and polarisation, the molecular 

 gives at once a reason for the effects upon the one theory, while upon the 

 other we must, in addition to previous assumptions, further assume a dif- 

 ferent elasticity of the ether in different directions within the doubly- 

 refracting medium. The same theory is applicable to Electricity and 

 Magnetism ; my own experiments on the influence of the elastic intermedium 

 on the voltaic-arc, and those of Faraday on electrical induction, furnish 

 strong arguments in support of it. My inclination would lead me to de- 

 tain you on this subject much longer than my judgment deems advisable : 

 I therefore content myself with offering it to your consideration, and, 

 should my avocations permit, I may at a future period more fully develope 



it 



Light, Heat, Electricity, Magnetism, Motion, and Chemical-affinity, are 

 all convertible material affections ; assuming either as the cause, one of 

 the others will be the effect : thus heat may be said to produce electricity, 

 electricity to produce heat ; magnetism to produce electricity, electricity 

 magnetism ; and so of the rest. Cause and effect, therefore, in their - ab- 

 stract relation to these forces, are words solely of convenience: we are 

 totally unacquainted with the ultimate generating power of each and all 

 of them, and probably shall ever remain so ; we can only ascertain the 

 normae of their action : we must humbly refer their causation to one omni- 

 present influence, and content ourselves with studying their effects and 

 developing by experiment their mutual relations. 



I have transposed the passages relating to voltaic action and 

 catalysis, but I have not added a word to the above quotation, 

 and, as far as I am now aware, the theory that the so-called im- 

 ponderables are affections of ordinary matter, that they are re- 

 solvable into motion, that they are to be regarded in their action 

 on matter as forces, and not as specific entities, and that they are 

 capable of mutual reaction, thence alternately acting as cause and 

 effect, had not at that time been publicly advanced. 



My original Essays being a record of lectures, and being pub- 

 lished by the managers of the Institution, I necessarily adhered 

 to the form and matter which I had orally communicated. In 

 preparing subsequent editions I found that, without destroying 

 the identity of the work, I could not alter the style ; although it 

 would have been less difficult and more satisfactory to me to have 

 done so, the work would not have been a republication ; and I 

 was for obvious reasons anxious to preserve as far as I could the 

 original text, which, though added to, is but little altered. 



The form of lectures has necessarily continued the use of the 



