xiv PREFACE TO THE FIFTH EDITION. 



inclination would lead me to detain you on this subject much 

 longer than my judgment deems advisable ; I therefore con- 

 tent myself with offering it to your consideration, and, should 

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



develop 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 

 abstract relation to these forces, are words solely of conve- 

 nience. 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 normse 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 quotations ; and, as far as I am now aware, 

 the theory that the so-called imponderables are affec- 

 tions of ordinary matter, that they are resolvable 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 Essay being a record of lectures, and 

 being published by the Managers of the Institution, I 



* See also an experiment shown at the London Institution, p. 196, post. 



