INTERMIXTURE OF EFFECTS. 493 



through all the interchanges. Were this proved, it would 

 establish what constitutes transformation, as distinguished 

 from the simple fact of reciprocal causation. The fact in issue 

 is simply the quantitative equivalence of all these natural 

 agencies ; whereby a given quantity of any one is convertible 

 into, and interchangeable with, a given, and always the same, 

 quantity of any other : this, no less, but also no more. It cannot 

 yet be said that the law has been fully proved of any case, ex- 

 cept that of interchange between heat and mechanical motion. 

 It does seem to be ascertained, not only that these two are 

 convertible into each other, but that after any number of con- 

 versions the original quantities reappear without addition or 

 diminution, like the original quantities of hydrogen and oxygen 

 after passing through the condition of water. If the same 

 thing comes to be proved true of all the other forces, in rela- 

 tion to these two and to one another, the law of Conservation 

 will be established ; and it will be a legitimate mode of ex- 

 pressing the fact, to speak of Force, as we already speak of 

 Matter, as indestructible. But Force will not the less remain, 

 to the philosopher, a mere abstraction of the mind. All that 

 will have been proved is, that in the phenomena of Nature, 

 nothing actually ceases without generating a calculable, and 

 always the same, quantity of some other natural phenomenon, 

 which again, when it ceases, will in its turn either generate a 

 calculable, and always the same, quantity of some third phe- 

 nomenon, or reproduce the original quantity of the first. 



In these cases, where the heteropathic effect (as we called it 

 in a former chapter)* is but a transformation of its cause, or 

 in other words, where the effect and its cause are reciprocally 

 such, and mutually convertible into each other ; the problem 

 of finding the cause resolves itself into the far easier one of 

 finding an effect, which is the kind of inquiry that admits of 

 being prosecuted by direct experiment. But there are other 

 cases of heteropathic effects to which this mode of investiga- 

 tion is not applicable. Take, for instance, the heteropathic 

 laws of mind ; that portion of the phenomena of our mental 

 nature which are analogous to chemical rather than to dyna- 



* Ante, ch. vii. 1. 



